CLINICAL ASSESSMENT OF THE REGULATION OF VENTILATION



Clinical assessment is currently limited to the chemical control of ventilation, as the precise role of sensory receptor influences is unknown and satisfactory techniques for their assessment do not exist. Depressed chemosensitivity should be sus­pected when one of the conditions listed in is present, and formal assessment of respi­ratory center function should be considered.

The rebreathing test is the commonest clinical method of assessing C02 sensitivity. Normally minute ventilation increases by an average of 2 L/ min/mm Hg C02 (range 1 to 8 L/min/mm Hg C02) (Figure 25-2A). Blunting of the C02 response oc­curs in idiopathic hypoventilation, obesity-hypo-ventilation syndrome, narcotic or sedative inges­tion, hypothyroidism, metabolic alkalosis, and primary neurological disorders. The reduced re­sponse in patients with COPD and C02 retention is discussed later. Chemosensitivity to hypoxia is technically more difficult to measure, and gen­erally there is a good relationship between reduced chemosensitivity to 02 and C02.