Chest
Volume 101, Issue 2, February 1992, Pages 509-515
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Clinical Investigations in Critical Care
Veno-arterial Carbon Dioxide Gradient in Human Septic Shock

https://doi.org/10.1378/chest.101.2.509Get rights and content

Recent reports have shown that venous hypercarbia, resulting in a widening of the veno-arterial difference in PCO2 (dPCO2), is related to systemic hypoperfusion in various forms of low-flow state. Although septic shock usually is a hyperdynamic state, other factors can influence the CO2 production and elimination, and thus dPCO2 in septic shock. This study examined the dPCO2 and acid-base balance together with cardiac output measurements and oxygen-derived variables in 64 adult patients with documented septic shock. For a total of 191 observations, a significant exoponential relation between dPCO2 and CO was found. At time of first measurement, 15 patients had an increased dPCO2 (above 6 mm Hg) and a higher mixed venous PCO2 (PvCO2) (47.2 ± 10.0 vs 35.9 ± 7.3 mm Hg, p<0.001). These patients had a lower cardiac index (2.9 ± 1.3 vs 3.8 ± 2.0 L/min•m1, p <0.01), a higher oxygen extraction ratio, but a similar VO2 than patients with normal dPCO2. The higher dPCO2 could also be related to an impaired CO2 elimination as indicated by a higher PaCO2 and a lower PaO2/FIO2 in these patients. Nonsurvivors had a significantly higher dPCO2 than survivors (5.9 ± 3.4 vs 4.4 ± 2.3 mm Hg, p<0.05) in the presence of similar cardiac output. The higher dPCO2 in these patients was probably related to the higher blood lactate levels (7.7 ± 5.3 mmol/L vs 4.5 ± 2.8 mmol/L, p<0.01) and the more severe pulmonary impairment (SaO2 90 ± 8 percent vs 95 ± 4 percent, p<0.001). Arteriovenous oxygen content difference (dAVO2) and VO2 were similar in survivors and nonsurvivors. In conclusion, dPCO2 patients with septic shock is related principally to cardiac output but apparently also to the degree of pulmonary impairment. Although dPCO2 is larger in nonsurvivors, its prognostic value is modest.

Section snippets

Patients

The study included 64 consecutive adult patients (49 male, 15 female) who developed a first episode of septic shock. Septic shock was defined by systemic hypotension (systolic arterial pressure <90 mm Hg), oliguria (urine output <20 ml/h), impaired mental status, and lactic acidosis (arterial lactate concentration >2 mmol/L) in the presence of an abnormal temperature (above 38.5°C or below 35.5°C), an abnormal white blood cell count (above 13,000/cu mm or below 4,000/cu mm), and a documented

RESULTS

The age of the 64 patients (49 male, 15 female) ranged from 27 to 89 years (mean, 61 ± 13 years). Shock survival was 58 percent (n = 37). The sources of infection were as follows: the lungs (n = 23), the abdomen (n = 20), the urinary tract (n = 5), the central nervous system (n = 5), and other sources (n = 11). Of the 64 patients, 32 (50 percent) had positive blood cultures.

DISCUSSION

The veno-arterial difference in PCO2 is the net result of CO2 production and clearance. The produced CO2 is transported mainly as HCO3 in plasma or in red blood cells. Only a small part of the CO2 is transported as carbamino-CO2 (5 percent) or dissolved in plasma and red blood cells (5 percept). CO2 is removed from the circulation by the lungs, 80 percent being mediated by the dehydration of bicarbonate. Under normal conditions, dCO2 does not exceed 6 mm Hg.21, 22

The CO2 is essentially

ACKNOWLEDGMENTS

We are thankful to Francis Cantraine, Ph.D., for his help in the statistical analysis of the data.

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