HYPOCAPNIA DOES NOT ALTER HEPATIC BLOOD-FLOW OR OXYGEN-CONSUMPTION INPATIENTS WITH HEAD-INJURY

Objective: To evaluate the effects of hypocapnia on the systemic and h epatic circulations and oxygenation values in patients with head injur y. Design: Open-label, prospective study. Setting: University hospital , department of anesthesiology and intensive care unit. Patients: Elev en mechanically ventilated patients with isolated head trauma and stab le hemodynamic status. Interventions: At the beginning of the study, e ach patient presented with normocapnic ventilation. Mechanical hyperve ntilation was then adjusted to obtain stable hypocapnia over an interv al of 24 hrs. Cardiac output and other systemic hemodynamic parameters were measured, using a pulmonary artery catheter. Hepatic parameters were measured via a catheter inserted into the hepatic vein. Total hep atic blood flow was determined by the Fick principle using a continuou s infusion of indocyanine green. Arterial and hepatic venous blood gas es were sampled to determine systemic and hepatic-splanchnic oxygenati on. Measurements were done at the end of the four phases: a) 30 mins o f normocapnia (N); b) 30 mins of hypocapnia (H0); c) 3 hrs of hypocapn ia (H3); and d) 24 hrs of hypocapnia (H24). Intracranial pressure and cerebral perfusion pressure were hourly monitored throughout the study . Measurements and Main Results: There were no significant changes in systemic hemodynamic parameters. The hepatic blood flow index did not differ from normocapnia (N 1.8 +/- 0.4 L/min/m(2)) to hypocapnia (H0 1 .6 +/- 0.3 L/min/m(2); H3 1.7 +/- 0.4 L/min/m(2); H24 1.7 +/- 0.4 L/mi n/m(2)). The ratio of hepatic blood flow index to cardiac index remain ed stable throughout the study. Hypocapnia did not affect hepatic spla nchnic oxygen delivery and consumption. Conclusions: Hypocapnic hyperv entilation does not alter hepatic hemodynamic parameters in patients w ith head injury. This result may be related to the lack of changes in cardiac output or in the hepatic vasoreactivity. Moreover, hypocapnia does not modify hepatic splanchnic oxygenation. Thus, in case of intra cranial hypertension, hypocapnia might be used without undesirable eff ect on the hepatic splanchnic perfusion.