The Effect of Nitrous Oxide on Chest Wall Function in Humans and Dogs

Abstract

The purpose of this study was to determine the effects of nitrous oxide (N (₂) O) on the chest wall of anesthetized humans and dogs.Six human subjects and six mongrel dogs were studied during 1 minimum alveolar anesthetic concentration halothane anesthesia before and during the substitution of 70% N₂ O for 70% N₂ in the inspired gas mixture. On a separate occasion, measurements also were made in pentobarbital-anesthetized dogs. Respiratory muscle activity was measured using electromyographic (EMG) electrodes. Chest wall configuration was determined by using fast three-dimensional computed tomography in dogs and by using respiratory impedance plethysmography in humans. N₂ O consistently decreased inspiratory ribcage displacement, a decrease attributable in dogs to decreased inspiratory activation of parasternal intercostal muscles; parasternal intercostal activity was not present in anesthetized humans. The decrease in ribcage motion decreased the tidal volume in humans, but not in dogs, because displacement of the diaphragm was better preserved in dogs, in association with changes in diaphragm EMG activation. N₂ O significantly increased phasic expiratory muscle activity in halothane-anesthetized humans and pentobarbital-anesthetized dogs. Thus, as has been demonstrated for other anesthetics, the actions of N₂ O are caused by alterations in the distribution and timing of neural drive to the respiratory muscles, rather than by a global depression of respiratory motoneuron drive. Implications: In this study, we examined the effects of nitrous oxide on breathing in halothane-anesthetized dogs and humans. Nitrous oxide affected breathing by changing the distribution and timing of neural drive to the respiratory muscles in a species-dependent manner, rather than by causing a global depression of their activity.

(Anesth Analg 1998;86:1058-64)

At an equivalent depth of anesthesia, as assessed by minimum alveolar anesthetic concentration (MAC), the PaCO₂ maintained during spontaneous breathing is generally less when a combination of a volatile anesthetic and nitrous oxide (N (₂) O) is inhaled compared with a volatile anesthetic alone ^[1-4]. This property is one of the many factors responsible for the enduring clinical popularity of N (₂) O. Most previous studies of N₂ O effects on breathing have focused on global measures of function, such as PaCO₂, minute ventilation, and ventilatory timing. These global measures are, in turn, determined by the actions of the respiratory muscles. Anesthetic drugs affect respiratory muscles in ways that depend on the particular drug, anesthetic depth, respiratory muscles, and species examined. Species differences in respiratory muscle responses to anesthetics may be particularly pronounced ^[5,6]. We have previously demonstrated that halothane-induced respiratory depression is caused by alterations in the distribution and timing of neural drive to the respiratory muscles, rather than by a global depression of respiratory motoneuron drive ^[7]. Whether the same principle is true for N₂ O is unknown because there are no previous studies of N₂ O effects on the function of individual respiratory muscles.

The purpose of this study was to determine how N₂ O changes chest wall function when it is added to the inspired gas of dogs and humans anesthetized with 1 MAC halothane. To further determine whether the background anesthetic affects responses to N₂ O, its effects were also examined in dogs anesthetized with pentobarbital.

Methods

This study was approved by our institutional review board and animal care and use committee. For human studies, six healthy men were studied after they gave their informed consent. For animal studies, six mongrel dogs (9-13 kg) were studied after the implantation of chronic electromyogram (EMG) electrodes. These dogs and humans have been the subjects of previous reports ^[5-8], which provide further details of the experimental techniques. The major difference in the experimental technique between humans and dogs was that chest wall motion was measured by using fast three-dimensional computed tomographic scans in dogs by using impedance plethysmography in humans. Tomography was not performed in humans because of constraints on allowable exposure to radiation in these subjects, who were scanned within the same anesthetic as a part of another protocol ^[8]. We have previously shown that impedance plethysmography is suitable to detect qualitative changes in the pattern of chest wall motion produced by anesthetics ^[8].

