feasibility of ambulatory, continuous 24-hour finger...

65

Feasibility of Ambulatory, Continuous 24-HourFinger Arterial Pressure Recording

Ben P.M. Imholz, Gerard J. Langewouters, Gert A. van Montfrans, Gianfranco Parati,Jeroen van Goudoever, Karel H. Wesseling, Wouter Wieling, and Giuseppe Mancia

We tested Portapres, an innovative portable, battery-operated device for the continuous, noninvasive, 24-hourambulatory measurement of blood pressure in the finger. Portapres is based on Finapres, a stationary devicefor the measurement of finger arterial pressure. Systems were added to record signals on tape, to alternatemeasurements between fingers automatically each 30 minutes, and to correct for the hydrostatic height of thehand. We compared the pressure as measured by Portapres with contralateral intrabrachial pressuremeasured with an Oxford device. Results were obtained in eight volunteers and 16 hypertensive patients. Timelost due to artifact was about 10% for each device. In two patients a full 24-hour Oxford profile was notobtained. In the remaining 22 subjects finger systolic, diastolic, and mean pressures differed +1 (SD 9), —8(6), and —10 (6) mmHg, respectively, from intrabrachial pressure. These diastolic and mean pressureunderestimations are similar to what was found earlier for Finapres, are typical for the technique, and aresystematic. Avoiding brisk hand movements resulted in fewer waveform artifacts. The hand had to be keptcovered to continue recording at low outside temperatures. Sleep was not disturbed by Portapres, and arterialpressure showed a marked fall during siesta and nighttime. There were no major limitations in behavior, andno discomfort that originated from continuous monitoring was reported. Measurements continued normallyduring physical exercise. Portapres provides for the first time continuous 24-hour, noninvasive ambulatoryblood pressure waveform monitoring and offers real and obvious advantages over current noninvasive andinvasive devices. (Hypertension 1993;21:65-73)

KEY WORDS • blood pressure determination • exercise • blood pressure monitors • monitoring,physiological • feasibility studies • hydrostatic pressure

I n clinical practice ambulatory blood pressure mon-itoring is usually performed by commercial, non-invasive devices that read blood pressure only

infrequently, such as every 10-30 minutes.1 These de-vices thus record only a fraction of the blood pressuresoccurring.2 To avoid artifactual recordings the subjecthas to be (nearly) immobile during the measurement,which means that readings during episodes of motionand exercise cannot be obtained.3 Furthermore, inter-mittent blood pressure readings cannot properly assessblood pressure variability.4 Finally, these devices oftendisturb the sleep of patients.

Since 1982 finger arterial pressure measurements byFinapres have allowed blood pressure to be measurednoninvasively on a beat-to-beat basis,5'6 and the pres-sures obtained correspond closely to intra-arterial mea-surements.7-'3 Finapres is a stationary device, but we

From the Department of Internal Medicine (B.P.M.I., G.A. vanM., W.W.) and TNO-Biomedical Instrumentation (G.J.L., J. vanG., K.H.W.), Academic Medical Centre, Amsterdam, The Neth-erlands, and the Cattedra di Semiotica Medica and Istituto diClinica Medica Generale e Terapia Medica, Universita di Milano;Centra di Fisiologia Clinica e Ipertensione (G.P., G.M.), OspedaleMaggiore, Milan, Italy.

Supported by grant (87 068) from the Netherlands Heart Founda-tion, and travel grants Nr 1.4.1/1 from EC Concerted Action.

Address for correspondence: Dr. W. Wieling, Department ofInternal Medicine, Academic Medical Centre, Room F4-257,Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands.

Received April 7, 1992; accepted in revised form July 20, 1992.

were impressed by its insensitivity to motion artifactduring exercise12 and orthostatic and Valsalva maneu-vers.10-11'13 This and clinical interest have stimulated thefurther development of Finapres to a portable, battery-operated version for 24-hour ambulatory blood pres-sure measurement, named Portapres.

For 24-hour ambulatory recording, the addition ofthree new systems was required: 1) a system to switchblood pressure monitoring between fingers at shortintervals since continuous monitoring on a single fingermay cause transient painful or numbing sensations inthe finger, usually occurring at intervals from 30 min-utes to 3 hours in awake subjects; 2) a system to monitorthe hydrostatic height of the measured finger withrespect to heart level; and 3) a system to record signalwaveforms and device diagnostics information on tape.

Our goal was to investigate the performance of thisnew device: for artifacts by measuring monitoring timelost per 24 hours, for blood pressure accuracy bycomparing with simultaneous intrabrachial blood pres-sure, for finger switching by assessing the pressuredifferences between fingers, and finally, for effects ofchanging hydrostatic height by observing subsequentchanges in finger pressure levels and pulse pressure.

MethodsSubjects

The study included eight male, normotensive volun-teers, aged 19-32 years, and 16 patients (three women,

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66 Hypertension Vol 21, No 1 January 1993

FIGURE 1. View of Portapres. The main unit (1) in thecenter contains, among other components, a TEAC HR-10Jcassette tape recorder (2) and a lithium battery pack (3) andcan be carried by a belt (4). A cable (5) connects to the patientfront-end box (6). Blood pressure is measured at a finger usingtwo finger cuffs (7) in alternation. The hydrostatic heightcorrection system consists of a reference transducer (8) car-ried at heart level, a liquid-filled tube, and a compliant bag(9) attached to the finger cuffs.

