ARTERIAL BLOOD GASES:

REST IN PEACE?

Anne-Maree KellyProfessor and DirectorJoseph Epstein Centre for Emergency Medicine Research @Western Health

PERMISSION TO USE

Professor Kelly gives permission for this material to be used for educational purposes (personal or group) on the basis that:•The original source is acknowledged•No liability is accepted by her for the currency or setting relevance of the content

Conflicts of interest

No conflicts of interest to declare.

I have not received industry funding for any of my blood gas research.

Why I go interested

I am a ‘woose’ I dislike needles and am averse to pain

My experience Late presentation of asthma and DKA because of fear of

ABG More severe illness, that was potentially preventable

Was there another way?

This session’s objectives

To understand the agreement performance of variables on arterial and venous blood gas analysis

To be aware of how venous blood gas analysis can be safely used in clinical decision-making

To be aware of grey areas and unanswered questions

Caveat

Discussion will be limited to comparisons between arterial and peripheral venous samples as these are the most relevant to Emergency Medicine practice

Blood gases in emergency medical care

Establishing acid-base status Mainly pH; but also bicarbonate

Measuring respiratory function/ ventilation Mainly pCO2; but also pH

‘Quick check’ potassium, haematocrit, some electrolytes

Why venous rather than arterial?

Less pain for patients Fewer complications, especially vascular and

infection Fewer needle-stick injuries to staff Easier blood draw Minimal training requirement

Key questions

Is my patient hypoxic? Does this patient have

respiratory failure? Is this patient a CO2

retainer? Do I need to provide

additional ventilatory support?

Is my treatment working?

Is my patient acidotic/ alkalotic?

What sort of acid-base disturbance do they have?

Is my treatment working?

Respiratory Disease Metabolic disease

Setting the context

JANE

26 year old, insulin dependent diabetic

2 days of vomiting and diarrhoea

Pulse 120 bpm, BP 100/-, BSL ‘Hi’

Would you: 1. Take an ABG to establish

acid/base status 2. Take an ABG to establish

acid/base & potassium status 3. Take a VBG to establish

acid/base status 4. Take a VBG to establish

acid/base and potassium status

Setting the context

JANE

26 year old, insulin dependent diabetic

2 days of vomiting and diarrhoea

Pulse 120 bpm, BP 100/-, bedside glucose ‘Hi’

VBG result: pH – 7.26 pCO2 – 16 mmHg HCO3 – 7.1 mmol/l K – 3.8 mmol/l BE -14

Is this data enough to guide clinical decision-making?

1. No 2. Yes Unsure

Setting the context

Would you: 1. Obtain an ABG for pO2, pCO2

and pH 2. Obtain a VBG for pCO2 and

pH 3. Obtain a VBG for pH and to

screen for hypercarbia 4. Proceed initially on clinical

features and pulse oximetry without blood gas analysis

TRAN

74 year old male; known COAD

Acute respiratory distress Pulse 118, BP 140/-,

respiratory rate 35, SpO2 (air) 86%

Setting the context

VBG result: pH – 7.16 pCO2 – 82.6mmHg HCO3 – 28.8 mmol/l

Is this data enough to guide clinical decision-making?

1. No 2. Yes Unsure

TRAN

74 year old male; known COAD

Acute respiratory distress Pulse 125, BP 140/-,

respiratory rate 35, SpO2 (air) 86%

Setting the context

VBG result: pH – 7.45 pCO2 – 42 mmHg HCO3 – 28.7 mmol/l

Is this data enough to guide clinical decision-making?

1. No 2. Yes Unsure

TRAN

74 year old male; known COAD

Acute respiratory distress Pulse 118, BP 140/-,

respiratory rate 30, SpO2 (air) 86%

What about this?

Statistical considerations

Outcome of interest is how closely venous and arterial values agree, not how well they correlate

Weighted mean difference gives an estimate of the accuracy between the methods

95% limits of agreement give information about precision

Arterial value

Venous value

95% LoA

Clinical considerations

There is limited data about the tolerance clinicians have with respect to agreement between arterial and venous values of blood gas parameters

Depending on this tolerance, the degree of agreement may be acceptable or unacceptable There is also considerable variation between clinicians

regarding this tolerance!

Issues with the evidence

A number of relatively small studies Patient cohorts are highly varied Patient groups of interest are those at high risk of

acidosis or hypercarbia Reporting does not always provide this detail Data is often dominated by patients with normal pH,

pCO2 and blood pressure

pH

13 studies Range from 44 to 346 patients

Various conditions DKA (3), COAD (4), trauma (1)

2009 patients Weighted mean difference of 0.033 pH units 95% limits of agreement generally within +/- 0.1 pH units

pH in illness subgroups

DKA 3 studies (265 patients) Weighted mean

difference = 0.02 pH units

95% limits of agreement = -0.009 to 0.02 pH units (1 study)

COAD 5 studies (643 patients) Weighted mean

difference= 0.034 pH units

95% limits of agreement generally +/- 0.1

Clinical bottom line

In patients without severe circulatory compromise, agreement between arterial and venous values for pH in both metabolic and respiratory conditions is close.

Level of a agreement is probably clinically acceptable to clinicians.

pCO2

8 studies 965 patients Various conditions

COAD 4 Weighted mean difference = 6.2 mmHg 95% limits of agreement: up to -17.4 to +23.9 mmHg

5/7 studies reporting LoA report LoA band >20mmHg

pCO2 COAD

4 studies 452 patients Weighted man difference = 7.26 mmHg 95% limits of agreement: up to -14 to +26mmHg

All 3 studies that report LoA have LoA band >20mmHg

Clinical bottom line

Agreement between venous and arterial pCO2 is NOT good enough for clinical inter-changeability

BUT WAIT ......