Human Studies

Respiratory impedance plethysmography (RIP) belts (Respitrace, Ardsley, NY) were placed around the upper ribcage and mid-abdomen of subjects lying supine and were calibrated using the method of Mankikian et al. ^[9]. An IV catheter was inserted, and the radial artery was cannulated to obtain samples for blood gas analysis (IL 1302; Instrumentation Laboratories, Lexington, MA). Inspiratory and expiratory gas flows were measured using a pneumotachograph (Fleisch, Richmond, VA) connected to a differential pressure transducer (Validyne, Northridge, CA). Gas flows were integrated to obtain changes in lung volume and were corrected to body temperature and pressure standard conditions. The pneumotachograph was calibrated both for gas mixtures containing 70% N₂ and 70% N₂ O.

Bipolar electromyogram electrodes were inserted into several respiratory muscles as previously described ^[7,8]. Electrodes were placed in the transversus abdominis and external oblique muscles at the anterior axillary line approximately 4 cm inferior to the costal margin, the parasternal intercostal muscle at the third right interspace, approximately 3 cm lateral to the midline, and the diaphragm ^[8]. EMG signals were amplified (Grass P511; Quincy, MA), bandpass-filtered between 30 and 3000 Hz, and recorded on digital audio tape.

Anesthesia was induced using inhaled halothane. The trachea was intubated with a 9.0-mm inner diameter endotracheal tube during deep halothane anesthesia, then the inspired halothane concentration was adjusted to maintain approximately 1 MAC end-tidal concentration of 0.9% +/- 0.2% (mean +/- SD). An esophageal balloon was placed in the mid-esophagus and validated by standard techniques ^[10]. After the performance of previously reported maneuvers ^[7,8], the pattern of breathing was permitted to stabilize with the subject breathing an inspired gas mixture of 30% O₂, halothane, and balance N₂, and measurements were recorded (control). Approximately 70% N₂ O was then substituted for N₂ in the inspired gas mixture. After 10 min, measurements were recorded (N₂ O). N₂ was then substituted for N₂ O, and the measurements repeated after 10 min (post-N₂ O).

Animal Studies

The trachea was intubated with a 9.0-mm tube via a chronic tracheostomy ^[5]. After the induction of anesthesia (see below), the femoral artery was cannulated to provide samples for arterial blood analysis and to measure arterial blood pressure. Inspiratory and expiratory gas flows were measured using the pneumotachograph as described.

EMG activity was monitored using chronic electrodes implanted in the parasternal intercostal muscle in the third right interspace, the triangularis sterni muscle in the fifth or sixth right interspace, the transversus abdominis muscle in the ventral axillary line midway between the costal margin and iliac crest, the external abdominal oblique muscle close to the transversus abdominis electrode, and the crural and costal portions of the left hemidiaphragm, as previously described ^[5].

Studies were performed in the same dogs on two occasions separated by at least 1 wk. Each dog was anesthetized on the first occasion using IV pentobarbital 25 mg/mL. Anesthesia was maintained with additional doses of pentobarbital in 15-mg increments when spontaneous movement of the limbs or head occurred. At least 10 min elapsed between these supplemental doses and any measurements. At this level of anesthesia, the lash reflex was still present. The dogs were anesthetized on the second occasion with inhaled halothane adjusted to produce stable end-tidal concentrations of approximately 1 MAC (taken as 0.87% in the dog).

The dogs were placed in the dynamic spatial reconstructor (DSR), a high-speed roentogen scanner that uses the computed tomography principle to provide three-dimensional images of the thorax. The dogs were placed in the supine position with the forelimbs positioned so that the humeri were approximately vertical. Approximately 2 h elapsed between the induction of anesthesia and the first measurements to allow the breathing pattern to stabilize. The dogs spontaneously breathed an inspired gas mixture of 30% O₂, balance N₂ (and halothane in the dogs so anesthetized). Scanning was then performed for 20 s; the other variables described above were recorded simultaneously (control). Approximately 70% N₂ O was then substituted for N₂ in the inspired gas mixture. After 10 min, the measurements were repeated (N₂ O). N₂ was then substituted for N₂ O, and a third set of measurements was obtained after 10 min (post-N₂ O; DSR scans were not obtained at this point). To determine the relaxed configuration of the chest wall (in the absence of any respiratory muscle activity), the lungs were then mechanically ventilated with a tidal volume of 40 mL/kg at a frequency of 25 min for 4 min. After electrical silence of the respiratory muscles was confirmed, scans were performed after 10 s of apnea ^[11].

