Effect of chronic pulmonary denervation on ventilatory responses to exercise

To assess the role of intrapulmonary receptors on the ventilatory responses to exercise we studied six beagle dogs before and after chronic pulmonary denervation and five dogs before and after sham thoracotomies. Each exercise challenge consisted of 6 min of treadmill exercise with measurements taken during the third minute at 3.2 km/h, 0% grade, and during the third minute at 5.0 km/h, 0% grade. Inspiratory and expiratory airflows were monitored with a low-dead-space latex mask and pneumotachographs coupled to differential pressure transducers. Both pre- and postsurgery, all dogs exhibited a significant arterial hypocapnia and alkalosis during exercise. Denervation of the lungs had no significant effect on minute ventilation at rest or during exercise, although there was a lower frequency and higher tidal volume in the lung-denervated dogs at all measurement periods. Breathing frequency increased significantly during exercise in lung-denervated dogs but to a lesser magnitude than in the control dogs. The changes that occurred in breathing frequency in all animals were due predominantly to the shortening of expiratory time. Inspiratory time did not shorten significantly during exercise following lung denervation. We conclude from these data that intrapulmonary receptors which are deafferented by sectioning the vagi at the hilum are not responsible for setting the level of ventilation during rest or exercise but are involved in determining the pattern of breathing.

Respiratory arrhythmias and airway CO2, lung receptors, and central inspiratory activity

The purpose of this study was to determine whether hypocapnia affects heart rate secondary to an effect on pulmonary receptors. Dogs were anesthetized and placed on cardiopulmonary bypass. Interrelationships among airway CO2, central inspiratory activity, and lung receptor effects on respiratory-related heart rate changes (respiratory arrhythmias) were studied after vagal efferent activity was increased secondary to baroreceptor stimulation. Hypocapnia, isolated to the lungs, produced an increase in the magnitude of the respiratory arrhythmias observed. Two mechanisms may produce these results. Hypocapnia affects pulmonary receptors, which 1) reflexly alter heart rate and 2) modulate breathing frequency, thus altering the dynamics of the respiratory arrhythmias that were produced. The results also suggested that the reflex increase in heart rate in response to lung inflation and the Hering-Breuer expiratory-facilitatory reflex are either produced by different pulmonary receptors or by the same pulmonary receptors but may be mediated by different central mechanisms.

Effects of chronic right-to-left cardiac shunt on hypoxic sensitivity of mongrel dogs

Resting ventilation (VI), blood gases, hypoxic sensitivity, and the ventilatory responses to intravenous sodium cyanide (NaCN, 100 micrograms/kg), doxapram (DOX, 500 micrograms/kg), and dopamine (DOPA, 20 micrograms/kg) were analyzed in four normal mongrel dogs (group I-N) and seven mongrel dogs with chronic (5-11 yr) right-to-left cardiac shunt (group II). The group I-N animals were also studied during steady-state isocapnic hypoxia (group I-H). The shunt procedure used for these studies produced a model for ventilatory studies during chronic shunt hypoxemia. The increases in VI per percent decrease in O2 saturation, which occurred during a four-breath N2 test, were 30, 43, and 13 ml X kg-1 X min-1 in groups I-N, I-H, and II, respectively. The decrease in hypoxic sensitivity of the group II animals, compared with groups I-N and I-H, occurred in the presence of an increase in PaCO2 from 21.9 to 26.0 Torr during the four-breath N2 test. A decrease in PaCO2 from 34.7 to 30.0 and from 33.6 to 30.4 Torr was observed in groups I-N and I-H. The response to DOX, a general analeptic agent, was greatest in group II and least in group I-N. However, the ventilatory responses to NaCN and DOPA were not sufficiently different among the three groups to suggest a difference in carotid body function as assessed by these drugs.

