Diaphragm Electromyogram Research Articles

The relation of ventilatory failure to pulmonary,respiratory muscle and central nervous system disturbances in calves with an experimentally produced pneumonia

To explore the pathophysiology of respiratory failure in an experimental pneumonia, a Pasteurella haemolytica broth culture was injected intratracheally into 12 calves, which were then studied over a period of 10 h. Measurements were made of inspired minute ventilation (VE), ventilatory pattern [inspiratory time (TI), expiratory time (TE), respiratory rate (RR) and tidal volume (VT)], transdiaphragmatic pressure (Pdi), occlusion pressure at the airway opening 100 milliseconds after onset of inspiration (Pawo100ms), arterial blood gas tensions and pH and recorded diaphragmatic electromyogram (EMGdi) and rectal temperature (Tr). On and after the third hour after inoculation, the animals varied in respect of clinical signs, Tr, RR, VE, Pawo100ms/EMGdi, and arterial gases and pH. In benign cases, diminished alertness, laboured respiration and fall of arterial oxygen pressure (PaO2) worsened up to 7 h after inoculation, but then progressively improved, VE being maintained at approximately 150% baseline throughout the study (10 h). Neither arterial carbon dioxide pressure (PaCO2) nor pH was altered. Moderate cases resembled benign cases in respect of laboured respiration, VE, PaO2 and PaCO2; however, pH was at first maintained at preinoculation levels, but declined thereafter. In severe cases, the animals were drowsy between hours 3 and 7, and became comatose between hours 8 and 10; in contrast to both benign and moderate cases: (1) RR was reduced by hour 5, (2) there was no trend towards recovery of PaO2 and pH, (3) VE, Pdi, Pawo100ms and Pawo100ms/EMGdi were severely decreased, and (4) PaCO2 increased. These results suggest that pneumonia does not alter ventilatory neuromuscular pump function in calves, unless concomitant cardiovascular collapse occurs. It is not clear whether fatal ventilatory failure is caused mainly by deterioration in ventilatory muscle fibre processes or structures, altered central nervous system adjustment of ventilatory timing, or cardiovascular dysfunction. However, inspiratory pressures fall when excitation to the diaphragm is still growing, which suggests peripheral respiratory muscle fatigue.

Influence of bipolar esophageal electrode positioning on measurements of human crural diaphragm electromyogram.

We aimed to describe how the human diaphragm electromyogram (EMGdi) is filtered at different positions within the esophageal hiatus, i.e., in the close proximity of the diaphragm, when obtained with an array of bipolar electrode pairs mounted on an esophageal catheter. We defined the electrically active crural diaphragm musculature that covers the esophagus as the electrically active region of the diaphragm (EARdi) and its center as the EARdi center. EMGdi signals were obtained via a multiple-array esophageal catheter consisting of seven sequential electrode pairs with three different electrode configurations. Subjects (n = 5) performed voluntary contractions of the diaphragm at functional residual capacity. Visual inspection of the signals revealed reversal of signal polarity on either side of the EARdi center. Extreme correlation values (r values close to -1 at 0-ms time offset) were observed for the correlation of signals on either side of the EARdi center. The root mean square (RMS) was reduced at the EARdi center; moving peripherally from the EARdi center (caudally and cephalad), the RMS increased to a peak (range of 2.1-4.1 dB for the different electrode configurations) and then decreased for the most peripheral electrode pairs. From a position where the RMS values peaked, center frequency values increased at the EARdi center (range of 26-29 Hz for the different electrode configurations). Computer simulation yielded similar data to the experimental results. We conclude that electrode positioning within the EARdi severely influences center frequency and RMS values and that the center of the EARdi can be identified via cross-correlation analysis.

Effect of Shengmai San injection on diaphragmatic function of rabbits) injection on diaphragmatic function of rabbits

By means of stimulating bilateral phrenic nerves in rabbits, the effect of Shengmai San (SMS) injection on function of normal and fatigue diaphragm was observed with the measuring of diaphragm evoked potential (DEP), and the analysis of the power spectrum of diaphragmatic electromyogram (EMGdi) at spontaneous breathing. The results were as follows: (1) After the adminstration of 2 ml/kg of SMS, amplitude of DEP, central frequency, and the high/low frequency ratio were markedly increased. (2) Injecting SMS 2 ml/kg in 30 minutes before injury could effectively get rid of diaphragmatic fatigue following electrical stimulation of the phrenic nerves. (3) Injecting SMS in 2 ml/kg after fatigue could enhance the recovery of the diaphragmatic fatigue.

