Methodology and Equipment

For assessment of ventilatory responses to hypercapnia or hy-poxia (36), the subject inhales a gas mixture that causes a change in either arterial Po2 or Pco2. A plot of Po2 (or Pco2) against ventilation (or, for Po2 response the algebraic constants describing a hyperbola) are compared with normal values. The induced change in blood gases may be continuous (rebreathing methods) or a few discrete points (steady state methods). Usually Pco2 is held constant while Po2 is changed and vice versa. Standard methods are available for measuring ventilatory responses during rebreathing (37, 38).

Steady state or quasi-steady state tests (39) are done simply by having the subject inhale a prepared mixture of gases, usually for 5 minutes (40). Judgments about the safety of inducing hypoxemia or acidosis are made clinically for individual patients. In chronically hypoxemic patients, transient responses to inhalation of pure oxygen may be useful and are safe (40).

For the measurement of P0.1, it is essential to close the airway exactly at the point of zero flow. This is usually done by separating the inspiratory and expiratory lines with one-way valves and then closing the inspiratory line while the subject is exhaling. Conscious subjects must be unable to anticipate occlusions, which must be done silently and unexpectedly. obstruction can be simply performed by inflating a balloon within the lumen of the inspired line or by closing a valve. A sensitive transducer and timer are used to record pressure at 0.1 second.

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