VBG or ABG in ER?

Emerg Med J. 2007 Aug; 24(8): 569–571.

Emerg Med J. 2007 Aug; 24(8): 569–571.

Emerg Med J. 2007 Aug; 24(8): 569–571.

Emerg Med J. 2007 Aug; 24(8): 569–571.

Emerg Med J 2014;0:1–3

Key questions for blood gas analysisRespiratory conditions

Is my patient hypoxic? Does my patient have respiratory failure?Is this patient a CO2 retainer? Do I need to provide additional ventilatory support?Is my treatment working?

Metabolic conditionsIs this patient acidotic/alkalotic?What sort of acid–base disturbance do they have?Is my treatment working?

CASE 1: DIABETIC KETOACIDOSISJane 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 abdominal exam.

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 lineThe 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 lineOn 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 ventilation.

CASE 2: A VARIATIONOn 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 VBG analysis.

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 significant hypercarbia.

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 ABG.

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 GASESPvCO2, 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 ABGAlthough 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  Central Peripheral 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

Misleading resultsThere 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 hypotensive patients. Second, periodic correlation of the venous measurements with arterial measurements should be performed whenever venous measurements are used for serial monitoring.

Carbon Monoxide ToxicityABG are no longer considered necessary as venous and arterial CO-Hb levels will be within ±2%

Ann Emerg Med 1995;33:105-109. Relationship between venous and arterial carboxyhemoglobin levels in patients with suspected carbon monoxide poisoning.

DKANo 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.

DKAETCO2 can be used for bedside assessment of DKAETCO2 of ≥35 is 100% sensitive to rule out DKAAn 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.

摘要VBG 的 PvCO2, pH & HCO3 與 ABG 差異不大，可用來評估 ventilation 和 acid-base statusVBG pCO2< 45 mmHg 可排除臨床有意義的 hypercarbiaVBG 的 PvO2 無臨床價值，但可用 pulse oximerty 來評估 oxygenation (O2 saturation)venous 與 artery 的 Hb-CO 差異不大，可相互取代大部分的臨床情況下，可以根據 VBG 決定病患的診斷與處置。除非病患血壓不穩或休克，或 VBG data 無法解釋臨床症狀，需再抽 ABG 確認

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