Diaphragmatic activity during REM sleep in the adult cat.

Diaphragmatic electromyograms from five adult cats were studied to determine whether diaphragmatic activity, like central respiratory activity, increases in rapid-eye-movement (REM) sleep. Breaths with inspiratory durations between 250 and 2,000 ms were analyzed. 1) There was a greater slope of the moving time average of diaphragmatic activity in REM than in non-REM (NREM) sleep. These greater slopes occurred whether the route of breathing was through the upper airways or through an endotracheal tube and may have resulted from early recruitment of motor units. 2) Mean diaphragmatic activity was also greater, but other variables (peak activity, the area under the curve of diaphragmatic activity, mean intratracheal pressures, inspiratory airflow rates, and tidal volumes) were not greater in REM than in NREM sleep. 3) Diaphragmatic activity was similar in REM sleep and active wakefulness. 4) Across states, slope of the moving time average varied with the duration of inspiration: greater slopes were associated with shorter breaths. These results are consistent with an increase in central respiratory drive in REM sleep that increases the rate of rise of diaphragmatic activity.

Effects of cooling the ventrolateral medulla on diaphragm activity during NREM sleep

Dysfunction through cooling of neurons near the ventrolateral medullary (VLM) surface results in apnea in the anesthetized state, whereas similar neuronal dysfunction in the awake state only modestly decreases breathing. The purpose of this study was to investigate effects on breathing, as measured by diaphragm electromyogram (EMG di), of VLM neuronal dysfunction during NREM sleep, a naturally occurring change in state. In six goats, thermodes for cooling were chronically implanted between the first hypoglossal rootlet and the pontomedullary junction (area M and area S). During wakefulness and NREM sleep, bilateral VLM cooling (thermode temp = 20°C) for 30 sec decreased EMG di mean activity and minute EMG di ( p < 0.05) and lengthened the time between diaphragm contractions. During NREM sleep, reductions in mean and minute EMG di during cooling tended to be greater than during waking, but not significantly. However, following carotid body denervation, VLM cooling caused prolonged apnea during NREM sleep but only a brief apnea in the awake state. The data suggest that either intact VLM neuronal mechanisms or intact carotid afferents are necessary for sustained EMG di activity during NREM sleep.

4-Aminopyridine antagonizes saxitoxin- and tetrodotoxin-induced cardiorespiratory depression

Antagonism of saxitoxin- and tetrodotoxin-induced lethality by 4-aminopyridine was studied in urethane-anesthetized guinea pigs instrumented for the concurrent recordings of medullary respiratory-related unit activities (Bötzinger complex and Nu. para-Ambiguus), diaphragmatic electromyogram, electrocorticogram, Lead II electrocardiogram, blood pressure, end-tidal CO 2 and arterial O 2/CO 2/pH. The toxin (either saxitoxin or tetrodotoxin) was infused at a dose rate of 0.3 μg/kg/min (i.v.) to produce a state of progressive cardiorespiratory depression. The animals were artificially ventilated when the magnitude of integrated diaphragm activities was reduced to 50% of control. Immediately after the disappearance of the diaphragm electromyogram, the toxin infusion was terminated, and 4-aminopyridine (2 mg/kg, i.v.) was administered. The therapeutic effect of 4-aminopyridine was striking in that the toxin-induced blockade of diaphragmatic neurotransmission, vascular hypotension, myocardial anomalies, bradycardia and aberrant discharge patterns of medullary respiratory-related neurons could all be promptly restored to a level comparable to that of control condition. The animals were typically able to breathe spontaneously within minutes after 4-aminopyridine. At the dose level used to achieve the desired therapeutic responses, 4-aminopyridine produced no sign of seizure and convulsion. Although less serious side-effects such as cortical excitant/arousal and transient periods of fascicular twitch could be observed, these events were of minor concern, in our opinion, particularly in view of the remarkable therapeutic effects of 4-aminopyridine.

Analysis of diaphragm EMG signals: comparison of gating vs. subtraction for removal of ECG contamination.

