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

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 (PulseCOTM). 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).