Phrenicotomy in the rat: acute changes in blood gases, pH and body temperature.

Adult male rats were used to compare blood gases, pH and body temperature (Tb) before and after acute bilateral phrenicotomy. Under anaesthesia a femoral artery was catheterised and ties were placed round the phrenic nerves of seven rats (PNX group), while in five rats the ties were placed in the vicinity of the phrenic nerves (SHAM group). Twenty-four hours after surgery arterial blood samples were collected during quiet wakefulness (QW) and grooming (G), before and 1 h after the ties were pulled, and analysed for PO2, PCO2 and pH. No changes were detected in the SHAM samples taken before and after the ties were pulled. In the PNX group a significant decrease in Tb occurred (QW, 0.6 degrees C; G, 1.5 degrees C). Following PNX PaO2 decreased by 11.2 mmHg (QW) and 10.0 mmHg (G); PaCO2 increased by 2.6 mmHg (QW) and 2.4 mmHg (G) and pH fell by 0.04 (QW) and 0.03 (G). All changes except in PaCO2 (QW) were significant. It is concluded that the changes in Tb, blood gases and pH which follow phrenicotomy in the rat are due to an increase in dead space ventilation (VD) and a small reduction in alveolar ventilation (VA) associated with a faster, shallower pattern of breathing.

After phrenicotomy the rat alters the output of the remaining respiratory muscles without changing its sleep-waking pattern.

The respiratory activity of selected rib cage and abdominal wall muscles was studied in intact and bilaterally phrenicotomized (PNX) rats during non-rapid eye movement sleep (nREMS) and REMS. The polygraphic method was used to identify the animal's sleep-waking states before and after PNX. Electromyographic (EMG) recordings were made from the following muscles: the parasternals of the 1st, 3rd, and 5th interspaces; the external and internal intercostals of the 1st, 6th and 10th interspaces; the levator costae attaching to the 10th rib; the scalenus medius; and, the abdominal wall muscles, the external and internal obliques and rectus abdominis. After PNX, all rib cage muscles contracted exclusively during inspiration and all but one increased their activity. The exception was the internal intercostal muscle of the 10th interspace; its activity decreased. The external and internal oblique muscles, both of which were active during expiration in nREMS, also increased their output after PNX: rectus abdominis became an inspiratory muscle. The persistence of phasic activity of respiratory muscles during REMS varied not only from muscle to muscle but from one REMS epoch to another. The sleep-waking pattern of the PNX rat differed in only a minor way from that of the intact rat. Therefore, we conclude that the rat with total paralysis of its diaphragm uses mainly its neurometabolic mechanisms to achieve an adequate level of alveolar ventilation and not neurobehavioral mechanisms.

Alteration in breathing of the awake rat after laryngeal and diaphragmatic muscle paralysis.

The respiratory rate (f), tidal volume (VT) and ventilation (V) were measured in 3 groups of rats: 10 rats before and after cutting both recurrent laryngeal nerves (RLNX), 10 rats before and after bilateral phrenicotomy (PNX) and 5 sham transected (SHAMX) rats. All rats were exposed to air and gas mixtures, deficient in O2 and/or enriched with CO2. The barometric method was used to measure ventilatory parameters. The sham operation did not affect breathing pattern or ventilation. In RLNX rats, breathing the various gas mixtures exhibited no changes in V because f uniformly increased as VT declined. Therefore, loss of the neural control of the respiratory functions of the larynx in awake rats exposed to selected gas mixtures has no untoward effects on alveolar ventilation. Changes in ventilation of PNX rats, compared with SHAMX rats, depends on the gas composition breathed. With increasing severity of hypoxia and/or hypercapnia, PNX rats show a marked reduction in alveolar ventilation over that of the SHAMX rats. Thus, when the diaphragm is no longer able to participate in ventilatory responses, gas exchange is likely to become deficient.

Proprioceptive, chemoreceptive and sleep state modulation of expiratory muscle activity in the rat.

The purpose of this study was to assess the respiratory and tonic activity of the abdominal muscles and the postinspiratory activity of the diaphragm (stage 1 expiration) in rats during sleep while they breathed air, hypercapnic, and hypoxic gas mixtures. ECoG and neck EMG recordings enabled the differentiation to be made between nonrapid eye movement sleep (nREMS) and rapid eye movement sleep (REMS). EMGs of the rectus abdominis, internal and external oblique, and diaphragm muscles were displayed on a CRT and polygraph. During nREMS the rectus abdominis showed no respiratory activity, whereas the oblique muscles showed activity confined to stage 2 expiration. This activity was modulated by proprioceptive (sleep postures) and chemoreceptive activation (5% CO2 in air and 10-12% O2 in nitrogen): tonic activity was not consistently affected by such inputs. During REMS tonic activity disappeared, whereas phasic activity either remained unchanged or was abolished. If phasic activity ceased it could reappear periodically during the same REMS epoch. While breathing air, rats in nREMS showed postinspiratory diaphragmatic activity which was sustained or slightly increased while breathing a hypoxic gas mixture but was virtually abolished during hypercapnia. In REMS postinspiratory discharges almost disappeared. The data support the conclusion that the diaphragm provides expiratory braking and that the external and internal oblique muscles contribute to active exhalation during nREMS as well as priming the diaphragm for the next inspiration by improving its length-tension relationship. A three-phase neural respiratory pattern generator operates in nREMS: it changes temporarily to a two-phase system while breathing CO2 and during REMS due to the inhibition of the postinspiratory phase.

