The involvement of expiratory termination in the vagally mediated facilitation of ventilatory CO2 responsiveness during hyperoxia

Low-dose acetazolamide does affect respiratory muscle function in spontaneously breathing anesthetized rabbits

Patients with chronic obstructive pulmonary diseases (COPD) and/or central sleep apnea are sometimes treated with the carbonic anhydrase inhibitor acteazolamide to improve blood gas values. Studies have shown that this agent may have a complicated effect on lung ventilation, because carbonic anhydrase has a widespread distribution within the body, particularly in tissues involved in the control of breathing. To investigate whether acetazolamide may have (neuro)muscular effects on respiration, we measured the responses of ventilation, phrenic nerve activity, and transpulmonary pressure to changes in arterial PCO2 before and after intravenous administration of a low-dose (4.6 +/- 0.2 mg x kg(-1), mean +/- SEM) of this inhibitor in anesthetized spontaneously breathing rabbits. The agent decreased the mean resting end-tidal PCO2 by 1 kPa and increased ventilation from 258 +/- 15 to 292 +/- 14 ml x min(-1) x kg(-1) (p < or = 0.05). The ventilatory and tidal volume responses to CO2 were reduced, and the response curves were shifted to lower PCO2 values. At the level of phrenic activity, however, the response was shifted leftward without altering CO2 sensitivity. With an unchanged lung compliance, the slopes of the relationships between tidal volume and phrenic activity and that between the tidal change in transpulmonary pressure and phrenic amplitude were both reduced by about 40%, indicating an action of acetazolamide on (neuro)muscular level. The results raise the suggestion that treatment of some hypercapnic COPD patients with acetazolamide may have undesired clinical implications, particularly in those with already weakened respiratory muscles.

Some effects of vagal blockade on abdominal muscle activation and shortening in awake dogs

1. The mechanisms of abdominal muscle activation are thought to be different during expiratory threshold loading (ETL) compared with hypercapnia. Our objectives in the present study were to determine the effects of removing excitatory vagal feedback on abdominal muscle activation, shortening and pattern of recruitment during ETL and hypercapnia. Six tracheotomized dogs were chronically implanted with sonomicrometer transducers and fine wire EMG electrodes in each of the four abdominal muscles. Muscle length changes and EMG activity were studied in the awake dog during ETL (6 dogs) and CO2 rebreathing (3 dogs), before and after vagal blockade. 2. Following vagal blockade, the change in volume (increase in functional residual capacity, FRC) during ETL was greater and active phasic shortening of all the abdominal muscles was reduced, when shortening was compared with a similar change in lung volume. Similarly, at comparable minute ventilation, abdominal muscle active shortening was also reduced during hypercapnia. The internal muscle layer was recruited preferentially in both control and vagally blocked dogs during both ETL and hypercapnia. 3. The degree of recruitment of the abdominal muscles during ETL and hypercapnia in awake dogs is influenced by vagal feedback, but less so than in anaesthetized dogs. These results illustrate the importance of the vagi and abdominal muscle activation in load compensation. However, vagal reflexes are apparently not contributing to the preferential recruitment of the internal muscle layer. In awake dogs during vagal blockade abdominal muscle recruitment still occurs by extravagal mechanisms.

Respiratory reflexes in the anesthetized miniature swine

To assess the suitability of the miniature swine for studies of the control of breathing we evaluated the response of these animals to commonly used respiratory stimuli. Hanford miniature pigs were anesthetized with alpha chloralose and allowed to breathe spontaneously. Rapid lung inflations induced a prolonged expiratory pause proportional to load. Mechanical stimulation of the upper airways induced coughing. Central venous injections of C-fiber stimulants produced bradycardia, hypotension with apnea and/or rapid shallow breathing. CO2 rebreathing increased ventilation primarily through an increase in tidal volume; inspiratory time was not changed. Bilateral vagotomy caused a slower, deeper pattern of breathing, and significantly attenuated the ventilatory response to CO2; all other reflexes were abolished by vagotomy. Cooling the vagus nerves caused reversible blockade of the cough, inflation and C-fiber mediated reflexes in that order. We conclude that the pig can serve as a useful animal in which to study the control of breathing.

The effects of CO2 and fixed acid on the O2-Hb affinity of rabbit and cat blood

The action of respiratory and metabolic acid-base disturbances on the O2-Hb affinity was studied in rabbits and cats. Blood samples of both species were exposed to in vitro pH-changes, which were either achieved by variation of PCO2 (2.8-8.3 kPa) at constant lactic acid concentration, or by addition of lactic acid (5-14 mmol X 1(-1) at constant PCO2. The PO2 at halfsaturation (P50) and the Hill's n were determined from O2-Hb dissociation curves (ODC) in a range between 20 and 80% SO2. Under standard conditions (T = 311 K, PCO2 = 5.33 kPa, pH = 7.4), the average P50 value was 4.66 +/- 0.05 kPa in rabbits, that is slightly higher than reported by others, and 5.17 +/- 0.03 kPa in cats. The average values of Hill's n were 2.91 +/- 0.02 and 2.95 +/- 0.03 for rabbits and cats, respectively. When plasma pH was varied by CO2, the resulting classical CO2 Bohr factor phi CO2 = delta lgP50/delta pH50 was distinctly higher in cats (-0.560 +/- 0.006, n = 25) than in rabbits (-0.504 +/- 0.014, n = 22), although in the latter species being even higher than reported elsewhere. Concomitant metabolic acidosis did not significantly affect phi CO2, but shifted the P50 at a given plasma pH to lower values. Substitution of lactic acid with equimolar amounts of sodium lactate left both phi CO2 and P50 unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)