The diaphragm electromyogram (EMGdi) conveys information relating to the mechanisms of respiration; however, electrocardiogram (ECG) contamination can compromise the accuracy of data derived from this signal. We examine the EMGdi recorded from anesthetized spontaneously breathing dogs via implanted electrodes to assess the extent of the error introduced by the ECG contamination and the effectiveness of ECG gating in reducing this error. Because ECG subtraction has been shown to generate accurate results for such applications, it is used as the gold standard. Analysis of variance methods are employed to compare results derived from the EMGdi data after ECG subtraction with corresponding results derived from the original data and from the data after ECG gating. Estimates of EMGdi variables obtained by using subtraction and gating techniques were not significantly different, indicating that gating can be employed on these signals to reduce ECG contamination without affecting the accuracy of the derived data. Results also show that at EMG-to-ECG power ratios > 13.3 dB, ECG contamination does not significantly affect estimates of the EMGdi variables.

Development of concentration thresholds for fetal swallowing and arginine vasopressin stimulation in the preterm ovine fetus

We previously demonstrated that fetal swallowing was stimulated by intracarotid saline injections in the near term (130 ± 1 days gestation) ovine fetus. In order to examine the ontogeny of the regulation of thirst-mediated swallowing behavior and arginine vasopressin (AVP) secretion, the present study examined fetal swallowing and AVP secretory responses to hypertonic NaCl and sucrose in preterm (115 days gestation) ovine fetuses. Five fetuses were chronically prepared with thyrohyoid, nuchal and thoracic esophagus, and diaphragm electromyogram (EMG) electrodes, an esophageal flow probe, and vascular catheters. Fetuses received intracarotid injections (0.15 ml/kg) of increasing concentrations of NaCl (0.15–2.55 M), administered at 2-min intervals. The threshold saline concentration for swallowing was defined as the minimum NaCl dose eliciting swallow responses (within 20 s) following 4 of 5 injections at each dose. In 2 fetuses, the threshold dose was 0.75 M (confirmed by 4 of 5 injections resulting in swallows at the next higher dose). In the other 3 animals, no swallowing threshold was observed (i.e., no dose through 2.55 M elicited swallows to 4 of 5 injections). There were no significant increases in AVP secretion. On a subsequent day, equimolar sucrose injections did not affect fetal swallowing activity or AVP secretion. Together with previous reports, these studies indicate that plasma osmolality dipsogenic mechanisms become functional in some ovine fetuses by 115 days gestation, while all fetuses exhibit intact osmotic dipsogenic responses at 130 days gestation. These results further suggest an acute process of development of dipsogenic responsiveness, rather than by gradually increasing sensitivity to osmotic stimuli. © 1996 Wiley-Liss, Inc.

Development of concentration thresholds for fetal swallowing and arginine vasopressin stimulation in the preterm ovine fetus

We previously demonstrated that fetal swallowing was stimulated by intracarotid saline injections in the near term (130 +/- 1 days gestation) ovine fetus. In order to examine the ontogeny of the regulation of thirst-mediated swallowing behavior and arginine vasopressin (AVP) secretion, the present study examined fetal swallowing and AVP secretory responses to hypertonic NaCl and sucrose in preterm (115 days gestation) ovine fetuses. Five fetuses were chronically prepared with thyrohyoid, nuchal and thoracic esophagus, and diaphragm electromyogram (EMG) electrodes, an esophageal flow probe, and vascular catheters. Fetuses received intracarotid injections (0.15 ml/kg) of increasing concentrations of NaCl (0.15-2.55 M), administered at 2-min intervals. The threshold saline concentration for swallowing was defined as the minimum NaCl dose eliciting swallow responses (within 20 s) following 4 of 5 injections at each dose. In 2 fetuses, the threshold dose was 0.75 M (confirmed by 4 of 5 injections resulting in swallows at the next higher dose). In the other 3 animals, no swallowing threshold was observed (i.e., no dose through 2.55 M elicited swallows to 4 of 5 injections). There were no significant increases in AVP secretion. On a subsequent day, equimolar sucrose injections did not affect fetal swallowing activity or AVP secretion. Together with previous reports, these studies indicate that plasma osmolality dipsogenic mechanisms become functional in some ovine fetuses by 115 days gestation, while all fetuses exhibit intact osmotic dipsogenic responses at 130 days gestation. These results further suggest an acute process of development of dipsogenic responsiveness, rather than by gradually increasing sensitivity to osmotic stimuli.

Respiratory response to positive inspiratory pressure in the cat: effects of CO2 and vagal integrity.