13 men), aged 20-60 years, having essential hyperten-sion of varying degree. Two patients received antihyper-tensive combination therapy; the remaining patientsreceived monotherapy. Treatment was discontinued inall cases at least 2 weeks before the measurementsbegan. The volunteers and eight hypertensive patientswere investigated in the Academic Medical Centre ofthe University of Amsterdam. Another eight hyperten-sive patients were investigated with the same equipmentand under the same protocol in the University Hospital(Ospedale Maggiore) of Milan. Because we appliedintra-arterial and finger devices on contralateral arms,each subject was required to have an average of sixauscultatory systolic and diastolic pressures agree towithin 5 mm Hg when measured simultaneously on theleft and right upper arm by two observers reading asingle mercury column.

Each participant gave his or her informed consent tothe study. The study protocol was approved by the ethicscommittees of the institutions involved.

Ambulatory Finger Blood PressurePortapres provides an indirect measurement of blood

pressure in a finger based on the arterial volume-clampmethod5'6 and the physiocal (physiological calibration)criteria for the full unloading of the arteries14 in amanner identical to Finapres. As shown in Figure 1, themodel 1 prototype consisted of a small patient front endunit connected to a larger main unit. The front end box,which is worn on the back of the hand or on the wrist,measures 30x50x70 mm and weighs 200 g. It containspreamplifiers, a pressure sensor, a cuff pressure controlvalve, and a pneumatic switch to pressurize one of twopermanently mounted cuffs on the annular and middlefingers, alternating every 30 minutes. This schemaavoids the discomfort associated with long-term blood

pressure measurement in a single finger and therebyallows measurement to continue for 24 hours. At eachalternation, the temperature-compensated pressuretransducer is rezeroed, avoiding any appreciable zerodrift in the Portapres measurements.

The larger main box measures 250x210x60 mm,weighs approximately 3 kg, and is worn on the abdomensecured by two belts, or hung from a shoulder with onebelt. This box contains a lithium battery pack, a highlyefficient air pump, a cassette tape recorder, a microcom-puter, and various analog electronic components. Thefront panel comprises a 32-character liquid crystaldisplay, a marker button, and a small number of re-cessed buttons to interact with device operation. Thebattery pack provides the power necessary to recordpressure for 24 hours. The pump, which is poweredfrom the battery pack, supplies air pressure to thepatient front end unit.

A separate recording system measures the hydrostaticheight of the hand, continuously tracking pressurechanges induced by moving the hand. A liquid-filledtube is connected to a pressure transducer at one endand a compliant bag at the other. The bag is attached tothe finger, and the transducer is fixed at the level of theintra-arterial transducer. Its output is separately re-corded but is also automatically added to the fingerpressure signal. Addition is based on previous observa-tions that changes in finger height appear negated inmean finger pressure, although with a delay. In theheight recording system, therefore, a low pass filter isused, with a similar time constant (2 seconds) as thephysiological delay time constant. Thus, ambulatoryfinger arterial pressure is corrected continuously for anyheight changes occurring between the finger and theintra-arterial reference transducer at heart level. Giventhe time constant, however, the system cannot correctfor shocks and abrupt movements.

A built-in four-channel instrumentation cassette re-corder (TEAC HR-10J, Teac Corporation, Japan) isused for storage of the height-corrected finger arterialpressure signal (Portap), the hydrostatic height signalwith superimposed amplitude-coded marker signals(Height), and the simultaneous intra-arterial pressuresignal (LAP). The fourth channel of the recorder is usedfor the compensation of tape flutter.

The microcomputer controls and monitors the entiresystem. It includes an algorithm to start the fingermeasurement; clock timers; and algorithms to digitizesignal waveforms, to recognize and measure heartbeats,to implement physiocal, to perform adaptive cuff pres-sure control, to recognize artifacts, to switch betweenfingers, and to display, record, and output data, devicediagnostics information, and markers.

The analog system includes a modern battery powermanagement system and an automatic, patient-adaptiveservocontroller. A monitor cable can be connected tothe main unit for visual inspection of the recordedsignals on an external oscilloscope or strip chart re-corder during the 24-hour run.

Intra-arterial PressureIntra-arterial pressure was measured through a Tef-

lon cannula with a length of 11 cm and an internaldiameter of 1 mm. After local anesthesia with 1%lidocaine, the cannula was inserted into the brachial



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Imholz et al Phasic Ambulatory Finger Blood Pressure 67

artery of the nondominant arm by the Seldinger tech-nique. The cannula was connected via a 70-cm-longpolyethylene tube to the transducer of a commercialOxford Medilog Mark II system (Romulus TechnologyLtd, Brunei University, Uxbridge, UK).15 To obtainimproved zero stability, power to the Oxford/Akers B22transducer was supplied from Portapres via a specialbridge amplifier. For storage of the intrabrachial signalwe used one channel of the Portapres built-in TEACrecorder.