EMG signals recorded on tape were processed with a third-order Paynter filter to provide a 100-ms moving time average (MTA). Five successive breaths during each sampling period were analyzed. The MTA tracings were digitized, and the mean EMG activity was calculated as the area under the MTA signal divided by the duration of the signal ^[7]. EMG activity was expressed as a fraction of the mean MTA activity measured before N₂ O administration (control). To further quantify the EMG activity of inspiratory agonists (the parasternal intercostal muscles and the diaphragm), which continuously increases during inspiration, the mean rate of MTA increase was also calculated. This parameter has the advantage of a lesser dependence on the duration of electrical activity compared with the mean activity ^[5,7]. If inspiratory EMG activity in the diaphragm persisted into expiration, only the portion of the signal before the onset of expiratory flow was used to calculate this mean ^[7].

Details of image processing to define chest wall boundaries and validation of the DSR in measuring chest wall motion have been previously described ^[8,11,12]. To summarize, scan segments from five successive breaths were gated together to obtain volume images of the thorax at both end-inspiration and end-expiration composed of cubic volume elements (voxels) with edge lengths of 1.3 mm. Images were processed to define each point in the image as being in the thoracic cavity, the abdominal cavity, or the background. Thoracic volume (V_th) was determined by counting the number of voxels in the thoracic cavity. Changes in V_th between any two scans were partitioned into volumes displaced by the diaphragmatic and ribcage surfaces.

All data are expressed as mean +/- SD. Paired comparisons were performed using a t-test. Multiple comparisons were performed using analysis of variance, with Dunnett's test for post hoc comparisons. Significance was assumed at the P < 0.05 level.

Results

Ventilation, Timing, and Gas Exchange

For halothane-anesthetized humans, the addition of N₂ O to the inspired gas significantly decreased tidal volume and significantly increased breathing frequency, significantly decreasing minute ventilation (Table 1). The increase in breathing frequency was associated with a proportional decrease in inspiratory and expiratory times; therefore, there was no change in the ratio of inspiratory time to the total period of breathing. N₂ O significantly increased PaCO₂ without significantly changing PaO₂.

Table 1: Ventilation, Timing, and Arterial Blood Gases

For halothane-anesthetized dogs, N₂ O did not change tidal volume, breathing frequency, minute ventilation, inspiratory time, ratio of inspiratory time to the total period of breathing, or arterial blood gases (Table 1). For pentobarbital-anesthetized dogs, N₂ O significantly increased the breathing frequency without changing tidal volume, producing a significant increase in minute ventilation (Table 1). Inspiratory time was significantly reduced, with no change in the ratio of inspiratory time to the total period of breathing. N₂ O significantly increased PaO₂ but did not affect PaCO₂.

EMG Activity

No activity was noted in the PS of anesthetized human subjects under any conditions (Table 2). The addition of N₂ O to inspired gas increased the rate of increase of MTA activity in the costal diaphragm without significantly changing the mean MTA activity or the duration of activity. N₂ O significantly increased the mean transversus abdominus activity without changing the duration of this activity (Table 3).

Table 2: Inspiratory EMG Activity

Table 3: Expiratory EMG Activity

For dogs anesthetized with halothane, N₂ O decreased the mean MTA activity in the parasternal intercostal by decreasing the duration of activity with an unchanged rate of MTA increase (Table 2). N₂ O increased the mean MTA in the costal diaphragm by increasing the MTA rate of increase with unchanged duration. A similar trend was present in the crural diaphragm but was not statistically significant. In the absence of N₂ O, there was no activity in muscles with expiratory actions (the triangularis sterni, transversus abdominis, and external oblique). N₂ O produced activity in the transversus abdominis of two dogs and the triangularis sterni of one dog.

