Methods of studying the control of breathing in experimental animals and man

Effect of posture change on control of ventilation

To clarify the control mechanism of ventilation during posture change, ventilatory parameters, PETCO2, and ventilatory response to CO2 were examined in 11 healthy male subjects at supine (0 degrees) and 75 degrees head-up tilt positions. Minute expiratory ventilation (V.E), tidal volume (VT), respiratory frequency (f), end-tidal and transcutaneous PCO2 and CO2 output (V.CO2), and ventilatory response to CO2 were measured during a steady state condition. V.E (V.A) and VT increased significantly at 75 degrees tilt with significant decrease in PETCO2 from 40.1 mmHg (0 degrees) to about 36.1 mmHg (75 degrees). Transcutaneous PCO2 also decreased during tilt, by 3.3 mmHg. Physiological dead space (VD/VT) and V.CO2, however, remained unchanged, and ventilatory equivalent (V.E/V.CO2, V.A/V.CO2) increased significantly. The CO2-ventilatory response curve shifted upward (or leftward) without significant change in the response slope. At 75 degrees tilt, EMG activity of gastro-cnemius muscle increased. These findings suggested that PETCO2 decreased because of increased V.E (V.A) with a leftward shift of CO2-ventilatory response curve. Various signals such as afferents from lower extremities might have net stimulatory effects on a CO2-ventilation control system to reset the controlled level of PETCO2 to a lower range, but without significant change in CO2-ventilatory response during upright position.

CO2-ventilatory response of the anesthetized rat by rebreathing technique

The ventilatory response to CO2 in rats under sodium pentobarbital anesthesia has been measured using the rebreathing technique. The animal rebreathed through a tracheal cannula for a period of 4 min from an apparatus of 200-400 ml capacity, containing 5-6% CO2 in O2. Pco2 in the rebreathing apparatus (PappCO2), instantaneously Vt, f, and Ve were monitored before, during, and after rebreathing. During the rebreathing run, PappCO2 and Paco2 rose linearly from 35-40 to 65-70 mm Hg; there was no significant difference between PappCO2 and Paco2 at any time during rebreathing. Vt and Ve increases almost linearly with the rise of PappCO2, while f increased to a maximum within 2 min of rebreathing. In the rat, Vt regulation seemed to operate exclusively as a proportional control system in response to linearly increasing CO2 stimulus. The slopes of PappCO2, Vt or Ve response curves varied considerably during the time course of the experiment, depending upon the level of anesthesia, even though there was no large change in Ve in the control periods which were under hyperoxic conditions. However, a significant linear relationship was seen between f in the respective control period and the slope of PappCO2-Vt response at various levels of anesthesia. We concluded that the rebreathing technique can be applied in small experimental animals and that changes in the sensitivity of the respiratory control system to a CO2 stimulus by anesthesia can be easily monitored by repeating the rebreathing test.

Effect of halothane anesthesia on end-tidal PCO and pattern of respiration in the rat

End-tidal Pco2 (petco2) and ventilation of the rat anesthetized with halothane were measured. The Petco2 measured by an infrared analyzer agreed well with the simultaneously measured Paco2 in the range from 20-60mm Hg (2.7-8.0 kPa). When the level of anesthesia was deepened by increasing the halothane concentration from 0.9-3.0%, minute ventilation was decreased progressively accompanied by a rise in Petco2 and the CO2 output was reduced. Halothane induced a progressive decrease in frequency of respiration (f) with almost constant or even slight increase in tidal volume (VT)). Decrease in f was caused largely by the prolongation of expiratory duration (TE). Changes in inspiratory duration (T1) were small and the mean VT/T1 ratio remained unaltered at different levels of anesthesia. These changes in respiratory pattern induced by halothane anesthesia contrasted with the simultaneous decrease in VT, f and VT/T1 ratio and apparent changes of both T1 an TE reported in other species. Species differences in effects of anesthesia on ventilation were discussed. Restraining the rat on a stereotaxic apparatus with head holders, especially with ear bars, elicited an initial transient stimulation of ventilation which was followed by a strong depression.