Hyperventilation associated with low pH of cerebrospinal fluid after intracranial haemorrhage

Cerebrospinal Fluid and Arterial Acid-Base Equilibrium of Spontaneously Breathing Patients with Aneurismal Subarachnoid Hemorrhage

BACKGROUND

Hyperventilation resulting in hypocapnic alkalosis (HA) is frequently encountered in spontaneously breathing patients with acute cerebrovascular conditions. The underlying mechanisms of this respiratory response have not been fully elucidated. The present study describes, applying the physical-chemical approach, the acid-base characteristics of cerebrospinal fluid (CSF) and arterial plasma of spontaneously breathing patients with aneurismal subarachnoid hemorrhage (SAH) and compares these results with those of control patients. Moreover, it investigates the pathophysiologic mechanisms leading to HA in SAH.

METHODS

Patients with SAH admitted to the neurological intensive care unit and patients (American Society of Anesthesiologists physical status of 1 and 2) undergoing elective surgery under spinal anesthesia were enrolled. CSF and arterial samples were collected simultaneously. Electrolytes, strong ion difference (SID), partial pressure of carbon dioxide (PCO₂), weak noncarbonic acids (A_TOT), and pH were measured in CSF and arterial blood samples.

RESULTS

Twenty spontaneously breathing patients with SAH and 25 controls were enrolled. The CSF of patients with SAH, as compared with controls, was characterized by a lower SID (23.1 ± 2.3 vs. 26.5 ± 1.4 mmol/L, p < 0.001) and PCO₂ (40 ± 4 vs. 46 ± 3 mm Hg, p < 0.001), whereas no differences in A_TOT (1.2 ± 0.5 vs. 1.2 ± 0.2 mmol/L, p = 0.95) and pH (7.34 ± 0.06 vs. 7.35 ± 0.02, p = 0.69) were observed. The reduced CSF SID was mainly caused by a higher lactate concentration (3.3 ± 1.3 vs. 1.4 ± 0.2 mmol/L, p < 0.001). A linear association (r = 0.71, p < 0.001) was found between CSF SID and arterial PCO₂. A higher proportion of patients with SAH were characterized by arterial HA, as compared with controls (40 vs. 4%, p = 0.003). A reduced CSF-to-plasma difference in PCO₂ was observed in nonhyperventilating patients with SAH (0.4 ± 3.8 vs. 7.8 ± 3.7 mm Hg, p < 0.001).

CONCLUSIONS

Patients with SAH have a reduction of CSF SID due to an increased lactate concentration. The resulting localized acidifying effect is compensated by CSF hypocapnia, yielding normal CSF pH values and resulting in a higher incidence of arterial HA.

Cerebral metabolism following brain injury. I. Acid-base and pO2 changes

Acid-base parameters and oxygen tension were studied in arterial blood and lumbar CSF of 45 subjects in three groups of controls, head injuries, and postoperative cases. CSF in both groups of head injury and postoperative cases showed changes of metabolic acidosis with the arterial blood changes of respiratory alkalosis. Both groups had significant hypoxaemia. CSF acidosis is an indication of cerebral metabolic disturbance. It is proportional to the severity of brain damage, and permits a prognosis for further clinical progress. It is recommended that patients with severe brain lesions should be put on controlled ventilation to relieve them of respiratory overwork and to combat cerebral oedema.

Cerebrospinal fluid and arterial lactate, pyruvate and acid-base balance in patients with intracranial hemorrhages

Lactate and pyruvate concentrations and acid-base balance in cerebrospinal fluid (CSF) and arterial blood were determined in patients with intracranial hemorrhages (28 subarachnoid hemorrhages and 15 intracerebral hemorrhages). A greater increase in CSF lactate and lactate-pyruvate ratio (L/P ratio) was observed in patients with impairment of consciousness, focal neurological deficits, poor prognosis, or CSF pressures higher than 300 mm H2O. A combination of CSF lactate greater than 2.5 mM per liter, L/P ration above 20, bicarbonate less than 20.4 mEq per liter, pH below 7.276, or arterial PCO2 below 31.5 mm Hg seems to indicate poor prognosis from intracranial hemorrhage. The mechanism of hyperventilation in acute cerebrovascular diseases and of CSF pH regulation in acid-base disturbances was also discussed.

The mechanics of breathing and stabilization of ventilation in patients with unilateral cerebral infarction

The mechanics of breathing and responses of the respiratory system to CO 2 during resistance breathing were studied in 12 patients with clinical evidence of unilateral cerebral infarction and compared with the findings in normal subjects. The respiratory abnormalities in patients with cerebrovascular disease consisted of noncompliant lungs and high levels of elastic and nonelastic work while rebreathing CO 2 . There were no differences in responses of minute ventilation to CO 2 in the two groups. Unlike normal subjects, the patients did not increase their level of respiratory work to compensate for the addition of a modest external resistance to airflow, and the ventilatory response to CO 2 , consequently, became less than in the normal subjects. This failure to maintain tidal volumes and minute ventilation while breathing through an external resistance may account for some instances of respiratory deterioration among patients with cerebrovascular disease.

Mechanism of hyperventilation in acute cerebrovascular accidents

Lumbar cerebrospinal fluid and arterial blood acid-base state were assessed in 19 patients within 24 hours of an acute cerebrovascular accident. Those with haemorrhage into the C.S.F. showed a lower C.S.F. pH and higher C.S.F. lactate than those without haemorrhage but the Pco(2), was similar in the two groups, suggesting that this greater C.S.F. acidity was not responsible for a greater degree of hyperventilation. In those without haemorrhage an inverse relation was found between C.S.F. pH and arterial Pco(2), suggesting that a non-chemical ventilatory drive-for example, due to central neurological damage-was responsible for the acid-base changes observed.