After mounting a cassette tape, all recorder channelswere calibrated by a 0-2 V (0-200 mm Hg) squarewave. After a warm-up period of at least 30 minutes,16

the Oxford transducer was calibrated by an accurate,servo-stabilized pneumatic pressure staircase of 0-100-200-100-0 mm Hg of 5 minutes duration generated byPortapres. The zero offset could thus be nulled and thesensitivity adjusted to make the calibrations of bothpressure signals exactly identical. The zero offset waschecked and recorded at approximately 4 and 20 hoursinto the 24-hour measurement period. The calibrationsequence was repeated at the end of the 24-hour period.No changes in transducer sensitivity were seen. Thezero offset usually showed an upward drift during themeasurements. The zero offset observed at the endstage ranged from -5.2 to +13.4 mmHg with anaverage value for the group of 1.4 mm Hg and standarddeviation of 3.6 mm Hg. To correct intra-arterial pres-sures for zero drift, we used half the end-stage zerooffset for each subject throughout the entire 24-hourrecording.1517 Assuming linear drift, the 24-hour meansare thus correct.

The dynamic performance of the invasive system wasmeasured by applying stepwise pressure changes of100-mm Hg amplitude and 10-msec rise time to theOxford transducer and recording the responses on ahigh speed electronic strip chart recorder (modelTA4000, Gould Inc., Cleveland, Ohio). The averageoverall resonance frequency of the combined systemwas 19 Hz (range 14-30 Hz). This represents an im-provement over the frequency response of an unmodi-fied Oxford Medilog Mark II system, which is near 10Hz,1517 but is still insufficient to guarantee a correctmeasurement of intrabrachial systolic pressure levels atall times.18

ProtocolAll hypertensive patients but not the volunteers were

hospitalized at least 1 day before the 24-hour protocolbegan. The cannulation of the brachial artery wasperformed between 9 and 10 AM in the morning of thefirst day. After the completion of another experimentalprotocol, to be reported elsewhere, Portapres was in-stalled on the hand contralateral to the intrabrachialsite. Properly sized finger cuffs according to the Por-tapres manual were carefully wrapped around the an-nular and middle fingers of the subject. Beginning onthe annular finger, measurements started at 1 PM andcontinued until 1 PM the next day. During the recordingperiod the subjects were free to move within the hospi-tal and to engage in the usual activities of inpatients notconfined to bed. In addition, a number of activities werescheduled for all subjects in the following sequence:from 2 to 3:30 PM, siesta; from 4:45 to 5:15 PM, cycling at50 W, 50-60 rpm; from 10 PM to 6 AM, sleep; from 10 to

TABLE 1. Time Lost Due To Artifacts

Type of artifact

Device-related

Flushing and zeroing

Faulty connection

Pump artifacts

Empty battery pack

Fall of Portapres

Loose air tube

Other technical

Patient-related

Flow disturbances

Repeated start-ups

Movements

Total

IAP

Minutes

104

412

2,047

630

137

3,330

%

0.3

1.2

5.9

1.8

0.4

9.6

Portap

Minutes

164

262

29

600

1,251

390249

2,945

%

0.5

0.8

0.1

1.7

3.6

1.1

0.7

8.5

Portap, hydrostatic height-corrected finger pressure recordedwith Portapres; IAP, zero drift corrected intrabrachial pressureobtained with the Oxford device. Percentage of time lost is withrespect to the total registration time in the 24 subjects of 34,556minutes.

10:30 AM, first walk outside hospital; from 11 to 11:30AM, second walk outside hospital. The subjects wereinstructed not to bend or compress the cuffed fingers.The first six subjects (all normotensive volunteers) weregiven no instructions with respect to the movement ofthe instrumented hand during the recording. The re-maining subjects, however, were asked to keep the handmore stable near heart level, also during physicalactivity.

Data AnalysisOff-line analysis of the 24-hour recordings was done

on an IBM-compatible personal computer system. Be-fore A/D conversion, the signals were zero- and gain-adjusted using the tape-recorded calibration. The tapewas then replayed at eight times real-time and A/Dconverted with 0.25 mm Hg resolution at 100 Hz real-time equivalent rate by means of DIGIFAST, a program ofthe FAST software package.19

From the IAP and then the Portap signals beat-to-beat results were obtained by BEATFAST.19 This programidentified for each heartbeat the time of the systolicupstroke, the levels of systolic, true integrated mean,and arterial end-diastolic pressure and height. Instan-taneous pulse rate was computed from the time intervalbetween upstrokes. In addition, the presence of physio-cal periods14 was identified and appropriately marked.Markers were detected in the Height signal and sepa-rately stored.

The comparison of Portap and IAP was performed ona beat-to-beat basis. During visual inspection of thePortap and IAP signals, periods of evident signal dis-turbances were removed. The conditions for rejectionare listed in Table 1. The remaining files were fedthrough a computer program that matched beats inPortap and IAP having the corresponding begin up-stroke instant.

