venous blood gases in the ed: eusem15

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

OR

Can venous blood gas analysis replace ABG in the ED?

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

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?◦ Searched the literature – limited data in paeds for VBG but no

data in adults◦ Decided to generate data to test my clinical questions

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

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

Establishing acid-base status◦ Mainly pH; but also bicarbonate

Measuring respiratory function/ ventilation Mainly pCO2; but also pH

‘Quick check’ potassium, haematocrit, some electrolytes

Less pain for patients Fewer complications, especially vascular and

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

Respiratory Disease Metabolic disease

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?

ELISSA WOULD YOU?

18 year old Known asthmatic, previous

admissions 2 day exacerbation Arrival by ambulance Pulse 120, SpO2 on oxygen

93%, able to speak in short phrases, tight wheeze

Obtain an ABG for pO2, pCO2 and pH?

Obtain a VBG for pCO2 and pH?

Obtain a VBG for pH and hypercarbia screen?

Proceed without blood gas based on clinical assessment

Clinical features VBG RESULT




VBG result◦ pH 7.35◦ pCO2 35mmHg (4.7 kPa)

Is this data enough to guide initial clinical decision-making?◦ Yes◦ No◦ Unsure

TRAN WOULD YOU?

74 year old Known COPD Acute respiratory distress Pulse 118, BP 140 Respiratory rate 35 SpO2 (air) 86%





Clinical features VBG result


VBG result◦ pH 7.16◦ pCO2 82.6mmHg (11 kPa)◦ Bicarbonate 28.8mmmol/l


Clinical features A different VBG result


VBG result◦ pH 7.45◦ pCO2 42mmHg (5.6 kPa)◦ Bicarbonate 28.8mmmol/l

What about this?

JANE WOULD YOU?

26 year old Insulin dependent

diabetic 2 days of vomiting and

diarrhoea Pulse 125, BP 100 Bedside glucose ‘Hi’





Clinical features VBG result




VBG result:◦ pH: 7.26◦ pCO2 16mmHg (2.1 kPa)◦ Bicarbonate 7.1 mmol/l◦ Potassium 3.8 mmol/l◦ Base excess -14


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

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 considerable variation between clinicians regarding

this tolerance!

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

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

COAD◦5 studies (643 patients)◦Weighted mean difference= 0.034 pH

units◦95% limits of agreement generally +/-

0.1

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)

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 most clinicians.

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

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

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

BUT WAIT ......

Author, year No. Screening cut-off

Sens. Spec. NPV %ABG avoided

Kelly, 2002 196 45 100 57 100 43Kelly, 2005 107 45 100 47 100 29Ak, 2006 132 45 100 * 100 33McCanny, 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

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.

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 but requires validation

8 studies 1211 patients Various conditions

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

COAD◦2 studies (643 patients)◦Weighted mean difference= -1.34 mmol/l◦95% limits of agreement: none reported

DKA◦1 study (21 patients)◦Weighted mean difference = -1.88 mmol/l◦95% limits of agreement = -2.8 to 0.9 mmol/l

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

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.

2 studies in DKA comparing BG 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.

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

Clinical decision-making isn’t just about the numbers

Clinical aspects of assessment are also important

Particularly the case in acute respiratory disease

ELISSA VBG RESULT




VBG result◦ pH 7.35◦ pCO2 35mmHg (4.7 kPa)

Is this data enough to guide initial clinical decision-making?◦Yes◦ No◦ Unsure

TRAN VBG result


VBG result◦ pH 7.16◦ pCO2 82.6mmHg (11 kPa)◦ Bicarbonate 28.8mmmol/l


TRAN A different VBG result


VBG result◦ pH 7.45◦ pCO2 42mmHg (5.6 kPa)◦ Bicarbonate 28.8mmmol/l


JANE VBG result




VBG result:◦ pH: 7.26◦ pCO2 16mmHg (2.1 kPa)◦ Bicarbonate 7.1 mmol/l◦ Potassium 3.8 mmol/l◦ Base excess -14


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

absolute 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

Questions?Questions

venous blood gases in the ed: eusem15

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