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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 (PCO 2 ), 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  &lt; 0.001) and PCO 2  (40 ± 4 vs. 46 ± 3 mm Hg,  p  &lt; 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  &lt; 0.001). A linear association ( r  = 0.71,  p  &lt; 0.001) was found between CSF SID and arterial PCO 2 . 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 PCO 2  was observed in nonhyperventilating patients with SAH (0.4 ± 3.8 vs .  7.8 ± 3.7 mm Hg,  p  &lt; 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.

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