Systemic arterial pH servocontrolled ventilator simulation of the respiratory control system

The terminology 'isocapnic hyperpnea' has been used to describe the ability of the respiratory control system to increase ventilation in response to inhalation of low levels of CO2 without an apparent change in the error signal (arterial pH or PCO2). Recently a control system for the systemic arterial pH (pHa) servocontrol of mechanical ventilation has been developed. The combination of proportional and integral control used produced a system by which the desired set point was maintained with virtually a zero steady-state error. The purpose of these experiments was to use this system to produce isocapnic hyperpnea in response to low levels of inspired CO2 and thus to demonstrate how, through integral control, a biological system could produce a particular response without an apparent change in the controlled variable. Adding 1.0 to 3.5% CO2 to the inspired gas of dogs connected to the pHa servocontrolled ventilator produced increases in minute ventilation with little or no change in pHa or PaCO2. Whether such a control system has any relevance to the physiological control system is questionable. It does however allow a unique way of investigating the possibilities by which the physiological system may work.

Pulmonary denervation in the dog

To produce a chronically lung-denervated animal model, a single-stage surgical procedure was performed on five beagle dogs. A left thoracotomy allowed hilar stripping of the pulmonary contributions to the left vagus nerve and transection of the right vagal trunk. The criterion for denervation was defined as the absence of the Hering-Breuer reflex (HBR). The five denervated dogs (DD) as well as five control dogs (CD) were evaluated for presence of the HBR under pentobarbital sodium (Nembutal, 30 mg/kg) anesthesia. Between the 3rd and 8th wk postsurgery, the HBR was clearly abolished in the DD but present in the CD. By the 12th-14th wk postsurgery, the HBR was again present in the DD. This relatively uncomplicated surgical procedure effectively produced a lung-denervated animal model. However, reinnervation occurred relatively soon, thus demonstrating the importance of regular frequent evaluation of the HBR when using this and any other surgically produced pulmonary denervation model.

Interrelationships among airway CO2, airway pressure, and breathing frequency

The breathing frequency response to changes in airway CO2 of a vascularly isolated lobe of the canine lung has previously been shown to be primarily dependent on CO2-mediated changes in airway pressure. This study was carried out to determine what contribution changes in airway pressure make in the whole lung airway CO2-mediated breathing frequency response. Mongrel dogs were anesthetized and placed on cardiopulmonary bypass. Diaphragm electromyogram (EMG) was used to monitor respiratory center output and to trigger ventilation of the lungs. Isoproterenol administered to the lungs prevented hypocapnic airway constriction but only partially blocked the decrease in breathing frequency, suggesting that in the whole lung preparation, airway CO2 in part alters breathing frequency through a direct effect on pulmonary receptors. At constant positive end-expired pressures (1-6 Torr), 0% airway CO2 produced greater increases in expiratory time than 10% CO2. Thus airway CO2 can affect breathing frequency in the absence of CO2-related changes in airway pressure at pressures that would produce lung volumes similar to those observed at end expiration in the intact animal. An argument is presented that the receptors directly affected by CO2 are probably not located in the airways constricted by hypocapnia.

Location of lung receptors mediating the breathing frequency response to pulmonary CO2

Pulmonary stretch receptors are thought to mediate the breathing frequency (bf) response to changes in pulmonary CO2. However, the location and distribution of these receptors is disputed. The purpose of this study was to determine what contribution the extrapulmonary receptors make in the pulmonary CO2 bf response. Mongrel dogs were anesthetized and placed on cardiopulmonary bypass. The diaphragm electromyogram was used to monitor respiratory center output and to trigger a ventilator. Exposure of an upper airway segment to CO2 or positive end-expired pressure failed to produce changes in the bf. Denervation of the upper airway down to but not including the hilum caused similar insignificant changes in the CO2 bf response. Lungs collapsed by suction showed minimal Hering-Breuer inhibition when compared with inflated lungs. Bronchial arterial perfusion with hypocapnic followed by hypercapnic blood failed to produce changes in the bf while similar perfusion of the pulmonary arterial system resulted in significant increases in bf. It appears that the receptors mainly responsible for the pulmonary CO2 response are located in the more peripheral regions of the lung.