For each matched beat the Portap systolic, mean, anddiastolic difference with the corresponding IAP beat



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was calculated (Portap-IAP) and corrected for Oxfordzero drift. Thus, an overestimation of intra-arterialpressure by Portapres is shown as a positive number.This formed the primary data for statistical comparison.Blood pressures and blood pressure differences werethen averaged by calculating mean and standard devia-tion each half hour in synchrony with finger alternation.These averages were used to present 24-hour bloodpressure profiles and to assess differences between theannular and middle fingers. Separate averages for eachsubject were also computed over the entire 24-hourperiod, and over daytime (nonsleep period), nighttime(sleep), siesta, cycling, and walking, and were pooled forthe group.

Effects of Hydrostatic HeightTo assess whether hydrostatic height, mean pressure

level, and pulse rate affect differences between finger andintrabrachial pressures, the multiple regression of themean pressure differences (Portap—LAP) on these vari-ables was calculated. This was done only for the first sixsubjects (normotensive volunteers) in which the move-ment of the hand was not restricted. In addition, theextent of relative pulse pressure changes (pulse waveamplification20) was studied by computing the multipleregression of the pulse pressure ratio (Portap/IAP) onheight, pressure, and rate. In each of the six subjects a10-20-minute period with marked movements of the handwas selected, comprising at least 600 heartbeats.

Statistical EvaluationMeans and standard deviations were computed and

reported as required by the Association for the Ad-vancement of Medical Instrumentation (AAMI),21 andparametric tests, in particular the / test, were used totest differences in mean values for significance. The95% confidence intervals (CI) and the limits of agree-ment are also based on the / distribution. Multiplelinear regressions were computed to verify the effects ofhydrostatic height and tested for significance with Pear-son's product-moment correlation.

ResultsFigure 2 shows a typical continuous intrabrachial and

finger pressure registration in subject 9 of Table 2. ThePortapres signal obtained during the 24 hours wasvisually similar to the intra-arterial pressure recordedsimultaneously from the contralateral side. The ex-pected fall in blood pressure during the night wasevident in both tracings and so was the blood pressurerise occurring during the exercise periods and the bloodpressure fall occurring during the afternoon siesta.

No subject complained of any disturbance related tothe wearing or the alternate half-hourly inflation of thefinger cuffs during either the day or night.

Artifact RejectionAfter visual inspection of the recordings, 9.6% of the

total intrabrachial and 8.5% of the total finger recordingtimes were rejected for the various reasons mentionedin Table 1. Pump artifacts accounted for nearly two-thirds (5.9%) of the rejections in the intrabrachialOxford recording. In fact, due to Oxford problems insubjects 13 and 22, measurements were available foronly the first 11 and 5 hours, respectively, and no

1h

FIGURE 2. Representative tracings of a continuous 24-hourintrabrachial (IAP) and finger (Portap) pressure measured inpatient 9 of Table 2. The registration runs from 1 PM on thefirst day until 1 PM on the next. The height signal is the bottomtrace. The event marks in this trace each 30 minutes indicateswitching between the finger cuffs. Coded error messages andmanual diary markers are also superimposed on the heightsignal.

24-hour profile could be obtained. The largest loss ofmonitoring time on the finger with Portapres was due toreduced circulation during pinch off of the arterialsystem in the arm (3.6%) and to ambient cold. Thiscauses repeated automatic start-ups of Portapres(1.1%), recognized by the typical cuff pressure searchstaircase generated by the computer at start-up andbroken off if finger circulation is detected as insufficient.A "marching artifact" was seen in two of the first sixsubjects during walking, probably caused by swinging ofthe arm. Walking produced oscillations in finger pres-sure of amplitudes that did not exceed one-third of thepulse pressure. Since intrabrachial and finger artifactsdid not always occur at the same time, of the totalrecording period simultaneous IAP and Portap signalswere available for comparison during 86.7% of the time.

Portap—IAP DifferencesTable 2 lists the intrabrachial pressures and the

deviations of Portapres values from intrabrachial, aver-aged per subject, over all the matched beats in the24-hour period. Note that within a subject all fingerpressure levels usually deviated in the same directionfrom the intrabrachial levels. For example, subject 2 hadrather positive systolic, diastolic, and mean pressuredifferences, whereas subject 9 had the opposite. Itappears that differences within a subject tend to besystematic.

Group averages are summarized in the bottom linesof Table 2 and exclude the partial results of subjects 13and 22. Systolic offset is essentially zero (CI, -3.2 to+4.6) but with high scatter (standard deviation) at 8.7mm Hg. Mean and diastolic pressures show offsets of-8.0 (O, -10.6 to -5.4) and -10.3 mm Hg (CI, -13.1to -7.5), respectively, but with scatter less than 7mm Hg. The computed limits of agreement betweendevices are -16.7 to +18.1 for systolic, -19.6 to +3.6



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TABLE 2.

