65 year old male known COPD, presents to ED as priority 1 with SOB and sats 60% RA. History not known as patient not able to talk.
Respiratory acidosis – pH7.04 CO2 83mmHg
Compensation – Patients HCO3 <30mmHg and when looking at old VBG’s CO2 was normal therefore expected HCO3 calculated as an acute respiratory acidosis – For every 10mmHg increase in CO2 above 40, there is a 1mmol/l increase in HCO3 = 28.3mmol/l
AG = Na- (HCO3 + Cl) = 8
A-a gradient:
PAO2 = 713xFiO2 – (CO2x1.25) =182
PaO2 =153mmHg
A-a gradient 29 (normal age/4+4=20)
Description: There is a severe respiratory acidosis present which is not adequately compensated for or there could be a mild underlying metabolic acidosis, with a normal AG.The patients A-a gradient is mildly elevated.The electrolytes and lactate on the ABG are within normal limits and the glucose is slightly elevated.
Interpretation: In this clinical context the respiratory acidosis is most likely due to a severe exacerbation of the patients COPD requiring urgent NIV. A differential diagnosis would include underlying pneumothorax, APO, pneumonia or PE. With the later 3 DDx you would expect a higher A-a gradient, rather than the mildly elevated A-a gradient of 29.The possibility of an underlying NAGMA is unlikely in this clinical context and it is more likely that the patient has not fully compensated for his respiratory acidosis.The raised glucose is likely due to a stress response.
ABG and VBG Correlation: When a VBG is done on a patient the following needs to be kept in mind when interpreting the VBG. pH, HCO3 and B/E all correlate well between an ABG and VBG. With respect to PCO2 if the VBG <45mmHg hypercapnia can be excluded. Above 45mmHg levels can be inaccurate and ABG is more accurate. The differences between ABG and VBG CO2 usually are 5mmHg, but can be as much as 20mmHg, as seen in this case.CO2 values are non correlative in cases of septic shock. PO2 values between ABG and VBG also differ markedly. Arterial PO2 is typically 36.9 mm Hg greater than the venous with marked variability
Lactate values above 2 are also inaccurate on a VBG
ABG’s are a painful procedure for patients and VBG’s can be used as a substitute if the above is kept in mind. ABG might be necessary in the following cases
- to determine PaCO2 in severe shock
- to determine PaCO2 if hypercapnic (i.e. PaCO2 >45 mmHg)
- to determine arterial lactate >2mM (rarely necessary)