Measurement of dead space ventilation using a pHa servo-controlled ventilator

A control system for the systemic arterial pH (pHa) servo control of mechanical ventilation has recently been developed. If pHa is maintained constant by the change, separation of minute volume into alveolar ventilation and physiological dead space ventilation (VE = fVA VDp) can be manipulated to show that VDp = (VE1 - VE 2)/(f1 - fe) where f1 and f2 are different ventilator frequencies and VE1 and VE2 are expired minute volumes at these frequencies. Also, added dead space can be measured. VDadded = (VE2 - VE1)/f where VE1 and VE2 are the minute volumes before and after the dead space was added. The validity of these equations was tested in the anesthetized dog. The measured added dead space was in close agreement with the volume of dead space which was added and with that measured by another independent method. The measurement of VDp, probably as a result of tidal volume-related changes in VDp, did not agree as well with VDp measured by an independent method.

Accessory muscle activity and respiration

The relationship between the accessory muscle activity (sternohyoid and sternothyroid) and respiration was studied in canines. These animals do not have an omohyoid muscle such as found in primates. Therefore, chair-trained monkeys that have all three accessory muscles were used in a portion of the study. Findings in canines supported those previously reported. The sternothyroid muscle fired spontaneously with the onset of inspiration, but there was no similar activity in the sternohyoid. The activity of both muscles was observed after sectioning the recurrent laryngeal nerves and inducing hypoxia and hypercarbia. The severely hypoxic animals produced some negligible activity in the sternohyoid muscle. The data obtained from the chair-trained monkeys showed no consistent accessory muscle activity during normal respiration with recordings taken immediately after electrode placement, at 24 hours, and one week later. Respiratory activity was consistent in the omohyoid and sternothyroid but not in the sternohyoid muscle when partial airway obstruction and hypoxia were induced.

Segmental ventricular adjustments to brief periods of ischemia in the dog

The changes in left ventricular segmented contractile force induced by brief periods of ischemia (15-90 s) and subsequent reperfusions were analyzed in anesthetized dogs. Segmental coronary artery occlusion (left anterior descending or left circumflex) produced a decrease in segmental ventricular function in the occluded area and an increase in contractile force in the myocardial segment away from the occluded area. With reperfusion, a transient overshoot in contractile force above preischemic control levels was observed in the occluded segment. This overshoot was shown not to be dependent on adrenergic mechanisms but appears to indicate changes in calcium permeability.

Effect of verapamil on pulmonary reflexes in the vascularly isolated canine lung: physiology

Breathing frequency (BF) may be affected by changes in the percent inspired CO2 administered to vascularly isolated lungs. Pulmonary CO2 probably affects BF, in part, through a secondary effect of CO2 on airway smooth muscle. To further determine the role of pulmonary mechanics in the pulmonary CO2-mediated BF response, Verapamil, a Ca++ blocking agent which blocks hypocapnic airway constriction, was administered to the vascularly isolated lungs of the dog. Verapamil blocked the hypocapnic airway constriction which occurred when pulmonary CO2 was reduced; however, the decrease in BF was not only blocked but in some animals there was an increase in BF. Also, the decrease in BF produced by hyperinflation of the lungs (Hering-Breuer reflex) was either blocked or an increase in BF occurred after administration of Verapamil.