Differential effects of hypoxia on sleep of warm- and cold-acclimated rats.

We studied the effect of different levels of hypoxia (10, 12 or 13, 15, and 18% O2) on the sleep-waking pattern (SWP) and the maximum-minimum core temperature of warm-acclimated (WA) and cold-acclimated (CA) rats at their neutral temperature, 29 degrees C. Whereas the SWP of WA rats showed a trend toward increasing disruption as the degree of hypoxia increased, CA rats exhibited no such trend. The effect was chiefly on the frequency of state changes and less on epoch durations. The SWP of WA rats was more vulnerable to hypoxia than that of CA rats. Maximum and minimum body temperatures of WA and CA rats were not significantly affected by O2 lack down to 10% inspired O2. We conclude that in the rat 1) hypoxia primarily affects the neural mechanism that governs the frequency of changes in sleep-waking states; 2) the extent of alterations in SWP's depends on the ambient temperature to which the rats are acclimated; and 3) hypoxia does not significantly affect deep body temperature at the animal's neutral temperature.

The labile respiratory activity of ribcage muscles of the rat during sleep.

1. Sleep-waking states of chronically implanted rats were identified polygraphically while recording the integrated electromyogram (e.m.g.) of extrinsic (scalenus medius and levator costae) and intrinsic (external and internal interosseous intercostal and parasternal) muscles of the thoracic cage. Rats breathed air, air enriched in CO2 (5%) or air deficient in O2 (10% O2 in N2) and were free to adopt any desired posture. 2. In non-rapid eye movement (non-r.e.m.) sleep, the scalenus medius and intercostal muscles of the cephalic spaces were always inspiratory; intercostal muscles of the mid-thoracic spaces were commonly expiratory while the more caudal ones were only occasionally expiratory. Expiratory activity, when present in quiet wakefulness, extended for a variable period of time into non-r.e.m. sleep and always disappeared in r.e.m. sleep regardless of the ribcage muscle under study. 3. Inspiratory activity, when present in non-r.e.m. sleep, was unaffected, partially attenuated or abolished at entry into r.e.m. sleep. The peak integrated e.m.g. activity of ribcage muscles was measured as a function of posture, gas mixture breathed and ribcage site: (a) the greater the degree of curled-up posture, the greater the respiratory activity of scalenus medius, an effect augmented by CO2 but depressed by hypoxia, and (b) the more caudally placed ribcage muscles exhibited respiratory activity which was essentially unaffected by posture and gas mixture inspired. 4. The presence or absence of tonic activity in ribcage respiratory muscles during non-r.e.m. sleep was unrelated to posture. When tonic activity was present, it always disappeared in r.e.m. sleep. When expiratory activity was present in non-r.e.m. sleep, it too always disappeared in r.e.m. sleep. Inspiratory activity present in non-r.e.m. sleep was variably affected at entry into r.e.m. sleep; it was unchanged, partially attenuated or abolished. 5. It is concluded that thoracic cage muscles exhibit marked variability in their respiratory activity depending on posture, sleep-waking states and gas mixture breathed. It is postulated that the presence of tonic and/or expiratory activity in ribcage muscles during non-r.e.m. sleep reflects an increase in functional residual capacity (F.R.C.).

Respiratory functions of the inferior pharyngeal constrictor and sternohyoid muscles during sleep.

We studied the respiratory activity of the inferior pharyngeal constrictor and sternohyoid muscles of the rat during non-rapid eye movement (non-REM) and REM sleep. Each animal carried chronically implanted electrodes for recording the integrated EMG activity of respiratory muscles as well as the electrocorticogram (ECoG) and postural tone (dorsal neck EMG). The latter permitted polygraphic identification of sleep states. Curled up postures enhanced inspiratory activity of both upper airway muscles during non-REM sleep, an effect which CO2 breathing failed to augment except in the well curled up position. Hypoxia reduced their activity. During REM sleep, the inferior pharyngeal constrictor and sternohyoid muscles retained their inspiratory activity. No tonic activity could be detected in either muscle. We conclude that the inferior pharyngeal constrictor and sternohyoid muscles safeguard upper airway patency in the two main sleep states.

Unity of costal and crural diaphragmatic activity in respiration.

In chronically implanted rats, we examined the respiratory EMG activity of the two parts of the diaphragm, costal and crural, during sleep and wakefulness. Their activity was compared and contrasted with that of the EMG activity of the cricothyroid muscle. Whether in wakefulness, while grooming and drinking, or in nonrapid eye movement (non-REM) sleep, and independent of the gas mixture breathed (4 to 5% CO2 or 10% O2 in nitrogen), the two parts of the diaphragm paused during REM apnea episodes whereas the cricothyroid muscle ceased its activity or exhibited sustained activity. We conclude that the diaphragm, mainly an inspiratory muscle, acts as a single functional unit when under the respiratory control system. The cricothyroid muscle functions as an inspiratory and/or expiratory muscle, also under the respiratory control systems. Both muscles in the rat come under other neural control mechanisms governing nonrespiratory functions, e.g., swallowing, defecation, and coughing, but not vomiting.

