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Key questions for blood gas analysis
Is my patient hypoxic?
Does my patient have
Is this patient a CO2 retainer?
Do I need to provide additional
Is my treatment working?
Is this patient
What sort of acid–base
disturbance do they have?
Is my treatment working?
CASE 1: DIABETIC KETOACIDOSIS
Jane is a 26-year-old insulin-dependent diabetic.
She attended ED with a 2-day history of nausea,
vomiting and diarrhoea.
On clinical examination, pulse was 120/min, BP
100 mmHg, RR 30/min, and there were no
specific abnormalities on cardiorespiratory or
Bedside glucose is ‘Hi’.
VBG result was pH 7.26, pCO2 16 mmHg,
HCO3 7.1 mmol/L, K 3.8 mmol/L, BE −14
mEq/L and lactate 7.2 mmol/L.
Clinical bottom line
The clinical picture is one of moderately severe DKA.
Agreement between ABG and VBG pH is close
enough for clinical interchangeability.
Even allowing for the width of the 95% limits of
agreement, pCO2 and bicarbonate are low and
lactate is high consistent with a metabolic acidosis
with a significant lactic acidosis.
The bedside glucose is ‘Hi’.
These are sufficient to confirm the
diagnosis of DKA and guide initial treatment.
Given the accuracy of VBG pH, resolution
of acidosis can be reliably tracked using
VBG pH alone.
CASE 2: ACUTE RESPIRATORY DISEASE
Tran is a 74-year-old man with known COAD.
He presented to ED with a 1-day history of
worsening dyspnea following a ‘cold’.
On examination, he was SOB at rest, only
able to speak in short phrases or words.
Pulse was 125/min, BP 140 mmHg, RR 35,
oxygen saturation on air 86%, and on
chest examination there was generally
reduced breath sounds with scattered
rhonchi but nothing focal.
VBG analysis showed pH 7.16, pCO2 82.6
mmHg and HCO3 28.8 mmol/L.
The clinical bottom line
On clinical grounds alone it is clear that Tran is
hypoxic with significant work of breathing.
The evidence is that the venous pH will be an
accurate reflection of arterial pH.
Even allowing for the wide limits of agreement,
pCO2 is high and coupled with the pH and near
normal bicarbonate is sufficient evidence of
acute hypercarbia and respiratory failure.
This is sufficient evidence to confirm a
diagnosis of acute respiratory failure
requiring careful oxygen management and
ventilatory support with non-invasive
CASE 2: A VARIATION
On examination, Tran can speak in short
sentences, has a pulse of 110/min, BP of
140 mmHg and RR of 30/min with oxygen
saturation on air of 86%. His chest findings
are the same.
This time the VBG shows pH 7.45, pCO2
42 mm Hg and HCO3 28.7 mmol/L.
The clinical question there is whether Tran has
clinically significant hypercarbia not identified by the
Four studies have explored whether there is a VBG
level of pCO2 that reliably rules out clinically
significant hypercarbia. Those studies have included
529 patients and established that a screening cut-off
of VBG pCO2 of 45 mmHg rules out clinically
Pooled sensitivity was 100% (95% CI 97% to 100%)
and negative predictive value 100% (97% to 100%).
In this variation of the scenario, Tran is
hypoxic but not in acute respiratory failure
and not significantly hypercarbic at the time
of the test.
That is not to say that if too high a level of
oxygen was given he would not develop
hypercarbia but the same would be true of an
The vast majority of patients can
be managed using VBG, if the
result is discordant with the
clinical situation, do an ABG
analysis to check.
VBG and other alternatives to ABG
Literature review current through: Sep 2016.
This topic last updated: Feb 29, 2016.
VENOUS BLOOD GASES
PvCO2, venous pH, and venous serum
HCO3 concentration are used to assess
ventilation and/or acid-base status
SvO2 is used to guide resuscitation during
severe sepsis or septic shock, a process
called Early Goal-Directed Therapy
PvO2 has no practical value
Correlation with ABG
Although ABG is more accurate than VBG for the
assessment of oxygenation, measurement of PCO2, pH,
and HCO3 are similar with some minor adjustments
Estimated corrections for converting VBG to ABG
pH ＋ 0.03 to 0.05 ＋ 0.02 to 0.04
pCO2 － 4 to 5 mmHg － 3 to 8 mmHg
HCO3 ＝ － 1 to 2 meq/L
There are conflicting data regarding the
correlation between ABG and VBG in patients
with hemodynamic instability.
First, clinicians should be wary of VBG
results and preferentially obtain an ABG in
Second, periodic correlation of the venous
measurements with arterial measurements
should be performed whenever venous
measurements are used for serial monitoring.
Carbon Monoxide Toxicity
ABG are no longer considered necessary as
venous and arterial CO-Hb levels will be
Ann Emerg Med 1995;33:105-109.
Relationship between venous and arterial carboxyhemoglobin
levels in patients with suspected carbon monoxide poisoning.
No need to perform ABG. VBG is sufficient
difference in pH from VBG vs ABG will be
±0.02 pH units
Emery Med Australas 2010; 22: 493 – 498.
Review Article – Can Venous Blood Gas Analysis Replace
Arterial in Emergency Medical Care.
American Journal of Nephrology 2000; 20:319-323.
Comparison of Blood Gas and Acid-Base Measurements in
Arterial and Venous Blood Samples in Patients with Uremic
Acidosis and Diabetic Ketoacidosis in the Emergency Room.
ETCO2 can be used for bedside assessment of DKA
ETCO2 of ≥35 is 100% sensitive to rule out DKA
An ETCO2 of ≤21 is 100% specific to diagnosis DKA
BCM Emerg Med. 2016; 16 (1).
Diagnostic value of end tidal capnography in patients with
hyperglycemia in the emergency department.