Correction of blood pH attenuates changes in hemodynamics and organ blood flow during permissive hypercapnia

Objectives: To determine whether changes in cardiac output, regional blood flow, and intracranial pressure during permissive hypercapnia are blood pH- dependent and can be attenuated by correction of intravascular acidemia. Design: Prospective, controlled study. Setting: Research laboratory. Subjects: Female Marino ewes. Interventions: Animals were instrumented with a pulmonary artery catheter, femoral arterial and venous catheters, a catheter in the third cerebral ventricle, and ultrasonic flow probes on the left carotid, superior mesenteric, and left renal arteries 1 wk before experimentation. At initiation of the protocol, ewes underwent endotracheal intubation and mechanical ventilation under general anesthesia. Minute ventilation was reduced to induce hypercapnia with a target PaCO₂ of 80 torr (10.7 kPa). In the pH-uncorrected group (n = 6), arterial blood pH was allowed to decreased without treatment. In the pH-corrected group (n = 5), 14.4 mEq/kg of sodium bicarbonate was given intravenously as a bolus to correct arterial blood pH toward a target arterial pH of 7.40 (dose calculated by the Henderson-Hasselbalch equation). Measurements and Main Results: Arterial blood pH, PCO₂, cardiac output, intracranial pressure, end carotid, superior mesenteric, and renal artery blood flow rates were measured at normocapnic baseline and at every hour during hypercapnia for 6 hrs. In the pH-uncorrected group, arterial blood pH decreased from 7.41 ± 0.03 at normocapnia to 7.14 ± 0.01 (p < .01 vs. normocapnia) as blood PCO₂ increased to 01.2 ± 1.8 torr (10.8 ± 0.2 kPa). In the pH-corrected group, arterial blood pH was 7.42 ± 0.02 at normocapnia and was maintained at 7.37 ± 0.01 while PaCO₂ was increased to 50.3 ± 0.9 torr (10.7 ± 0.1 kPa). Significant increases in cardiac output occurred with the initiation of hypercapnia for both groups (pH-uncorrected group: 4.3 ± 0.6 L/min at normocapnia va. 6.8 ± 1.0 L/min at 1 hr [p < .05]; pH-corrected group: 4.1 ± 0.4 at normocapnia vs. 5.7 ± 0.4 L/min at 1 hr [p < .05]). However, this increase was sustained only in the uncorrected group. Changes in carotid and mesenteric artery blood flow rates, as a percent of baseline values, showed sustained significant increases in the pH-uncorrected groups (p < .05) and only transient (carotid at 1 hr) or no (superior mesenteric) significant change in the pH-corrected groups. Conversely, significant increases in renal artery blood flow were seen only in the pH-uncorrected group during the last 2 hrs of the experiment (p < .05). Organ blood flow, as a percent of cardiac output, did not change significantly in either group. Intracranial pressure increased significantly in the pH-uncorrected group (9.0 ± 1.5 mm Hg at normocapnia va. 26.8 ± 5.1 st 1 hr, p < .05), and remained increased, while showing no significant change in the pH-corrected group (8.5 ± 1.6 mm Hg at normocapnia to 7.7 ± 4.2 st 1 hr). Conclusions: Acute hypercapnia, induced within 1 hr, is associated with significant increases in cardiac output, organ blood flow, and intracranial pressure. These changes can be significantly attenuated by correction of blood pH with the administration of sodium bicarbonate, without adverse effects on hemodynamics.