Effect of anodal blockade of myelinated fibers on vagal C-fiber afferents

The effects of monopolar cathodal, monopolar anodal, and bipolar anodal polarizing currents on vagal A- and C-fiber activity were studied in anesthetized dogs. Monopolar cathodal polarization consistently produced excitation of spontaneous and evoked A- and C-fibers. Monopolar anodal and bipolar anodal polarizations differentially blocked A-fibers as a function of fiber diameter. The specific purpose of this study was to compare the effects of monopolar anodal and bipolar anodal currents on C-fiber excitability. Small fiber preparations were dissected from the vagal trunk, cut centrally, and placed on recording electrodes. A constant-current stimulus pulse was applied to the nerve at various distances from the blocking electrodes. The stimulus current strength was increased until an isolated C-fiber spike was observed. This value was defined as 100% of threshold. The stimulus current was then reduced to zero, the blocking current was increased slowly to 100 microA, and the procedure repeated. Threshold data obtained in this manner for each set of stimulation electrodes was plotted as a function of distance from the blocking electrode(s) for both modes of anodal blockade. No significant change in C-fiber excitability was observed with bipolar anodal blockade, whereas excitability was significantly (P less than or equal to 0.05) decreased using the monopolar technique. Thus, monopolar anodal block may reduce the possibility of asynchronous C-fiber discharge, which has been associated with a bipolar block of A-fibers.

Cardiac, aortic, pericardial, and pulmonary receptors in the dog

Afferent nerve activity from left and right atrial, left and right ventricular, interventricular septal, papillary muscle, pericardial, aortic and pulmonary vascular receptors was recorded from the left T3 white ramus communicans and the innominate, dorsal, recurrent, ventromedial, craniovagal and caudovagal cardiac nerves in alpha-chloralose-anesthetized dogs. The receptors were localized and the nature of the stimuli required to excite these receptors was also determined. Some ventricular receptors were excited during maximal contraction of the myocardium. Other ventricular and aortic receptors were stimulated by elevation of intracardiac or aortic pressure. The pericardial, atrial, and papillary muscle receptors were excited by stretching the surrounding tissue. The discharge patterns of these receptors were not always synchronous with the events of the cardiac cycle. The stimuli required to excite each type of receptor (ventricular, atrial, etc.) and their resultant discharge patterns were not identical for all of the receptors. Excitation of cardiac receptors with sympathetic afferents resulted in 1--2 spikes per cardiac cycle, whereas receptors with vagal afferents resulted in bursts of spikes per cardiac cycle.

Breathing frequency responses to pulmonary CO2 in an isolated lobe of the canine lung

Recent studies have indicated that the breathing frequency responses to inspired CO2 in part result from changes in pulmonary stretch receptor activity. Pulmonary CO2 may alter frequency by direct inhibition of stretch receptor discharge, or secondarily, by changes in airway mechanics. The vascularly isolated left lower lobe (LLL) of the canine lung was used to determine the effect of hypocapnic airway constriction on the pulmonary CO2 reflex. The upper and middle lobes of the left lung were removed and the right vagus nerve sectioned. Blood was recirculated through the LLL. Diaphragm electromyogram was used as an index of respiratory center activity and to trigger ventilation of the left lower lobe. Lobar hypocapnia increased peak airway pressure and reduced respiratory rate. However, infusion of isoproterenol or the use of a mechanical overflow system to block the airway pressure response prevented the frequency changes associated with CO2. Although both the direct and mechanical effects of CO2 on stretch receptors may contribute to the reflex, in the LLL preparation the mechanical effects predominate.

Halothane, tracheal compliance and upper-airway mechanoreceptors

This study was designed to define the effects of halothane on the compliance of the trachea. An isolated in-situ tracheal preparation was studied in 14 mongrel dogs anesthetized with pentobarbital. Compliance of the closed tracheal segment was measured with continuous intraluminal pressure recordings during repeated injections of known volumes of air: Slow-adapting neural activity observed in paratracheal branches of the recurrent laryngeal nerve accurately reflected pressure in the tracheal segment. Halothane at 0.5--4.0 per cent concentrations caused a significant (P less than 0.001) average 10 per cent increase in the compliance of the trachea. Stimulation of the efferent vagus caused a significant (P less than 0.001) average 8 per cent decrease in compliance of the trachea. After exposure to halothane, vagal stimulation still caused a significant decrease in compliance of the trachea.

