Search

Background: The partial pressure of carbon dioxide (pCO2) gap is the difference between pCO2 values in the arterial and mixed venous blood; values higher than six mmHg may be predictive of tissue hypoperfusion. It is still doubtful if central venous blood can be used to assess the gap. This study was aimed to compare the values obtained with blood collected from the superior vena cava and the pulmonary artery., Methods: Data were obtained from a previous study. In 30 patients who underwent on-pump myocardial revascularization, blood samples from a radial artery, the pulmonary artery, and the superior vena cava were collected eight times in the perioperative period. Two-hundred determinations were utilized to calculate the pCO2 gap from central and mixed venous pCO2., Results: The pCO2 gap was 8.7±2.6 mmHg in the superior vena cava, and 6.0±2.7 in the pulmonary artery. The difference between the two values fell within a wide interval between -4.1 mmHg and +9.5 mmHg in 95% of cases. pCO2 values were significantly higher in the superior vena cava than in the pulmonary artery (mean difference 2.7±2.4 mmHg; P<0.0001), causing a systematic bias between the two estimates; such bias increased during spontaneous breathing (P=0.0249)., Conclusions: pCO2 values are higher in the superior vena cava than in the pulmonary artery. As a consequence, the pCO2 gap calculated with the blood taken from the superior vena cava is overestimated, probably because it reflects the pCO2 arteriovenous difference of the upper part of the body, in particular of the brain.

Background: As a rule, central venous catheters (CVC) should not be positioned in the right atrium (RA) to avoid the risk of perforation and cardiac tamponade. However, in ICUs where ECG monitoring can detect any initial damage of the atrial wall, CVCs may probably be safely positioned in the RA. We investigated whether mixed venous saturation (SvO2) was better estimated by measuring central venous saturation (ScvO2) in the RA or in the superior vena cava (SVC) in patients undergoing cardiac surgery., Methods: A CVC and a pulmonary artery catheter (PAC) were positioned before surgical coronary revascularization in sixty patients. Under transesophageal echocardiographic guidance, CVC tips were randomly positioned inside the RA (group A) or the SVC (group C). In each patient, eight pairs of blood samples were collected from CVC and PAC distal ports and saturation measured. Cardiac arrhythmias that occurred in the first 48 postoperative hours and CVC tip position on chest X-rays were also registered., Results: ScvO2 and SvO2 correlated better in group A (r=0.95) than in group C (r=0.84). The 95% interval of confidence of the gap between ScvO2 and SvO2 was narrower in group A (-6.9/+ 3.2 vs. -11.6/+5.5; p<.01). The incidence of arrhythmias was equal in the two groups (16.7%). On chest X-rays, CVC tips were 5.4 (SD=3.6) cm below the tracheal carina in group A and 5.3 (SD=3.9) cm in group C., Conclusion: In monitored patients, positioning CVC tips in the RA rather than in the SVC may allow closer estimates of SvO2 and may be safe. Yet, safety should be confirmed by further studies with larger samples of patients.