Venous pCO2: A screening test for hypercarbia?

Author, year No. Screening cut-off

Sens. Spec. NPV %ABG avoided

Kelly, 2002 196

45 100 57 100 43

Kelly, 2005 107

45 100 47 100 29

Ak, 2006 132

45 100 * 100 33

McCanny, 2011

94 45 100 34 100 23

POOLED DATA

529

45 100 (95% CI 97-100)

53(95% CI 57-58)

100(95% CI 97-100)

35%(95% CI 32-41)

Data limited to studies in cohorts with respiratory disease

Using venous pH and CO2 to track progress? 1 study Average difference between change in pH (v-a) was 0.001

(LoA -0.7 to +0.7). Average difference between change in pCO2 (v-a) was

0.04mmHg (LoA -17.3 to +18.2). For both pH and pCO2, in the majority of cases the direction

of change was the same although the magnitude was variable.

Clinical bottom line

Agreement between venous and arterial pCO2 is NOT good enough for clinical inter-changeability

pCO2 on VBG is a reliable screening test for clinically relevant hypercarbia

In combination with clinical assessment, change in venous pH and pCO2 may be useful to monitor progress

Bicarbonate

8 studies 1211 patients Various conditions

COAD 2 Weighted mean difference = -1.3mmol/l 95% limits of agreement : up to +/- 5mmol/l

Bicarbonate in illness subgroups DKA

1 study (21 patients) Weighted mean

difference = -1.88 mmol/l

95% limits of agreement = -2.8 to 0.9 mmol/l

COAD 2 studies (643 patients) Weighted mean

difference= -1.34 mmol/l 95% limits of agreement:

none reported

Clinical bottom line

Limited data shows good agreement Evidence regarding 95% limits of agreement is

sparse Probably close enough agreement for classification

as high, low or normal Clinical acceptability may be context specific

Base excess

Two studies only In a sample of 103 patients (various conditions), they

report: mean difference of 0.089 95% limits of agreement -0.974 to +0.552

In 326 trauma patients mean difference -0.3 BE units 95% limits of agreement -4.4 to +3.9 BE units 20% did not fall within pre-defined clinical equivalence threshold

Current view: Agreement unclear. If accuracy is needed in critically ill, need ABG.

Potassium

2 studies in DKA comparing VBG vs serum K+ In both studies serum K+ is usually higher than BG

K+. Fu et al.

95% limits of agreement -0.96 to +1.19mmol/l 80% of patients had agreement within +/- 0.5mmmol/L

Roblas et al. Mean difference 1.13mmol/l (serum higher) 34% of patients had agreement within +/- 0.5 mmol/L.

Lactate

Limited data Systematic review by Bloom

Mean difference 0.25mmol/L LoA -2 to 2.3mmmol/L

Depending of where cut-off for clinical importance is set, significant misclassification rate

No data re accuracy of trend monitoring

Grey areas

Some data to suggest that AV agreement deteriorates in shock states

No data in mixed acid-base disorders Limited data in toxicological conditions

Is it just about the numbers?

Clinical decision-making isn’t just about the numbers

Clinical aspects of assessment are also important

Particularly the case in acute respiratory disease

Thinking again

JANE 26 year old, insulin dependent

diabetic 2 days of vomiting and

diarrhoea. Pulse 120 bpm, BP 100/-,

bedside glucose ‘Hi’

VBG result: pH – 7.26 pCO2 – 16mmHg HCO3 – 7.1 mmol/l K – 3.8 mmol/l

Is this data enough to guide clinical decision-making?

1. No

2. Yes

pH and bicarbonate agreement are good – clinically interchangeable

Be careful about error margin of K+ at extremes of the normal range – will always need serum value

Thinking again

VBG result: pH – 7.16 pCO2 – 82.6mmHg HCO3 – 28.8 mmol/l

Is this data enough to guide clinical decision-making?

1. No

2. Yes

TRAN 74 year old known COAD Acute respiratory distress Pulse 125, BP 140/-, respiratory

rate 35, SpO2 (air) 86%

pH is accurate - clinically interchangeable

pCO2 (even with LoA error) is clearly high

Clinically hypoxicHigh respiratory & pulse rateNeeds NIV

Thinking again

VBG result: pH – 7.45 pCO2 – 42 mmHg HCO3 – 28.7 mmol/l

Is this data enough to guide clinical decision-making?

1. No

2. Yes

TRAN 74 year old known COAD Acute respiratory distress Pulse 118, BP 140/-,

respiratory rate 30, SpO2 (air) 86%

pH is accurate – clinically interchangeablepCO2 is below screening cut-off for clinically relevant hypercarbiaDoes not need NIV

Take home messages

pH and bicarbonate probably close enough agreement for clinical purposes in DKA, acute respiratory failure,

isolated metabolic acidosis More work needed in toxicology, shock, mixed disease

pCO2 NOT enough agreement for clinical purposes, either as one-off or to monitor change Data suggests venous pCO2 is useful as a screening test

Base excess Agreement unclear

Potassium Beware the error margin at the extremes of the normal range

Lactate LoA wide and unclear if safe to use venous for trend

Questions?

Questions?

Vale ABG!