Effects of bronchoconstriction on breathing during normoxia and hypoxia in anesthetized cats

Contribution of Carotid Bodies on Pulmonary Function During Normoxia and Acute Hypoxia

Carotid bodies (CBs) are main peripheral chemoreceptors involved in breathing regulation. Despite the well-known role played by CBs on breathing control, the precise contribution of CBs on the regulation of lung mechanics remains controversial. Accordingly, we study changes in lung mechanics in normoxia (FiO₂ 21%) and hypoxia (FiO₂ 8%) in mice with or without functional CBs. For this, we used adult male mice that underwent sham or CB denervation (CBD) surgery. Compared to sham-operated mice, we found that CBD induced an increase in lung resistance (R_L) while breathing normoxic air (sham vs. CBD, p < 0.05). Importantly, changes in R_L were accompanied by an approximately threefold reduction in dynamic compliance (C_dyn). Additionally, end-expiratory work (EEW) was increased in normoxia in the CBD group. Contrarily, we found that CBD has no effect on lung mechanics during hypoxic stimulation. Indeed, R_L, Cd_yn, and EEW values in CBD mice were undistinguishable from the ones obtained in sham mice. Finally, we found that CBD induces lung parenchyma morphological alterations characterized by reduced alveoli space. Together our results showed that CBD progressively increases lung resistance at normoxic conditions and suggest that CB tonic afferent discharges are needed for the proper regulation of lung mechanics at rest.

Loading and unloading breathing during exercise: respiratory responses and compensatory mechanisms

To characterize the ventilatory responses to resistive loading or unloading, we studied the effects of breathing 79% helium-21% oxygen (He-O₂), 79% argon-21% oxygen (Ar-O₂) and 79% SF₆-21% oxygen (SF₆-O₂) on the volume-time parameters, end-tidal partial pressure of CO₂ (P_ETCO₂), mouth pressure (P_Im), work of breathing (W_I), central inspiratory activity (dP/dt_I), and electromyographic activity of parasternal inspiratory muscles (EMGps) in 10 normal subjects at rest and during short-time steady-state exercise. There were no significant changes in tidal volume (V_T), breathing frequency (f), inspiratory (T_I) and expiratory (T_E) durations, minute ventilation (V_E), and P_ETCO₂ when air was replaced by He-O₂ or SF₆-O₂ at rest. V_E and P_ETCO₂ were not significantly different after replacement of air by He-O₂ or SF₆-O₂ during exercise. However, inhalation of He-O₂ decreased in V_T and increased in f, whereas inhalation of SF₆-O₂ led to the opposite effects compared with air during exercise. Both at rest and exercise, P_Im, W_I, dP/dt_I and EMGps were significantly less during He-O₂ breathing and higher during SF₆-O₂ breathing (P < 0.01) from the first respiratory cycle after room air was replaced by He-O₂ or SF₆-O₂. Ar-O₂ breathing did not affect the time-volume parameters both at rest and during exercise compared with air. The increase in P_Im, W_I, and dP/dt_I was observed at Ar-O₂ inhalation during exercise relatively to air conditions (P < 0.05). We conclude that internal resistive loaded (SF₆-O₂) or unloaded (He-O₂) breathing changes the neuromuscular output required to maintain constant ventilation. The mechanisms of load or unload compensation seem to be mediated by afferent information from lung and respiratory muscle receptors as well as by segmentary reflexes and properties of muscle fibers.

A computed tomographic scan assessment of endotracheal suctioning-induced bronchoconstriction in ventilated sheep

This study was directed at assessing changes in bronchial cross-sectional surface areas (BCSA) and in respiratory resistance induced by endotracheal suctioning in nine anesthetized sheep. Cardiorespiratory parameters (Swan-Ganz catheter), respiratory resistance (inspiratory occlusion technique), BCSA, and lung aeration (computed tomography) were studied at baseline, during endotracheal suctioning, and after 20 consecutive hyperinflations. Measurements performed initially at an inspired oxygen fraction (FI(O(2))) of 0.3 were repeated at an FI(O(2)) of 1.0. At an FI(O(2)) of 0.3, endotracheal suctioning resulted in atelectasis, a reduction in BCSA of 29 +/- 23% (mean +/- SD), a decrease in arterial oxygen saturation from 95 +/- 3% to 87 +/- 12% (p = 0.02), an increase in venous admixture from 19 +/- 10% to 31 +/- 19% (p = 0. 006), and an increase in lung tissue resistance (DR(rs)) (p = 0. 0003). At an FI(O(2)) of 1.0, despite an extension of atelectasis and an increase in pulmonary shunt from 19 +/- 5% to 36 +/- 2% (p < 0.0001), arterial O(2) desaturation was prevented and BCSA decreased by only 7 +/- 32%. A recruitment maneuver after endotracheal suctioning entirely reversed the suctioning-induced increase in DR(rs) and atelectasis. In three lidocaine-pretreated sheep, the endotracheal suctioning-induced reduction of BCSA was entirely prevented. These data suggest that the endotracheal suctioning-induced decrease in BCSA is related to atelectasis and bronchoconstriction. Both effects can be reversed by hyperoxygenation maneuver before suctioning in combination with recruitment maneuver after suctioning.

Lung mechanics and end-expiratory lung volume during hypoxia in rats

We investigated whether an hypoxia-induced increase in airway resistance mediated by vagal efferents participates in the increase in end-expiratory lung volume (EELV) observed in hypoxia. We also assessed the contribution of the end-expiratory activity of the diaphragm (DE) to this phenomenon. Therefore, we measured EELV, total lung resistance (RL), dynamic lung compliance (Cdyn), DE, and minute ventilation (VE) in anesthetized rats during normoxia and hypoxia (10% O(2)) before (control) and after administration of atropine or saline. In the control group, hypoxia increased EELV, Cdyn, DE, and VE but slightly decreased RL. These changes were unaffected by saline or atropine, except that, in the atropine-treated rats, hypoxia did not change RL. These results suggest that 1) the increase in EELV observed in hypoxia cannot result from an increase in airway resistance; 2) the increased and persistent activity of inspiratory muscles during expiration is the most likely cause of the increase in EELV during hypoxia; and 3) the decrease in RL induced by hypoxia could result from the increase in lung volume including EELV.

Bronchoconstriction and delayed rapid shallow breathing induced by cigarette smoke inhalation in anesthetized rats

Bronchomotor and ventilatory responses to inhalation of cigarette smoke (50% concentration, 6 ml) were studied in anesthetized and vagotomized Sprague-Dawley rats. Low-nicotine cigarette smoke did not cause any detectable delayed response, whereas high-nicotine cigarette smoke induced rapid, shallow breathing, and a marked increase in airway resistance (RL). The increase in f reached a peak (delta f = 43 +/- 8%) at the 5th breath after the onset of smoke inhalation, preceding both the decrease in VT (delta VT = -27 +/- 4%) and the increase in RL (delta RL = 89 +/- 19%); the latter 2 reached their peaks at approximately the 10th breath, displaying a similar temporal pattern of responses between them. The bronchomotor response to high-nicotine cigarette smoke was slightly attenuated but not prevented by prior administration of isoproterenol (0.2 mg/kg, intravenously [iv]), nor was the smoke-induced rapid, shallow breathing. In contrast, prior administration of mecamylamine (0.9 mg/kg, iv) completely abolished both the bronchomotor and ventilatory responses to smoke inhalation, indicating that nicotine is the primary causative agent responsible for these changes.