Patient

1

2

3

4

5

6789

10

11

12

13*

14

15

16

17

18

19

2021

22*

23

24

GroupSD

Pressures and

SBP

125

108

110

126

119

109

135

136

134

143

140

170

147

146

142

142

151

161

129

145

136

161

130

112

134

16

Pressure

IAP

DBP

77

66

61

77

69

61

81

76

82

84

82

99

97

85

77

88

96

98

81

8981

103

8275

80

10

Differences

MAP

96

83

79

96

88

80

100

100

104

107

103

125

117

110

103

109

119

125

100

111

102

125

102

91

102

13

ASBP

Mean

-3.8

18.0

4.5

-5.6

-12.0

3.8

0.5

8.5

-16.8

-8.9

2.5

-0.1

-16.0

-12.9

12.6

-4.4

7.9

10.4

-4.2

0.6

4.0

-30.5

5.36.5

0.7

8.7

SD

12.8

15.7

11.0

10.8

11.4

12.3

12.3

13.2

9.1

11.5

17.8

16.0

12.0

12.6

13.1

10.7

16.9

17.2

10.9

12.1

12.8

12.2

10.7

13.9

12.9

2.4

ADBP

Mean

-9.6

3.5

-4.7

-12.6

-17.0

-6.4

-14.5

-11.1

-13.9

-11.4

-11.5

-9.9

-24.0

-9.9

-6.6

-14.2

-12.7

3.1

-1.1

-3.1

-4.7

-33.9

-4.4

-0.9

-8.0

5.8

SD

8.1

9.8

7.4

6.8

6.2

7.5

7.2

7.1

5.3

6.7

12.0

13.6

7.9

7.7

6.9

5.6

7.7

10.7

7.1

8.7

7.7

6.5

7.40.5

7.9

2.0

AMAP

Mean

-13.6

2.5

-7.6

-15.2

-20.2

-9.2

-16.3

-12.7

-18.1

-16.3

-12.9

-14.5

-26.0

-14.6

-7.6

-15.6

-12.4

3.9

-5.6

-5.7

-7.4

-37.6

-6.7

-0.0

- 1 0 3

6.4

SD

6.8

10.4

6.7

6.2

5.8

6.7

5.8

5.8

5.6

6.6

12.3

13.6

8.2

7.8

6.4

5.6

8.6

12.0

6.6

7.5

6.9

6.2

6.39.6

7.7

2.4

IAP, intrabrachial pressures; SBP, systolic blood pressure; DBP, diastolic blood pressure; MAP, mean arterialpressure; SD, standard deviation. Pressures and pressure differences are in millimeters of mercury. IAP are averagedover 24 hours per patient and again averaged for the group. The differences between finger and intrabrachial pressureare for systolic (ASBP), diastolic (ADBP), and mean (AMAP) levels. Both mean and standard deviation are given.

•Subjects excluded from group average IAP. For these subjects no 24-hour profile was obtained due to Oxford devicemalfunction.

for mean, and -23.1 to +2.5 mm Hg for diastolicpressure, and are larger than observed.

Figure 3 shows one subject's individual and the groupaverage pressure responses and finger-to-brachial pres-sure differences. The top panel represents subject 9 inTable 2 as an example of a finger and intrabrachial24-hour blood pressure profile based on half-hour av-erages. Differences (Portap-IAP) are separately plot-ted. The siesta and nighttime periods show drops in allpressure levels, whereas the walking periods show in-creased pressures. The pressure differences, thoughquite steady, do show apparently systematic variations.Differences are more negative during siesta and sleep,less negative during physical exercise. The group aver-age 24-hour profiles are shown in Figure 3 bottompanel, excluding subjects 13 and 22. Systematic pressuredifferences between the middle and annular fingers canbe seen as a repeating triangle with hourly periodicity.Trends in pressure difference during 24 hours follow thepattern seen in the top panel registration in subject 9.

Figure 4 shows the pressure differences averaged overthe 24-hour period and also separately group averagedfor a number of specific activities. Most activity-grouped

differences were quite similar. However, walking wasassociated with an overestimation of systolic intrabra-chial pressures in the finger and reductions in theunderestimation of mean and diastolic levels. Artifactlevels were higher, leading to a smaller number ofincluded subjects. For cycling, the upward movement offinger pressure with respect to intrabrachial pressurewas not as clear, partially because the timing of thisexcercise was such that the first quarter hour fell in onehalf hour average and the second quarter in the follow-ing half hour period and thus were averaged in withnormal resting values, and partially because the effecton blood pressure of cycling at 50 W is small.

Two-Finger AgreementPressure measured in the annular finger was higher

than in the middle finger. Group average differences forsystolic, diastolic, and mean pressure were 1.1 (SD 8.0),3.3 (5.4), and 2.4 (6.0) mm Hg, respectively. The aver-age differences are small compared with total differ-ences, and only the diastolic difference is significant(p=0.01; paired per subject t test since compared withcommon reference). However, the large standard devi-



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mmHg

180 -

160 -

140 -

120

100

80

60

40

20

0

- 2 0

- 4 0

Portip-lip

13:00 01:00 13:00

mmHg

180 -

160 -

140 -

120 -100 -80 -

60 -40 -20 -0 -

-20 --40

13:00 01:00 13:00

FIGURE 3. Panel A: An individual 24-hour intrabrachial(lap) and finger (Portap) blood pressure profile in patient 9 ofTable 2, composed of30-minute average values of systolic anddiastolic pressures. Intrabrachial traces are the continuouslines, and the finger traces are the dashed lines. Traces,continuous for systolic and dashed for diastolic, at the bottomrepresent the differences. Panel B: The 22-patient groupaverage 24-hour blood pressure profiles presented in the samemanner. Both panels show a substantial dip in pressure in theinactive siesta and sleep periods beginning at 2 PM and 10 PM.Blood pressure differences also dip in these periods and clearlyrise in the highest activity periods of outside walking at 10 AMand 11 AM.

ations indicate the presence of between-subject differ-ences. Indeed, for systolic pressures, five of the 24subjects had differences between fingers greater than 10mm Hg, and a further five had differences greater than5 mm Hg. For diastolic pressures, these numbers werethree and three subjects, for mean pressures, they werethree and three subjects also. A large pressure differ-ence between fingers did not correlate with a largepressure difference between finger and intrabrachialpressures.