Effects of inspiratory pressure support (IPS) on respiration and activity of inspiratory muscles were tested in eight anesthetized cats by recording the diaphragmatic electromyogram (EMGdi) and respiratory variables at four levels of positive inspiratory airway pressure (5, 10, 15, and 20 cmH2O); onset of IPS was triggered by the inspiratory effort of the animal. When IPS was applied with room air (IPSAir) the respiratory frequency (f) was reduced compared with spontaneous breathing and the tidal volume (VT) was significantly increased, which resulted in a fall of arterial PCO2 (PaCO2) at IPS airway pressures (Paw) above 5 cmH2O. Despite this increase in VT, the amplitude of the integrated EMGdi (Adi) was reduced during IPS at all levels of Paw. When arterial hypocapnia is corrected by addition of CO2 to the inspirate, the values of VT at any given Paw were virtually identical with those during IPSAir, but IPS-mediated changes in f and Adi were smaller than those during IPSAir. IPS was also performed after bilateral vagotomy. Vagotomy itself caused VT and Adi to increase, and f to decrease, during spontaneous breathing. In comparison with the corresponding treatment before vagotomy, IPSAir led to a less severe reduction in Adi. As a result, VT was more enhanced and Paco2 was more reduced after vagotomy than before, both during spontaneous breathing and during IPSAir at all levels of Paw. When, however, isocapnia was restored with IPS with CO2 in the vagotomized animal, diaphragmatic activity and f became very similar to their values during spontaneous breathing, whereas VT remained elevated as a result of the high positive airway pressure. Our data suggest that in anesthetized cats IPS leads to a diminution of diaphragm activity and that this reduction can be entirely attributed to 1) the hypocapnia, resulting from increased VT, and 2) the stimulation of pulmonary vagal afferent fibers at positive airway pressure.

Automatic assessment of electromyogram quality

Power spectrum analysis of the diaphragm electromyogram (EMGdi) is time consuming, and no criteria have been developed to objectively quantify contamination of the signal. The present work describes a set of computer algorithms that automatically select EMGdi free of the electrocardiogram and numerically quantify the common artifacts that affect the EMGdi. The algorithms were tested 1) on human EMGdi (n = 5) obtained with esophageal electrodes positioned at the level of the gastroesophageal junction, 2) on EMGdi obtained in mongrel dogs (n = 5) with intramuscular electrodes in the costal diaphragm, and 3) on computer-simulated power spectra. For authentic and simulated power spectra, indexes were obtained by the algorithms and were able to quantify signal disturbances induced by noise, electrode motion, esophageal peristalsis (in humans), and non-QRS complex-related electrocardiogram activity. With the index inclusion thresholds set to levels that allowed for a high signal acceptance rate with relatively small artifact-induced fluctuations (10-15%) of the EMGdi center frequency, the computer algorithms were found to be as reliable as or more reliable than other methods, including careful visual selection of the time domain signals by experienced analysts. In conclusion, the frequency domain application of computer algorithms offers a reliable and reproducible means to objectively quantify the sources that contaminate the interference pattern EMG.

Effect of atropine on the frequency of reflux and transient lower esophageal sphincter relaxation in normal subjects

Background & Aims Low basal lower esophageal sphincter (LES) pressure is believed to be an important mechanism of reflux. The effects of atropine on the frequency and mechanisms of gastroesophageal reflux under the experimental conditions of a low basal LES pressure in 13 normal subjects were studied. Methods LES pressure, esophageal pressures, esophageal pH, and crural diaphragm electromyogram were recorded simultaneously in the postprandial period for 30 minutes before and two 30-minute periods after the injection of atropine. Results Atropine reduced the basal LES pressure from 16.4 ± 3 to 8.7 ± 2 mm Hg. The frequencies of reflux in the control and two postatropine periods were 3.5 ± 0.5, 0.4 ± 0.2, and 0.8 ± 0.3, respectively ( P < 0.05). The frequencies of transient LES relaxations decreased from 3.5 ± 0.5 in the control to 0.4 ± 0.2 and 1.5 ± 0.4 in the two postatropine periods ( P < 0.05). Transient LES relaxation and associated inhibition of the crural diaphragm was the major mechanism of reflux under conditions of low LES pressure induced by atropine. Conclusions Atropine-induced low LES pressure does not predispose to reflux in normal healthy subjects. Atropine reduces the frequency of reflux by its inhibitory effect on the frequency of transient LES relaxation.