Respiratory roles of genioglossus, sternothyroid, and sternohyoid muscles during sleep.

We examined the respiratory activity of the genioglossus, sternothyroid, and sternohyoid muscles of the rat during nonrapid eye movement (non-REM) and REM sleep. Each animal carried implanted electrodes for recording the integrated EMG activity of respiratory muscles, the postural tone (EMG), and electrocortical activity (polygraphic identification of sleep-waking states). The three upper airway muscles exhibited inspiratory activity during non-REM sleep while rats breathed ambient air. Curled up postures promoted inspiratory activity of genioglossus and sternothyroid muscles, an effect enhanced by CO2 breathing but reduced by hypoxic breathing. During REM sleep, genioglossus and sternothyroid muscles lost their activity but the sternohyoid muscles retained their inspiratory activity. We conclude that the genioglossus and sternothyroid muscles contribute to upper airway patency during non-REM sleep, an effect CO2 augments but hypoxia reduces. The sternohyoid muscles have at least two functions during both sleep states: they contribute to maintenance of upper airway patency and to rib cage fixation, thereby optimizing the ventilatory action of the diaphragm.

The effects of hypoxia and CO2 on the sleep-waking pattern of the potoroo (Potorous tridactylus apicalis).

Four male potoroos (Potorous tridactylus apicalis) breathed 21% and 7% O2 with and without the addition of 5% CO2. The effects of these gas mixtures on the potoroo's sleeping-waking pattern (SWP) were studied. The SWP while breathing 21% O2/5% CO2 was unchanged when compared with that of breathing ambient air (21% O2). While breathing 7% O2, the SWP was severely disrupted: total sleep time (TST) and slow wave sleep (SWS) increased markedly. Brain temperature fell substantially. Paradoxical sleep (PS) was almost abolished and wakefulness (W) decreased. The addition of 5% CO2 to the O2 deficient gas mixture, i.e., 7% O2/5% CO2, restored the SWP to that obtained while breathing ambient air. It is concluded that CO2 neutralizes the disruptive effect which hypoxia has on the potoroo's SWP. It is hypothesized that this constitutes a homeostatic mechanism for stabilizing the SWP and is carried over from pouch life.

An electrophysiological analysis of sleep and respiration of rats breathing different gas mixtures: diaphragmatic muscle function.

The effects of breathing 21% O2, 21% O2 + 5% CO2, 10% O2 + 4% CO2 and 10% O2 on the sleep-waking rhythm, respiratory rate, diaphragmatic EMG, inspiratory (Ti) and expiratory (Te) times were studied in rats. They carried chronically implanted electrodes to permit polygraphic recordings of the ECoG, EOG and dorsal neck and integrated diaphragmatic EMG activity. Average respiratory rates, independent of state of consciousness varied depending on the gas mixture breathed. Sleep-waking times, expressed as percentages, were determined as a function of the gas mixture breathed. Oxygen deficiency caused PS deprivation which was partially alleviated by the addition of 4% CO2. Diaphragmatic EMG activity decreased during PS when rats breathed gas mixtures rich in CO2 but increased when they breathed 10% O2. In general, at a given frequency of breathing, Ti was shorter during PS than during SWS except when rats breathed 10% O2. It is concluded that: (1) regardless of the state of consciousness hypoxia is a more potent stimulus of respiratory rate than hypercapnia, (2) diaphragmatic effort is reduced when rats breathe CO2 enriched gas mixtures but is increased by hypoxia due to changes in upper airway resistance, and (3) low O2 content of an inspired gas disrupts the inspiratory and expiratory off-switch mechanisms, this disruption being prevented by the addition of CO2.

Respiratory EMG activity of the posterior cricoarytenoid, cricothyroid and diaphragm muscles during sleep.

The respiratory activity (EMGs) of the posterior cricoarytenoid (PCA), cricothyroid (CT) and diaphragm (D) were examined during slow wave sleep (SWS) and paradoxical sleep (PS).-Chronically implanted, free-to-move adult rats were used. In SWS, CT exhibited inspiratory or expiratory or inspiratory and expiratory bursts in each respiratory cycle. The latter was common during CO2 (4%) breathing. PCA manifested phasic inspiratory discharges along with tonic expiratory activity. The latter was augmented by breathing CO2. At onset of PS, inspiratory PCA and CT activity declined. In those PS epochs of irregular D activity, PCA and CT further declined during eye movements. In other PS epochs, D arrest coincided with co-activation of PCA and CT, occasionally CT activation alone. CO2 breathing did not affect the above described during PS. In PS, loss of PCA and CT's inspiratory activity during rhythmic D activity may contribute to obstructive apnea; PCA and CT co-activation with D arrest characterizes central type apnea.