Effects of Hydrostatic HeightThe regression of the differences in mean pressure

between finger and brachial pressures and of the pulsepressure ratio on the height of the finger in the first sixsubjects are given in Table 3. The regression slopesdiffer widely between subjects; however, they are statis-

20 -

y 10 •

| • •

-20 •

20 -

10 •

-10 -

-20 •

20 -

o 10 -

I-io-I- 2 0 •

T

24h day night ileita cyclN-22 N-24 N-22 N-22 N-22

walk1 walk*N-18 N-17

FIGURE 4. Bar graphs show average differences and stan-dard deviations for systolic, mean (MA.P.), and diastolicpressures for the 24-hour period as a whole and subdivided forthe specific conditions labeled Total number of subjectsincluded in the different activities is indicated. These numbersare not always 24 because of the artifact rejection explained inTable 1. For example, one entire night's intrabrachial bloodpressure recording had to be rejected because of pump artifact,another because of a broken pressure transducer. Initially,outside walking often disturbed Portapres registrations due torepeated start-ups and movements until measures were takento protect the hand from cold by placing it under the overcoat.

tically significant except in one subject each for meanpressure difference and pulse pressure ratio. Even so,the explained variance is only small. Regression of meanpressure difference on the level of mean pressure andregression of pulse pressure ratio on heart rate werealso significant in most subjects but explained evensmaller fractions of the variance.

DiscussionThe present study was performed to validate Por-

tapres, a device designed to measure ambulatory bloodpressure noninvasively on a beat-to-beat basis. Theresults provide information on the device feasibility.They also allow examination of problem points of thedevice components and a discussion of advantages forclinical and basic research in the field of ambulatoryintra-arterial blood pressure monitoring.

Feasibility of Ambulatory Blood Pressure Monitoringby Portapres

In our subjects the blood pressure signal provided byPortapres could be used for evaluation of the subject'sblood pressure more than 90% of the total recordingtime. Thus, monitoring time lost, at less than 10%, is a



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TABLE 3. Effect of Finger Height on Mean Pressure Differenceand Pulse Pressure Ratio

Patient

123456

Average

Minimum

Maximum

Slope

0.147

-0.010

0.053

0.094

0.115

0.268

0.111

-0.010

0.268

A Mean

%XP

1608

221245

170

45

•P

NS

Slope

0.12

-0.65

-0.57

-0.28

-0.45

-0.40

-0.37

-0.65

0.12

PPR

%XP

2192610138

132

26

*PNS

A Mean, regression of the difference between finger and intra-brachial mean pressure; PPR, regression of finger to brachial pulsepressure ratio on height signal; %XP, percentage of A mean andPPR variance explained by the height signal. Regression slopes arein millimeters of mercury/millimeters of mercury and percent/millimeters of mercury, respectively.

'Except for two not significant measurements atp=0.05 (NS),all slopes are significant atp<0.01 (»test), taking into account thatin the time span considered only approximately 100 independentsamples on the height signal are available.

factor of three better than the minimum requirement ofthe British Hypertension Society.22 No major accidentshappened, and a satisfactory 24-hour profile could beobtained in all subjects. The Portapres signal time lostwas partially due to avoidable technical problems suchas a reused battery pack running out of power or anunreliable signal caused by a fall of the device, and itwas partially due to accidental physiological conditionssuch as diminished finger blood flow due to cold duringoutside walking or to compression of the conduit arter-ies in the arm, unrelated to the Portapres design. Simpleprecautions such as limitation of hand movement andcovering of the hand by a mitella or overcoat whenoutside proved sufficient to reduce such artifacts. Evenwithout these precautions, however, the number ofsatisfactory blood pressure values was high. Indeed, itsoverall percentage was comparable to that obtained bythe intra-arterial Oxford Medilog Mark II recording,which also lost about 10% of the signal.

This proves the feasibility of blood pressure recordingby Portapres. This feasibility is emphasized by a numberof other findings. 1) During the 24-hour period ofregistration by Portapres none of the subjects reporteddiscomfort or other side effects from finger cuff infla-tion. This confirms that selection of a 30-minute inter-val of pressure monitoring on a single finger alternatedwith 30 minutes of normal circulation with the fingercuff deflated was appropriate. 2) All subjects were ableto perform the planned physical activities without diffi-culty. Subjects did not report any major behaviorallimitation despite the fact that Portapres is heavier thancurrent commercial noncontinuous devices. 3) In con-trast with reports on intermittent ambulatory bloodpressure monitoring devices, none of the subjects com-plained of sleep disturbances, and arterial pressurealways showed a marked fall during the nighttime,regardless of the fact that monitoring was continuousand alternated between fingers. The absence of re-

peated inflations of an arm cuff as with ordinary,24-hour blood pressure recorders probably facilitatedthe subjects' acceptance of Portapres. This is furtherenhanced by the low and uninterrupted noise of thePortapres pump.

