PULMONARY EXCHANGE AS RELATED TO ALTERED PULMONARY MECHANICS IN ANESTHETIZED DOGS

Activation of opioid micro-receptors in medullary raphe depresses sighs

Sighs, a well-known phenomenon in mammals, are substantially augmented by hypoxia and hypercapnia. Because (d-Ala(2),N-Me-Phe(4),Gly-ol)-enkephalin (DAMGO), a mu-receptor agonist, injected intravenously and locally in the caudal medullary raphe region (cMRR) decreased the ventilatory response to hypoxia and hypercapnia, we hypothesized that these treatments could inhibit sigh responses to these chemical stimuli. The number and amplitude of sighs were recorded during three levels of isocapnic hypoxia (15%, 10%, and 5% O(2) for 1.5 min) or hypercapnia (3%, 7%, and 10% CO(2) for 4 min) to test the dependence of sigh responses on the intensity of chemical drive in anesthetized and spontaneously breathing rats. The role of mu-receptors in modulating sigh responses to 10% O(2) or 7% CO(2) was subsequently evaluated by comparing the sighs before and after 1) intravenous administration of DAMGO (100 microg/kg), 2) microinjection of DAMGO (35 ng/100 nl) into the cMRR, and 3) intravenous administration of DAMGO after microinjection of d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP, 100 ng/100 nl), a micro-receptor antagonist, into the cMRR. Hypoxia and hypercapnia increased the number, but not amplitude, of sighs in a concentration-dependent manner, and the responses to hypoxia were significantly greater than those to hypercapnia. Systemic and local injection of DAMGO into the cMRR predominantly decreased the number of sighs, while microinjection into the rostral and middle MRR had no or limited effects. Microinjecting CTAP into the cMRR significantly diminished the systemic DAMGO-induced reduction of the number of sighs in response to hypoxia, but not to hypercapnia. Thus we conclude that hypoxia and hypercapnia elevate the number of sighs in a concentration-dependent manner in anesthetized rats, and this response is significantly depressed by activating systemic mu-receptors, especially those within the cMRR.

Dysfunction of the canine respiratory muscle pump in ascites

Ascites, a complicating feature of many diseases of the liver and peritoneum, commonly causes dyspnea. The mechanism of this symptom, however, is uncertain. In the present study, progressively increasing ascites was induced in anesthetized dogs, and the hypothesis was initially tested that ascites increases the impedance on the diaphragm and, so, adversely affects the lung-expanding action of the muscle. Ascites produced a gradual increase in abdominal elastance and an expansion of the lower rib cage. Concomitantly, the caudal displacement of the diaphragm and the fall in airway opening pressure during isolated stimulation of the phrenic nerves decreased markedly; transdiaphragmatic pressure during phrenic stimulation also decreased. To assess the adaptation to ascites of the respiratory system overall, we subsequently measured the changes in lung volume, the arterial blood gases, and the electromyogram of the parasternal intercostal muscles during spontaneous breathing. Tidal volume and minute ventilation decreased progressively as ascites increased, leading to an increase in arterial Pco2 and parasternal intercostal inspiratory activity. It is concluded that 1) ascites, acting through an increase in abdominal elastance and an expansion of the lower rib cage, impairs the lung-expanding action of the diaphragm; 2) this impairment elicits a compensatory increase in neural drive to the inspiratory muscles, but the compensation is not sufficient to maintain ventilation; and 3) dyspnea in this setting results in part from the dissociation between increased neural drive and decreased ventilation.

Preservation of pulmonary function in the ventilated neonatal piglet with normal lungs

Little attention has been focused on the progressive pulmonary deterioration which occurs in mechanically ventilated infants with normal or mildly abnormal lungs. We hypothesized that lung function would deteriorate over a 24-hr period in anesthetized neonatal piglets with normal lungs mechanically ventilated at 2 cm H2O PEEP (2PEEP group). We further hypothesized that an intermittent lung inflation procedure consisting of 15 out of 60 min of increasing lung distention (4, 8, 12 cm H2O PEEP), with the remaining 45 min at 2 cm H2O PEEP (Inflation group) would prevent this deterioration in lung function, similar to piglets mechanically ventilated continuously at 6 cm H2O PEEP (6PEEP). Results indicate that 2PEEP piglets experienced progressive deterioration in lung function, including dynamic lung compliance (-42%) and lung resistance (+55%). In contrast, inflation piglets and 6PEEP piglets had no deterioration in lung function. Hemodynamics were similar between groups, although they were the most stable in the 6PEEP group. Histopathological changes were not significantly different. We conclude that (1) prolonged mechanical ventilation at 2 cm H2O PEEP in neonatal piglets resulted in progressive deterioration in pulmonary function, (2) intermittent lung inflation or continuous 6 cm H2O PEEP prevented deterioration, and (3) functional changes occurred without changes in histopathology. Lung inflation strategies other than PEEP can be used to prevent deterioration in lung function which accompanies prolonged mechanical ventilation in anesthetized nonspontaneously breathing piglets with normal lungs.

