Oscillations in Cardiac Output in Hypoxia with Periodic Breathing and Constant End-Tidal PCO₂ at High Altitude (5,000 m)

Abstract

At an altitude of 5,000 m during an ascent to Everest Base Camp in 1994 CBW experienced episodes of heavy breathing during a steady walk up a shallow gradient. Recorded respiration during similar exercise in the laboratory at base camp (DJC, 5,340 m) showed similar episodes of heavy breathing and yet end-tidal PCO₂ remained steady. This led to the hypothesis that cardiac output (CO) oscillations, at the frequency of the hyper-ventilatory episodes, could be the cause of changing CO₂ delivery to the lung. The ventilatory periodicity would then sustain the constant PCO₂. The present study included measurement at rest and in mild exercise (35 W approx.) of changes in CO during a recent visit to the Hidden Valley in West Central Nepal at 5,000 m. PCO₂, breathing, SaO₂ and continuous arterial blood pressure (BP) were recorded. CO was derived from the BP record (PulseCO^TM). Subject 04 showed vigorous periodic breathing with constant end-tidal PCO₂ (PetCO₂) but his BP record was technically unsatisfactory. However, there was a satisfactory BP record in subject 01 with strong periodicity in the respiratory and SaO₂ records (period around 32 s). CO showed oscillations with amplitudes around 1.8 l per min (l min⁻¹, range 1.5–2.8) with an average period of 32.1 s (range 28–42 s). The oscillations in CO partially compensated for SaO₂ leading to near constant oxygen delivery (DO₂). There was some PetCO₂ oscillation. Subject 09 showed a run of seven more rapid CO oscillations during a resting record (period 25 s) resulting from respiratory gasps. The gasps preceded troughs in the CO record after a delay of approximately 21 s. PetCO₂ was constant. Further subjects showed considerable variation in CO though not periodic. Data concerning haemoglobin, arterial oxygen saturation (SaO₂) arterial oxygen content (CaO₂) and calculated DO₂ have also been provided for comparisons between air breathing, hypoxia at sea level and at altitude (both rest and mild exercise).