Severe pulmonary insufficiency:
- PaO2 < 55
- Pa CO2 > 45
Uncompensated hypercarbia ≡ unstable / high-risk e.g. for postoperative complications such as pulmonary and right heart failure:
- pH < 7.3 (if ↑ PaCO2)
- The first step is to characterise the result as acidaemia or alkalaemia: look at the pH using 7.40 as the reference point
- Next, evaluate the respiratory and metabolic components, the PaCO2 and HCO3, respectively.
- Next, assess for evidence of compensation of the primary acidosis or alkalosis by looking for the value (either PaCO2 or HCO3) that is not consistent with the pH.
- Lastly, assess the PaO2 for any abnormalities in oxygenation.
Include an electrolyte imbalance or anion gap in your synthesis of the information. For example: In a patient who presents with Diabetic Ketoacidosis, they will eliminate ketones, close the anion gap, but have a persistent metabolic acidosis due to a hyperchloremia.
If there is no concern about the respiratory system it is reasonable to take a venous blood gas (VBG) as opposed to an arterial blood gas (ABG) in the first instance when presented with an unwell patient. A classic example of this is DKA, where the key variables are pH, lactate, and potassium, all of which can be gathered from a VBG. However, where there is concern about a respiratory component to an illness, an initial ABG is necessary.
Despite considerable heterogeneity between studies, a systematic review and meta-analysis of studies comparing plasma venous blood gas (PVBG) with arterial blood gas (ABG) analysis in adult subjects showed little difference between the pH obtained from the PVBG and the ABG, with the arterial pH typically 0.03 higher than the venous pH (95% confidence interval 0.029-0.038).²
1. Metformin inhibits pyruvate carboxylase (PC) → inhibits hepatic gluconeogenesis146 → excess lactate84. Metformin also inhibits complex I of the mitochondrial electron transport chain (ETC)84 → increases NADH/NAD+ ratio → blocks the entry of pyruvate into the tricarboxylic acid (TCA) cycle147. LDH = lactate dehydrogenase
2. In vitro, nucleoside reverse transcriptase inhibitors (NRTIs) inhibit β-oxidation, the tricarboxylic acid (Krebs) cycle, and DNA γ-polymerase → mitochondrial dysfunction and loss of transcription of essential enzymes → hepatic steatosis (increased triglycerides), myopathy, pancreatitis, nephrotoxicity, and lactic acidosis68.
3. Linezolid may cross-react with mammalian cellular processes → disrupts mitochondrial protein synthesis involved in ETC75,148.
4. Propofol may inhibit coenzyme Q and cytochrome C at Complex IV in ETC, and also inhibit mitochondrial fatty acid metabolism88.
5. Isoniazid inhibits metabolism of lactate to pyruvate82. (Pham et al, 2015)
³
References
- Awasthi, Shilpi et al. “Peripheral venous blood gas analysis: An alternative to arterial blood gas analysis for initial assessment and resuscitation in emergency and intensive care unit patients.” Anesthesia, essays and researches vol. 7,3 (2013): 355-8. doi:10.4103/0259-1162.123234.
- Byrne AL, Bennett M, Chatterji R, Symons R, Pace NL, Thomas PS. “Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis..” Respirology. 2014;19(2):168.
- Pham AQ, Xu LH, Moe OW. Drug-Induced Metabolic Acidosis. F1000research. 2015;4. DOI: 10.12688/f1000research.7006.1.
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The interpretation of arterial blood gases – Australian Prescriber
