[abstract] VOLUNTARY CONTROL OF VENTILATORY, ARTERIAL PCO2, AND CEREBRAL BLOOD FLOW RESPONSES TO INCREMENTAL EXERCISE WHILE BREATHING O2 AT 2.0 ATA

Abstract

BACKGROUND: Convulsions while breathing O2 at increased pressure occur more rapidly during exercise than at rest (Br Med J 1947; 1:667-672). This was can be explained at least in part by the previous observation that incremental exercise while breathing O2 at 2.0 ATA is associated with progressive rises in arterial PCO2, cerebral blood flow (CBF), and brain PO2 (UHM 1995; 22(Supp):69-70). It is hypothesized that voluntary hyperventilation above that required by exercise will prevent the associated increments and delay the onset of convulsions. METHODS: The feasibility of this hypothesis was tested by measuring ventilatory, arterial PCO2, and middle cerebral arterial (MCA) flow velocity responses to bicycle ergometer exercise at 50, 125 and 200 watts during spontaneous ventilation (SV) and voluntary hyperventilation (VH) while breathing O2 at 2.0 ATA. RESULTS: In an ongoing series of 3 men to date, average ventilation rates at rest and during exercise, respectively, were 11.5, 27.3, 40.3 and 61.0 L/min during SV and 11.2, 36.7, 61.6 and 89.2 L/min during VH. Corresponding arterial PCO2 values at rest and during exercise were 35.4, 38.2, 40.8 and 43.5 mm Hg during SV and 36.5, 33.3, 32.1 and 35.0 mm Hg during VH. Associated exercise-induced increments in a CBF index (MCA velocity) were 15percent, 23percent and 55percent during SV and -6percent, -1percent and 13percent during VH. CONCLUSIONS: These preliminary results indicate that VH during exercise while breathing O2 at 2.0 ATA can prevent associated increments in CBF and brain PO2 at low to moderate workloads and can reduce them at high workloads. When supplemented by additional experiments, it is expected that they will provide insights regarding the most effective and efficient breathing patterns (rate vs. tidal volume) for preventing exercise-induced increments in CBF and brain PO2. They should also define the workload range over which the operational use of this technique is practical. Supported in part by USN NMRDC N00014-96-1-0776, NASA NAGW-4359, and NAGW-3628.