Objective: To study the effects of prolonged (6 hrs) hypercapnia on cerebral blood flow and cerebral metabolism in newborn lambs and to evaluate the effects on cerebral blood flow and cerebral metabolism on return to normocapnia after prolonged hypercapnia.
Design: Animal studies, using the newborn lamb, with comparison to control group.
Subjects: Newborn lambs of mixed breed, 1-7 days of age, were used for the study. Two groups of animals were studied: a hypercapnic group (n = 10) and a normocapnic control group (n = 5).
Setting: Work was conducted in the research laboratories at Children's National Medical Center, Washington, DC.
Interventions: Animals were anesthetized with pentobarbital, intubated, paralyzed, and mechanically ventilated. After baseline measurements were made, CO2 was blended into the ventilator gas until a PaCO2 of 75-80 torr (10-10.6 kPa) was obtained. Measurements were made 1 hr after the desired PaCO2 was achieved and after 6 hrs of hypercapnia. After 6 hrs of hypercapnia, the ventilator gas was returned to the baseline value, that is, normocapnia. Measurements were made 30, 60, and 90 mins after PaCO2 returned to baseline.
Measurements: Six measurements were made during the study. For each measurement, blood samples were drawn from the sagittal sinus and brachiocephalic artery catheters and were analyzed for pH, hemoglobin concentration, oxygen saturation, and blood gas values. Cerebral blood flow (CBF) was measured by using the radiolabeled microsphere technique. Cerebral oxygen consumption, fractional oxygen extraction, and oxygen transport values were calculated at each study period.
Main results: Increasing PaCO2 from 37 +/- 3 torr to 78 +/- 6 torr (4.9 +/- 0.4 kPa to 10.3 +/- 0.8 kPa) for 1 hr increased CBF by 355%. After 6 hrs of PaCO2 at 78 +/- 3 torr (10.3 +/- 0.4 kPa), CBF remained 195% above baseline. At 30 mins of normocapnia, CBF had returned to baseline and remained at baseline until the conclusion of the study, a total of 90 mins of normocapnia. Cerebral oxygen consumption did not change during hypercapnia or with return to normocapnia. Oxygen transport increased 331% above baseline after 1 hr of hypercapnia and stayed 180% above baseline after 6 hrs of hypercapnia. Fractional oxygen extraction decreased by 55% at 1 hr of hypercapnia and stayed 39% below baseline at 6 hrs of hypercapnia.
Conclusions: Healthy lambs seem to tolerate undergoing hypercapnia for 6 hrs with a return to normocapnia. The return to baseline of CBF and cerebral metabolism at normocapnia seen in our study with lambs may explain why prolonged hypercapnia appears to be well tolerated in mechanically ventilated patients. If these results can be extrapolated to human subjects, our study in lambs supports evidence that patients who have undergone permissive hypercapnia seem to be neurologically unaffected.