Differences in Ambulatory Blood Pressures FromPortapres and Medilog

We evaluated differences between blood pressure inthe finger measured by Portapres and in the brachialartery measured invasively by an Oxford Medilog MarkII. Since these measurement sites are separated byapproximately 45 cm, with finger pressure being arterialpressure at an end point of the arterial system, wecannot expect to measure identical pressures. A pres-sure gradient exists in the arterial system, causing distalmean pressure to be at most equal but usually belowmore proximally measured pressures.23 In addition, apulse wave amplification effect exists that increases thepulse amplitude along the arterial system,20 and therebysystolic pressure in particular. Such differences, whichare by no means stable or constant but depend on age,flow,9 blood pressure, and heart rate, can be calledphysiological. Further, technical systems such as Por-tapres or Oxford Medilog can add measurement differ-ences due to incorrect calibration, unstable baseline,incorrect flush rate, insufficient dynamic response,18 orin the case of Portapres, an erroneous arterial unload-ing set point.19 Such differences can be called instru-mental or error. In most cases, it has been impossible todifferentiate the source of an observed difference asphysiological difference or instrumental error. Wetherefore used the word difference throughout.

Comparison with intrabrachial pressure on a pairedbeat basis showed that Portapres underestimated dia-stolic and mean blood pressure to a degree that isregarded as too large by the AAMI21 criteria, i.e., largerthan ±5 (8) mm Hg. Systolic blood pressure levels wereestimated more closely but with larger scatter. Thedifferences within any one subject are rather constantand only show a small and slow, systematic fluctuation.It would appear that the systematic offsets could becompensated for in an average sense by correcting forgroup average offsets. The within-subject standard de-viation, although too large in all subjects for systolicpressures, is within AAMI limits for mean and diastolicpressures in 16 of the 22 subjects in whom a full 24-hourprofile was obtained.

Finally, although systolic blood pressure showed atendency to be overestimated at the highest heart ratevalues seen during the 24-hour period, the differencesin the estimation of diastolic and mean blood pressureactually decreased in ambulant conditions. This is insharp contrast with the performance of intermittentnoninvasive blood pressure monitoring devices, whichmay be accurate at rest but are often inaccurate to thepoint of utmost unreliability during exercise and ambu-lant conditions.2-3-24-26 Thus, the ambulatory blood pres-sure values provided by Portapres compare favorably foraccuracy and insensitivity to motion artifact with thosefrom other intermittent noninvasive systems, havingover them the real advantage of continuous assessmentof the changing blood pressure.



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Portapres ComponentsFinger switching altered the recorded blood pressure

values for undetected reasons, but the changes were toosmall as to fail to offer the 24-hour blood pressureprofile when group data were considered. Importantly,differences between fingers did not introduce any sub-stantial differences in the estimates of 24-hour bloodpressure standard deviations. The larger differencesobserved between fingers in some subjects can beremoved almost completely by taking half-hour averages(fifty-fifty) of pressures from each finger.

The system to measure hydrostatic height of thefinger showed a slight but statistically significant positivecorrelation between mean pressure differences andmeasured height (Table 3). Although at first sight thismight suggest a less than perfect functioning of thecompensation device, the large differences in the regres-sion slopes between subjects, although recorded withthe same device, indicate that the phenomenon isphysiological and not instrumental. We were aware thatraising the hand reduces pulse pressure, which is why weincluded this computation, but we were surprised to seethis positive effect on mean pressure. A possible expla-nation may be that raising the hand reduces flow, asobserved recently by Joyner.27 The reduced flow resultsin a reduced pressure gradient along the arteries andincreased peripheral mean pressure.9

Since this effect causes an increase in mean fingerpressure of approximately 1 mm Hg by raising the hand10 cm and a decrease in pulse pressure of 3% and sincesubjects moved their hands 70-100 cm in height, thiseffect alone causes a peak-to-peak deviation of 7-10mm Hg in mean pressure difference and a pulse pres-sure ratio scatter of 6%. After the first six subjects,therefore, we instructed the remaining 18 subjects tokeep the hand being measured near heart level. Toreach maximal recording accuracy and smallest rejec-tion of artifact beats, it seems important to limit thedegree of the hand excursion above and below the heartand to perform physical activity without sudden changesin hand level.

Since Portapres is essentially a Finapres device, Por-tapres results can be compared with those of Finapres.In a previous study28 in 40 patients we have seen thatFinapres systolic, diastolic, and mean blood pressurediffered by an average of -3.5, -4.4, and -8.0 mm Hg,respectively, from intrabrachial pressure, although inother studies, the systolic and diastolic offsets werelarger (for review, see Reference 28). In the presentstudy, the Portapres blood pressure levels differed fromintrabrachial levels by an average of +0.7, —8.0, and-10.3 mmHg, i.e., less for systolic, more for diastolicand mean pressures. However, none of the differenceswith the Finapres data from the previous study werestatistically significant (p=0.05). Thus, Portapres ambu-latory blood pressure levels show deviations similar tostationary Finapres.

