Mike McEvoy, PhD, RN, CCRN, NRPSenior Staff RN – Cardiothoracic Surgical ICUs

Albany Medical College – Albany, New YorkChair – Resuscitation Committee – Albany Medical Center

EMS Coordinator – Saratoga County, New YorkEMS Editor – Fire Engineering magazine

Check a Pulse! When to Question SpO2, NIBP & EtCO2 Readings

Learning Objectives

Upon completion of the presentation the participant will:

1. Recall two common sources of user error in non-invasive vital sign measurement

2. Discuss the methodology used to obtain a non-invasive blood pressure reading

3. State one response of a pulse oximeter when unable to detect a pulse

Talk Code = 711

Case # 1 - Desaturation• While charting…

• SpO2 alarms 74%

• Patient in no distress, good color

• Repositioning sensor yields same 74% sat

• ABG shows 98% sat

Well appearing patient, 74% SpO2

•Why me?

Case # 2 – O2 Sat Out Of Nowhere…

• Patient discharged 2 hours ago

• Mysteriouswaveformand 100% sat

Model of Light Absorption At Measurement Site Without Motion

ACAC Variable light absorption due pulsatile volume of arterial blood

DCDC Constant light absorption due to non-pulsatile arterial blood.

DCDC Constant light absorption due to venous blood.

DC Constant light absorption due to tissue, bone, ...

Abs

orpt

ion

TimeTime

Model of Light Absorption At Measurement Site With Motion

ACAC Variable light absorption due pulsatile volume of arterial blood

DCDC Constant light absorption due to non-pulsatile arterial blood.

AC Variable light absorption due to moving venous blood

DCDC Constant light absorption due to venous blood.

DC Constant light absorption due to tissue, bone ...

TimeTime

Abs

orpt

ion

Influence of Perfusion on Accuracy of Conventional Pulse Oximetry During Motion

Good Perfusion (Conventional PO)

SpaO2=98

SpvO2=88SpO2=93

Poor Perfusion (Conventional PO)

SpO2=74

SpaO2=98

SpvO2=50

Post Processor

R & IRDigitized, Filtered &

NormalizedR/IR

MEASUREMENT

CONFIDENCE % Saturation% Saturation

Conventional Pulse Oximetry Algorithm

3 options during motion or low perfusion:

1. Freeze last good value

2. Lengthen averaging cycle

3. Zero out

Next Generation Pulse Oximetry

Next Generation Pulse Oximetry

Masimo SET: Signal Extraction Technology

SET “Parallel Engines”

Masimo SET “Parallel Engines”

R/IR(Conventional Pulse

Oximetry)

Confidence Based

Arbitrator

0 50% 66% 97% 100%SpO2% SpO2%

Post ProcessorDigitized,

Filtered & Normalized

% Saturation

SSTTM

Proprietary Algorithm 4

DST DST SET – 97%SET – 97%

DSTTM

FSTTM

MEASUREMENT

CONFIDENCE

MEASUREMENT

CONFIDENCE

MEASUREMENT

CONFIDENCE

MEASUREMENT

CONFIDENCE

MEASUREMENT

CONFIDENCE

R & IR

A Solution for Patient Motion Discrete Saturation Transform (DST)

0 50% 66% 86% 97% 100%

SpOSpO2% %

Measure Through Motion Pulse Oximetry

Separating - accurate SpO2

Conventional Pulse Oximetry

0 50% 66% 86% 97% 100%

SpOSpO2% %

Averaging - inaccurate SpO2

Variable

Constant

Variable

Constant

In the presence of motion, SET separates the venous and arterial saturation values resulting in accurate saturation readings without false alarms (compared to conventional oximetry that averages the values to produce a reading)

Certainty…

Case # 3 – Smoke Inhalation

ED Triage Desk:

• 35 yo male presents with diff breathing

• States, “My furnace exploded.”

