Comparative value of peripheral and central venous pCO2 in predicting normal paCO2 during anaesthesia

The ability of venous pCO2 to predict arterial pCO2 within the normal range was tested by measuring pCO2 in blood sampled simultaneously from a large forearm vein (PER), from the superior vena cava (SVC), and from an artery in 35 anaesthetized patients. The relationship between arterial and both venous pCO2's were studied in a first series of 15 patients (ASA physical status class I-II) anaesthetized with methohexitone, fentanyl, pancuronium and nitrous oxide/oxygen, and in a second series of 20 patients scheduled for cardiac surgery anaesthetized with flunitrazepam, fentanyl, pancuronium and nitrous oxide. A marked correlation was found between arterial and both venous pCO2's samples in the normal patients (a/PER: r = 0.922; a/SVC: r = 0.940); in the patients with abnormal cardiovascular status the correlation observed was less pronounced (a/PER: r = 0.501; a/SVC: r = 0.507). In view of the similar correlation coefficients observed from the PER or SVC blood sampling sites, we conclude that the degree of accuracy of the prediction of paCO2 from the venous pCO2's is not modified by the origin of the venous blood. The differences between the coefficients of correlation found in the normal patients and in those with abnormal cardiovascular function indicate that venous pCO2 as estimate of paCO2 appears useful only in subjects with normal haemodynamic status.