The effects of reversing distending pressure sequences in the neonatal piglet

We studied different sequences of lung inflation in ventilated newborn piglets with normal lungs in order to determine the effects of sequence, magnitude and duration of distending pressure on pulmonary function, and/or hemodynamics. End-expiratory pressure was varied using a continuous negative extrathoracic pressure (CNEP) device. Three groups of ventilated piglets with normal lungs were exposed to 2 cmH2O increments of CNEP from -2 to -12 cmH2O, and to decrements from -12 to -2 cmH2O, or to only -6 cmH2O. Lung inflation sequence, magnitude of inflation pressure, and duration of inflation had significant effects on end-expiratory lung volume and lung compliance at numerically equivalent pressure levels. End-expiratory lung volume and lung compliance varied (at four and five of six inflation pressures studied) by as much as 68% and 104%, respectively. Hemodynamic effects of the lung inflation sequence were more variable; those found to be different at numerically equivalent pressure levels were associated with changes in lung compliance and ventilation. Differences in pulmonary mechanics can best be explained by the effects of lung inflation on alveolar recruitment versus overinflation.

The relation between end-tidal CO2 and discharge patterns of sympathetic preganglionic neurons

In 11 Nembutal-anesthetized, vagotomized, thoracotomized, paralyzed and artificially ventilated cats, the electrical activity of 32 single sympathetic preganglionic neurons (SPNs), dissected from the cervical nerve was recorded at various end-tidal CO2 levels, together with the activity of the phrenic nerve. Seven of these neurons were insensitive to CO2 changes, within a range of end-tidal CO2 values from 1;0 to 10;0%. All 7 had a background firing pattern without respiratory modulation, even at the highest CO2 levels tested, i.e., had the same firing frequency in both phases of the phrenic nerve activity cycle. Seventeen units were silent at low CO2 levels, began to discharge at particular CO2 levels (on the average, at 2.3% CO2) and increased their firing frequency (on the average, by 0.9 spikes/sec/% CO2) as end-tidal CO2 was raised above the threshold level. Their background discharge pattern was characterized by firing only in the inspiratory phase of the phrenic nerve activity cycle. Three units had firing which was CO2-independent within a range of low CO2 concentrations and which increased as CO2 concentration was increased above this range. These units fired throughout the phrenic nerve activity cycle but had their peak frequency in inspiration. Five units had a firing frequency which was highest at low CO2 and which decreased with increasing CO2 levels. These units had their peak frequency in expiration. These results show that the output of this SPN population is strongly influenced by CO2 within the range of concentrations tested. The finding that sensitivity to CO2 changes is a property only of SPNs with respiratory-modulated firing pattern suggests that the CO2-dependent input is relayed to these SPNs via the respiratory center; A comparison of data obtained under hypocapnic conditions with data obtained in previous studies in normocapnic cats with mid-cervical spinal cord transections suggests that brain stem inspiratory neurons represent a major excitatory input to this SPN pool.

Respiratory gas exchange in patients with spontaneous pneumothorax

Pulmonary gas exchange was studied in 12 patients with spontaneous pneumothorax by measuring the partial pressure of oxygen and carbon dioxide in arterial blood and expired gas when breathing air and 100% oxygen. The arterial oxygen tension was below 80 mm. Hg in nine patients, and the alveolar-arterial difference in oxygen tension was abnormally large in 10, but the physiological dead space was generally normal. There was a positive correlation between the size of the anatomical shunt and the extent of the pneumothorax as measured from the chest radiograph. Calculations indicated that the fall in arterial oxygen tension when breathing air could be fully accounted for by the increased anatomical shunt. After the air had been removed ventilation—perfusion relationships appeared to become more uneven, and the anatomical shunt was greater than would have been expected from the size of the lung. Observations during infusions of acetylcholine suggested that active vasoconstriction in poorly ventilated regions may have occurred to a slight or moderate degree in four out of eight patients.