Effect of chronic hypobaria on isolated tracheal preparation

Isolated tracheal preparation (ITP) has been studied from rabbits adapted to 24 days simulated hypobaria (PB = 395 mmHg), corresponding to an elevation of 17,000ft (5,000m). During this daily hypobaric exposure, the growth of these animals was continuous and final hematocrit reached 70%. After 24 days of hypobaric exposure, the sensitivity of ITP was tested to acute hypercapnia and acute anoxia. Acute hypercapnia was induced by aerating a muscle chamber with a gas mixture of 85% oxygen and 15% carbon dioxide. During that time, the PCO2 of the experimental solution increased from 24 to 66 mmHg with a corresponding change of pH from 7.39 to 7.10. ITP from rabbits exposed to chronic hypoxia reveals a greater sensitivity to acute hypercapnia as measured by active tension (AT max) and rate of tension development (dT/dt). Acute anoxia was induced by aerating a muscle chamber with a gas mixture of 95% nitrogen and 5% carbon dioxide. During 20 min of these conditions, the Po2 decreased from 520 to 25 mmHg. Although the contractility diminished at approximately equal rates in both groups in response to acute anoxia, preparation from animals exposed to hypobaria showed statistically a significantly greater ability to recover from the anoxic state.