For dogs anesthetized using pentobarbital, N₂ O decreased the mean MTA activity of the parasternal intercostal by decreasing the duration of activity, without affecting the rate of increase of MTA activity (Figure 1, Table 2). N (₂) O significantly increased the rate of increase of MTA activity in the costal diaphragm without significantly changing the duration or mean activity. N₂ O did not significantly change crural diaphragm activity. N₂ O significantly increased mean MTA activity in all three expiratory agonists without changing the duration of this activity (Table 3).

Figure 1: Representative electromyogram recordings from a pentobarbital-anesthetized dog before (left panel) and after (right panel) the addition of 70% N₂ O to the inspired gas. N₂ O decreased phasic inspiratory activity in the parasternal intercostal muscles and increased phasic expiratory activity in the triangularis sterni and transversus abdominis muscles.

For all conditions, all significant changes returned to control values after the discontinuation of N₂ O.

Chest Wall Motion

For humans anesthetized with halothane, the inspiratory change in total thoracic volume, estimated using RIP, was decreased by N₂ O (Table 4). N₂ O significantly reduced the percent ribcage contribution to total inspiratory V (_th) change; the calculated volume displaced by inspiratory ribcage expansion was reduced, whereas the calculated volume displaced by the abdomen-diaphragm was unchanged.

Table 4: Chest Wall Displacements

During both halothane and pentobarbital anesthesia in dogs, N₂ O did not affect the inspiratory change in total V_th, as measured using the DSR (Table 4). However, N₂ O significantly reduced the percent ribcage contribution to total inspiratory V_th change; the absolute volume displaced by inspiratory ribcage expansion was reduced, whereas the volume displaced by the diaphragm tended to increase (but not significantly). N₂ O did not significantly change the endexpiratory V_th (from 3 +/- 6 to -8 +/- 9 mL, relative to the relaxed volume of the thorax) in halothane-anesthetized dogs, but it did significantly reduce the end-expiratory V_th (from -55 +/- 15 to -82 +/- 16 mL; P < 0.05 relative to the relaxed volume of the thorax) in pentobarbital-anesthetized dogs.

Discussion

When administered alone to awake human subjects, the effects of up to 50% N₂ O on resting breathing are modest, characterized by increases in breathing frequency and unchanged or decreased PaCO₂^[13,14]. However, even low concentrations of N₂ O may significantly affect responses to hypercapnia and hypoxia ^[14-16], although the latter finding is controversial ^[17]. Under hyperbaric conditions, N₂ O at 1 atm markedly increases breathing frequency and decreases tidal volume ^[18].

Several studies have investigated the respiratory effects of N₂ O in human subjects when used as an adjunct to volatile anesthetics. In general, when the total MAC equivalent is maintained by adjusting the inspired concentration of volatile anesthetic, the PaCO₂ maintained during spontaneous breathing is lower when up to 70% N₂ O is substituted for nitrogen or oxygen in the inspired gas mixture ^[1-4]. This effect is most pronounced for enflurane and least pronounced for halothane ^[1]. In other studies (comparable to the protocol followed in the present study), N₂ O was added to the inspired gas while the inspired concentration of volatile anesthetic was kept constant. Hornbein et al. ^[2] added 70% inspired N₂ O to 1 MAC halothane in human volunteers. Consistent with our results in human subjects, N₂ O increased PaCO₂, decreased tidal volume, and increased breathing frequency. A similar pattern of results was observed by Eger et al. ^[3] for 1 MAC isoflurane, and by Wren et al. ^[19] during the withdrawal of N₂ O in children anesthetized with 1 MAC halothane. In contrast, Lam et al. ^[1] found that the addition of N₂ O to 1.1 MAC enflurane had little effect on ventilation.

