Pathogenesis of central nervous system disorder during artificial hyperventilation in compensated hypercarbia in dogs

Stability of brain intracellular lactate and 31P-metabolite levels at reduced intracellular pH during prolonged hypercapnia in rats

The tolerance of low intracellular pH (pHi) was examined in vivo in rats by imposing severe, prolonged respiratory acidosis. Rats were intubated and ventilated for 10 min with 20% CO2, for 75 min with 50% CO2, and for 10 min with 20% CO2. The maximum PaCO2 was 320 mm Hg. Cerebral intracellular lactate, pHi, and high-energy phosphate metabolites were monitored in vivo with 31P and 1H nuclear magnetic resonance (NMR) spectroscopy, using a 4.7-T horizontal instrument. Within 6 min after the administration of 50% CO2, pHi fell by 0.57 +/- 0.03 unit, phosphocreatine decreased by approximately 20%, and Pi increased by approximately 100%. These values were stable throughout the remainder of the hypercapnic period. Cerebral intracellular lactate, visible with 1H NMR spectroscopy in the hyperoxic state, decreased during hypercapnia, suggesting either a favorable change in oxygen availability (decreased lactate production) or an increase in lactate clearance or both. All hypercapnic animals awakened and behaved normally after CO2 was discontinued. Histological examination of cortical and hippocampal areas, prepared using a hematoxylin and eosin stain, showed no areas of necrosis and no glial infiltrates. However, isolated, scattered, dark-staining, shrunken neurons were detected both in control animals (no exposure to hypercapnia) and in animals that had been hypercapnic. This subtle histological change could represent an artifact resulting from imperfect perfusion-fixation, or it could represent subtle neurologic injury during the hypercapnia protocol. In summary, extreme hypercapnia and low pHi (approximately 6.5) are well tolerated in rats for periods up to 75 min if adequate oxygenation is maintained.(ABSTRACT TRUNCATED AT 250 WORDS)

The independent effects of hyperventilation, tolazoline, and dopamine on infants with persistent pulmonary hypertension

We studied the separate and combined effects of hyperventilation and administration of dopamine and tolazoline in five infants with pulmonary hypertension managed with indwelling pulmonary artery catheters. In five infants the right-to-left shunt reversed during ventilator-induced respiratory alkalosis (pH greater than 7.6). Response to drugs was variable and unpredictable. One infant could be oxygenated at normal pH during combined dopamine and tolazoline infusion. Other infants showed no response to drugs, or became worse during infusion. The ratio of pulmonary artery to systemic artery pressure averaged 1.14 with standard therapy, but decreased to 0.98 following respiratory alkalosis alone, to 0.87 following drug infusions, and to 0.70 following the combination of alkalosis and drug infusion. These changes were significant by analysis of variance at P less than 0.02, P less 0.001, and P less than 0.001, respectively. Systemic oxygenation was satisfactory in all cases when the pulmonary to systemic pressure ratio was less than 1.0.

Total brain ischaemia in dogs: cerebral physiological and metabolic changes after 15 minutes of circulatory arrest

Cross-clamping of the ascending aorta in dogs for 15 min produced severe neurological deficit, observed for up to 20 h. Immediately after restoration of the circulation, the intracranial pressure in the cisterna magna increased transiently to a mean peak of 22.8 Torr (SD +/- 1.7) because of a compensatory increase in systemic arterial pressure, without a fall in cerebral perfusion pressure. The intracranial pressure returned to control values 15-30 min after ischaemia and showed no secondary rise during the 8 h of observation. The electroencephalogram became isoelectric 34 +/- 6.5 s (mean +/-SD) after circulatory occlusion, and was abnormal when it reappeared 5 h 36 min (SD +/- 2 h 4 min) after the circulation was restored. The electrical impedance of the brain increased immediately after ischaemia and returned rapidly towards pre-ischaemic values during re-perfusion. The cerebral water had not increased measurably 4 h after ischaemia. After ischaemia, the lactate concentration in the cerebrospinal fluid increased to 4.7 mequiv./1(SEM +/-0.1) and the pH decreased to 7.17 (SEM +/-0.02); both returned to control values after 3.5 h. The cerebral glucose uptake was decreased 35 min after ischaemia, cerebral oxygen uptake remained unchanged but cerebral blood flow decreased (P less than 0.05 at 90 min). Immediately after cardiac arrest, recovery was impaired more by the presence of focal abnormal brain perfusion than by intracranial hypertension.