Systemic arterial blood pH servocontrol of mechanical ventilation

Servocontrol of mechanical ventilation using systemic arterial blood pH, measured by a dual-function pH/PCO2 intra-arterial sensor, as the controlled variable uas carried out in 30 dogs anesthetized with pentobarbital, 30 mg/kg. The control loop consisted of the animal, an intra-arterial dual-function pH/PCO2 sensor and sensor amplifier, a controller, and a Siemans-Elema 900 servoventilator. The system responded appropriately to changes in set-point pH from 7.30 to 7.50, as well as to infusions of lactic acid, which, with the control loop open, decreased systemic arterial blood pH 0.1 TO 0.2 PH units. Long-term (16 hr) ventilation of one dog with the systemic arterial blood pH servocontrol ventilator was shown to be feasible.

Respiratory inhibition with sympathetic afferent stimulation in the canine and primate

Inhibition of phrenic efferent nerve activity, diaphragm electromyogram (EMG), and external intercostal EMG was observed in halothane- and pentobarbital-anesthetized mongrel dogs and pentobarbital-anesthetized monkeys with stimulation of sympathetic afferents. The central end of the transected ventral limb of the left ansa subclavia, the sympathetic chain, or individual white rami were stimulated while simultaneously recording phrenic efferent nerve activity, diaphragm EMG, or the external intercostal EMG. Averaged phrenic efferent bursts or MEG were used to trigger an electronic respirator. In all of the dogs and monkeys, electrical stimulation of sympathetic afferent pathways resulted in inhibition of phrenic efferent nerve activity, diaphragm EMG, or external intercostal EMG. Although the exact origin of these fibers was not determined, the conduction velocities of these afferents were 4-7 m/s, which places them in the Adelta fiber-type range. The importance of these afferents in the regulation of respiration in the awake animal remains unknown.

Cardiac responses to stimulation of thoracic afferents in the primate and canine

Excitatory cardiovascular responses to electrically stimulated upper thoracic sympathetic afferent nerves were observed in halothane-anesthetized mongrel dogs and monkeys. The central end of the transected ventral limb of the left ansa subclavia was stimulated before and after several types of denervation. Significant increases in right and left ventricular maximum systolic pressures, systolic and diastolic systemic blood pressures, and aortic flow were observed. The carotid sinuses were denervated bilaterally and stimulation of the ansa was repeated. The cardiovascular responses to stimulation of the ventral ansa after carotid sinus denervation were greater in magnitude than those observed prior to denervation. This carotid sinus modulation of cardiovascular responses was observed in dogs and monkeys. Cardiovascular responses to stimulation of the ventral ansa after bilateral vagotomy were significantly less than the responses observed after carotid sinus denervation prior to vagotomy. However, the responses after vagotomy were statistically identical to responses obtained while stimulating the ventral ansa when the carotid sinuses and vagi remained intact.

Tracheal afferent nerves

The individual fibers of the inferior laryngeal nerve and anastomosing branch of the superior laryngeal nerve arising from the upper five to six tracheal rings were studied. In 30 mongrel dogs using simple and multifiber nerve preparations, afferent nerve activity was studied both in the intact trachea and in an isolated segment of trachea with artificial stimuli of graded pressure and flow. The adaptation rate, threshold, spontaneous firing frequency, maximum firing frequency, and rate of change in afferent discharge were recorded. Subsequently, these fibers were electrically stimulated to determine what reflex effects could be produced. Afferent nerve activity recorded from the upper trachea was found to be sensitive to tracheal pressure changes of 0.5 to 12 mm Hg. Nerve activity paralleled the frequency of pressure changes. Electrical stimulation of these fibers demonstrated reflex bradycardia, bradypnea and alteration of blood pressure. These data and that of other investigators indicate that upper airway mechano-receptors may be significant, not only in the initiation of upper airway protective reflexes, but also in the regulation of normal respiration.