Conventional versus slug CO2 loading and the control of breathing in anaesthetized cats

1. Conventional inspiratory CO2 loading (CCL) is accomplished by having the subject breathe CO2-enriched air. An alternative method of CO2 loading is to inject a bolus of CO2 at the start of each inspiration into the inspired air: slug CO2 loading (SCL). During SCL PCO2 in the conducting airways declines quickly towards 0 kPa in the course of inspiration, whereas PCO2 remains at a constant value equal to the inspiratory PCO2 during CCL. Therefore, CCL and SCL may stimulate the respiratory controller differently. 2. We compared the ventilatory responses to SCL and CCL in fourteen anaesthetized, spontaneously breathing cats. In each experimental animal we applied, in a fixed randomized order, five CCL experiments (fractional inspiratory CO2, FI,CO2 = 0.01-0.05), six SCL experiments (slugs of 50% CO2 ranging from 0.5 to 6 ml) and three control experiments in which no CO2 was loaded. Partial pressure of CO2 in arterial blood was determined from small blood samples (0.14 ml). In three cats we repeated the experiments after bilateral transection of the cervical vagi to evaluate the contribution of vagal receptors to the responses observed. 3. The average slope of the CO2 response curves for SCL was 2 times steeper than that for CCL (P less than 0.01). The larger minute ventilation for SCL for a particular value of arterial PCO2 (Pa,CO2) could not be attributed exclusively to the increased breathing frequency or the increased tidal volume (P greater than 0.10). Further, mean inspiratory flow (VI) was larger for SCL than for CCL at the same Pa,CO2 (P less than 0.01), also because the ratio TI/TE (inspiratory duration/expiratory duration) was smaller (P less than 0.01). After vagotomy the difference between SCL and CCL response curves was much reduced. 4. It is concluded that SCL and CCL affect the respiratory controller in a different way causing differences in breathing pattern and CO2 sensitivity between the two methods. Evidently, a mechanism based on CO2 sensitivity of pulmonary receptors is involved in the responses observed.