• Soot in mouth/nares

•O2 sat 98%

Carbon Monoxide (CO)

• Gas:• Colorless• Odorless• Tasteless• Nonirritating

• Physical Properties:• Vapor Density = 0.97• LEL/UEL = 12.5 – 74%• IDLH = 1200 ppm

Limitations of Pulse Oximetry

Barker SJ, Tremper KK. The Effect of Carbon Monoxide Inhalation on Pulse Oximetry and Transcutaneous PO 2. Anesthesiology 1987; 66:677-679

SpCO-SpO2 Gap:

The fractional difference between actual SaO2 and display of SpO2 (2 wavelength oximetry)

in presence of carboxyhemoglobin

SpCO-SpO2 Gap:

The fractional difference between actual SaO2 and display of SpO2 (2 wavelength oximetry)

in presence of carboxyhemoglobin

From Conventional Pulse Oximeter

From invasive CO-Oximeter Blood

Sample

[Blood]

Conventional pulse oximetry can not distinguish between COHb, and O2Hb

CO: The Leading Cause of Poisoning Deaths

30-50 % of CO-exposed patients presenting to Emergency Departments are misdiagnosed

Barker MD, et al. J Pediatr. 1988;1:233-43

Barret L, et al. Clin Toxicol. 1985;23:309-13

Grace TW, et al. JAMA. 1981;246:1698-700

Pulse CO-oximetry

Hgb Signatures: CO, Met, Hgb…

14,438 Patient Brown University Study

• Partridge and Jay (Rhode Island Hospital, Brown University Medical School), assessed carbon monoxide (CO) levels of 10,856 ED patients

• 11 unsuspected cases of CO Toxicity (COT) were discovered.Overall mean SpCO was 3.60%

• Occult COT was 4 in 10,000 during cold, 1 in 10,000 during warm months

• They concluded “unsuspected COT may be identified using noninvasive COHb screening and the prevalence of COT may be higher than previously recognized”

Non-Invasive Pulse CO-Oximetry Screening in the Emergency Department Identifies Occult Carbon Monoxide Toxicity. Suner S, Partridge R, Sucov A, Valente J, Chee K, Hughes A, Jay G. J Emerg Med 2008 Department of Emergency Medicine, Rhode Island Hospital, Brown Medical School, Providence, RI.

Pulse Oximetry

Problems:

•Accuracy

•Motion & artifact

•Dyshemoglobins

Case # 4 – Which Pressure Is Right?

78 yo trauma patient BP • A-line = 70/42 (50)• NIBP = 90/50 (52)

Blood Pressure Monitoring

Direct

Pressure

vs

Indirect

Flow

Errors in BP Measurement

Cuff Size:• Too large = BP• Too small = BP• 2/3 extremity length

Mid Heart Level:• Higher = BP• Lower = BP• Best sitting, arm @ side

How does NIBP work?

• Measures flow (pulsatile)

• Determines HR and MAP

• By formula, calculatesSBP and DBP

• Subject to same interferences as auscultated BP

• Important to confirm HR (if wrong, SBP and DBP wrong)

Mean Arterial Pressure (MAP)

• A clinical parameter useful in assessing perfusion

• Represents the average pressure within the arterial system throughout the cardiac cycle

• MAP = 2 (diastolic) + systolic

3

• 2/3 time in diastole only when HR = 70

•28

150

90

60

Waveform CapnographyAvailable for spontaneously breathing and for intubated patients

Case # 5 – Bad Day in OR

• 37 yo male cholecystectomy• No significant PMH, smooth induction• Shortly after incision, EtCO2 gradually declines• Manual BVM with good compliance & chest rise• ???

Circulation

The heart and lungs

are inextricably

linked together

Cardiac Arrest!• Little O2 delivery or consumption

• Little CO2 production or venous return

• Little O2 delivery or consumption

• Little CO2 production or venous return

In other words: CO2 production is largely

dependent on oxygen

consumption!

CO2 Clearance Reflects Perfusion

Case # 6 – Misplaced ETT?

• Cardiac arrest on med-surg floor• CRNA intubates without difficulty, visualizes

tube pass through cords • EtCO2 circuit connected = flatline• ???

Circuit Connector

Case # 7 – EtCO2 ≠ PaCO2

• Post CABG patient EtCO2 drops to 6

• ABG PaCO2 = 48 mmHg

•Why?

Another Cause of Low EtCO2

• Profound metabolic acidosis• pH = 6.93

Questions?

Slides available at: www.mikemcevoy.com