We found that the decrease in tidal volume and minute ventilation produced by the addition of N₂ O to human subjects anesthetized with halothane was attributable to a decrease in inspiratory ribcage expansion. The mechanism by which this occurred is not clear from our results. Parasternal intercostal muscle activity was absent under all conditions ^[8,20] and therefore cannot explain the changes caused by N₂ O. The pattern of respiratory muscle activities in the diaphragm and transversus abdominis muscles were changed, but the pattern of such changes do not readily explain the observed changes in ribcage expansion ^[21,22]. It should be noted that the pattern of chest wall motion is dependent on a complex interaction between several respiratory muscle groups that may not always be readily predicted. Inspiratory changes in esophageal pressure before and during the administration of N₂ O were similar (-5.6 +/- 1.6 and -6.1 +/- 1.9 cm H₂ O, respectively); therefore, differences in passive effects on the ribcage secondary to thoracic pressure are unlikely. It is possible that changes in the activity of other respiratory muscle groups not monitored were responsible for N₂ O-induced changes in chest wall motion. For example, we have previously shown that some subjects demonstrate phasic inspiratory activity in the scalene muscles during halothane anesthesia, a muscle group that is important in maintaining inspiratory ribcage expansion ^[23,24].

Respiratory muscle activity differs markedly in dogs and humans anesthetized with volatile drugs ^[5,6]. Halothane anesthesia suppresses parasternal intercostal muscle activity and produces phasic activity in expiratory agonists, such as the transversus abdominis, in most human subjects ^[8,20,23,25]. In contrast, parasternal intercostal muscle activity is preserved in dogs during halothane anesthesia, whereas phasic activity in expiratory agonists normally present while awake is suppressed ^[5,6]. The pattern of activity in dogs also depends on the anesthetic; during pentobarbital anesthesia, phasic expiratory muscle activity is preserved ^[5]. In our current study, we found that certain effects of N₂ O also depended on the species and on baseline anesthetic, although some patterns transcended both factors.

As in halothane-anesthetized humans, in dogs anesthetized with both halothane and pentobarbital, N₂ O tended to increase breathing frequency, a tendency reaching statistical significance for pentobarbital. Hall ^[26] reported similar results in halothane-anesthetized dogs. As in human subjects, N₂ O reduced inspiratory ribcage expansion during both anesthetic regimens in dogs. This decrease in ribcage motion in dogs could be attributed to a decrease in parasternal intercostal muscle activity. The change in ribcage motion was not sufficient to significantly decrease tidal volume in the dogs because displacement of the diaphragm tended to increase (although not significantly) in association with changes in diaphragm EMG activation. In other words, unlike humans, dogs were apparently able to compensate for decreases in inspiratory ribcage expansion caused by N₂ O to largely preserve tidal volume. In pentobarbital-anesthetized dogs, increases in phasic expiratory muscle activity produced by N₂ O may have assisted this compensation by reducing endexpiratory V_th. Such active expiration significantly contributes to the generation of tidal volume in both dogs ^[11] and humans ^[23] when expiratory muscle activation ceases at the onset of inspiration. However, the modest activation of expiratory muscle activity produced by N₂ O in human subjects was apparently not sufficient to preserve tidal volume. As a result, N₂ O decreased minute ventilation and increased PaCO₂ in the human, but not the dog.

In conclusion, although the specific pattern of changes varied somewhat with species and baseline anesthetic, N₂ O, when added to inspired gas, changed the distribution and timing of neural drive to the respiratory muscles. In general, these changes can be summarized as a decrease in parasternal intercostal activation (when present), an increase in phasic expiratory muscle activity (when present), and unchanged or increased activation of the diaphragm, accompanied by changes in the timing of such activity. Thus, as has been demonstrated for other anesthetics ^[7], changes in the pattern of ventilation produced by N₂ O are caused by alterations in the distribution and timing of neural drive to the respiratory muscles, rather than by global effects on respiratory motoneuron drive.

The authors thank Kathy Street and Darrell Loeffler for their excellent technical assistance, Janet Beckman for superb secretarial support, Brad Narr for performing preanesthetic medical evaluations, Don Erdman and Mike Rhyner for operating the DSR, Bill Lichty for assistance with EMG techniques, and Jamil Tajik for assistance with ultrasonography.