Portapres in Cardiovascular ResearchThe availability of a device for the noninvasive mea-

surement of ambulatory blood pressure on a continuousbasis carries obvious advantages for cardiovascular re-search. With current noninvasive methods, ambulatoryblood pressure can be assessed on an intermittent basis

only. The overall number of values that can be collectedin 24 hours (usually less than 100) represents just about1/1,000 of the values that occur over this period. Thisprevents the investigation and knowledge of 24-hourblood pressure phenomena of great basic and clinicalinterest such as acute changes in blood pressure in-duced by behavior, spectral blood pressure events,drug-induced blood pressure falls, or overall bloodpressure variability.

Also, conventional intra-arterial blood pressure mon-itoring by the Oxford Medilog method, although pro-viding a large and accurate body of registrations, isconfined to a few specialized centers worldwide and isnot devoid of risks for the patient.29 Obtaining the samelarge body of registrations by a noninvasive and reason-ably accurate device, such as Portapres, would gTeatlyadvance the potential for studying blood pressure con-trol mechanisms operating in daily life, in health anddisease. It may also offer a tool to accurately assess theantihypertensive efficacy of new drugs.

References1. O'Brien E, Cox JP, O'Malley K: Ambulatory blood pressure mea-

surement in the evaluation of blood pressure lowering drugs, (edi-torial review) / Hypertens 1989;7:243-247

2. Parati G, Mutti E, Ravogli A, Trazzi S, Villani A, Mancia G:Advantages and disadvantages of non-invasive ambulatory bloodpressure monitoring. / Hypertens 1990;8(suppl 6):S35-S38

3. White WB, Lund-Johansen P, Omvik P: Assessment of four ambu-latory blood pressure monitors and measurements by cliniciansversus intrabrachial blood pressure at rest and during exercise. AmJ Cardiol 1990;65:60-66

4. Mancia G, Parati G: Experience with 24-hour ambulatory bloodpressure in hypertension. Am Heart J 1988;116:1134-1140

5. Peniz J: Photoelectric measurement of blood pressure, volumeand flow in the finger, in Albert A, Vogt W, Helbig W (eds): Digestof the 10th International Conference on Medical and Biological Engi-neering. Dresden, FRG, International Federation for Medical andBiological Engineering 1973, p 104

6. Wesseling KH, de Wit B, Settels JJ, Klawer WH: On the indirectregistration of finger blood pressure after Penfiz. Funkt Biol Med1982;l:245-250

7. Molhoek GP, Wesseling KH, Settels JJ, van Vollenhoven E,Weeda HWH, de Wit B, Amtzenius AC: Evaluation of the Pefiazservo-plethysmo-manometer for the continuous, non-invasive mea-surement of finger blood pressure. Basic Res Cardiol 1984;79:598-609

8. Smith NT, Wesseling KH, de Wit B: Evaluation of two prototypedevices producing noninvasive, pulsatile, calibrated blood pressuremeasurement from a finger. J Clin Monit 1985;l:17-29

9. Wesseling KH, Settels JJ, van der Hoeven GMA, Nijboer JA,Butijn MWT, Dorlas JC: Effects of peripheral vasoconstriction onthe measurement of blood pressure in a finger. Cardiovasc Res1985;19:139-145

10. Imholz BPM, van Montfrans GA, Settels JJ, van der HoevenGMA, Karemaker JM, Wieling W: Continuous non-invasive bloodpressure monitoring; reliability of Finapres device during the Val-salva manoeuvre. Cardiovasc Res 1988;22:390-397

11. Parati G, Casadei R, Gropelli A, Di Rienzo M, Mancia G: Com-parison of finger and intrabrachial blood pressure monitoring inrest and during laboratory tests. Hypertension 1989;13:647-655

12. Idema RN, van den Meiracker AH, Imholz BPM, Man in't VeldAJ, Ritsema van Eck AD, Settels JJ, Schalekamp AH: Comparisonof Finapres non-invasive beat-to-beat finger blood pressures withintrabrachial artery pressures during and after bicycle ergometry.J Hypertens 1989;7:S58-S59

13. Imholz BPM, Settels JJ, van den Meiracker AH, Wesseling KH,Wieling W: Non-invasive continuous finger blood pressure mea-surement during orthostatic stress compared to intrabrachial pres-sure. Cardiovasc Res 1990;24:214-221

14. Wesseling KH: Finapres, continuous non-invasive ringer arterialpressure based on the method of Peflaz, in Meyer-Sabellek W,Anlauf M, Gotzen R, Steinfeld L (eds): Blood Pressure Measure-ment Darmstadt, FRG, Steinkopff Verlag, 1990, pp 161-172



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Wesseling, W Wieling and G ManciaB P Imholz, G J Langewouters, G A van Montfrans, G Parati, J van Goudoever, K H

Feasibility of ambulatory, continuous 24-hour finger arterial pressure recording.

Print ISSN: 0194-911X. Online ISSN: 1524-4563 Copyright © 1993 American Heart Association, Inc. All rights reserved.

is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231Hypertension doi: 10.1161/01.HYP.21.1.65

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