Effects of muscle-to-electrode distance on the human diaphragm electromyogram.

It has been suggested that esophageal recordings of the diaphragm electromyogram (EMGdi) are influenced by changes in chest wall configuration. Whether the changes are of physiological or artifactual origin is unclear. For example, the distance between the esophageal electrode and the diaphragm is likely to alter with chest wall configuration and may lead to misinterpretations of EMGdi. The aims of this study were 1) to evaluate and quantify the effect of the muscle-to-electrode (ME) distance filter on EMGdi, as obtained with a multiple-array esophageal electrode, 2) to take advantage of the ME distance filter to locate the position of the diaphragm with respect to the electrode, and 3) to evaluate the influence of lung volume and chest wall configuration on EMGdi center frequency (CF) while controlling for the ME distance filter and signal quality. Five subjects performed six static contractions of the diaphragm at each of seven chest wall configurations, as evaluated by the method of K. Konno and J. Mead (J. Appl. Physiol. 22: 407-422, 1967). EMGdi was measured with seven pairs of electrodes mounted on an esophageal catheter. The pair of electrodes whose EMGdi power spectra were the least filtered by the ME distance was assumed to be closest to the diaphragm. The results of the study indicated that 1) EMGdi power spectra were strongly affected by the distance between the diaphragm and the electrodes. CF decreased by approximately 1 Hz/mm displacement away from the electrode pair closest to the diaphragm; and 2) no systematic relationship was found between changes in chest wall configuration and CF, when CF was measured from the electrode pair closest to the diaphragm. We conclude that the EMGdi CF can be reliably measured with a multiple-array esophageal electrode that covers the span of diaphragmatic excursion and by selecting the pair of electrodes that is the closest to the diaphragm.

Effect of Aminophylline on High-energy Phosphate Metabolism and Fatigue in the Diaphragm

Diaphragmatic fatigue causes respiratory failure, for which aminophylline has been used as therapy. Because the mechanism of action of aminophylline in reversing diaphragmatic fatigue is unclear, we used in vivo 31P magnetic resonance spectroscopy (MRS) to determine the relation between diaphragmatic activation, force output, and aerobic metabolism. Bilateral phrenic stimulation was used to pace the diaphragm in pentobarbital-anesthetized piglets (6-10 weeks old; n = 44). Esophageal and abdominal pressures were measured to calculate transdiaphragmatic pressure (Pdi) (Pdi = abdominal pressure-esophageal pressure) as an index of force output. Activation was determined by the amplitude of the compound action potential of the diaphragmatic electromyogram. Aerobic metabolism was assessed with a 31P MRS surface coil on the right hemidiaphragm with the animal in a 4.7-T magnet. The animals were divided into four groups based on aminophylline loading dose: saline, aminophylline 10 mg/kg (A10), aminophylline 20 mg/kg (A20), and aminophylline 40 mg/kg (A40). After aminophylline loading the diaphragm was paced for 25 min followed by a 10-min recovery. Aminophylline concentrations were 12.2 +/- 0.7, 21.9 +/- 2.4, and 44.9 +/- 3.6 mg/l in the A10, A20, and A40 groups, respectively. Compound action potential amplitude decreased in all groups by 30% after 25 min of pacing. Conversely, Pdi remained at 100 +/- 3% of the initial value after 5 min of pacing in the A40 group but decreased to 75 +/- 3% in the saline group. Pdi recovered completely (103 +/- 17%) in the A40 group but remained depressed (72 +/- 6%) in the saline group. Pdi values were intermediate in the A10 and A20 groups. MRS data revealed inadequate energy supply/demand ratio in the saline group such that the ratio of inorganic phosphate to phosphocreatine (Pi/PCr) increased to 1.01 +/- 0.09 after 5 min of pacing. Pi/PCr remained unchanged in the A40 group and was intermediate in the A10 and A20 groups. beta-Adenosine triphosphate and intracellular pH did not differ among groups or as a function of pacing. Diaphragmatic blood flow increased from a resting value of 35-60 to 300-410 ml.min-1 x 100 g-1 during pacing in all groups and was not affected by aminophylline dose. Aminophylline, in a dose-dependent fashion, delays the onset of fatigue and improves recovery from fatigue. Delayed fatigue is associated with improved aerobic metabolism as reflected in a low Pi/PCr ratio.

Expiratory Supraglottic Obstruction During Muscular Relaxation

Some reports have suggested occurrence of expiratory upper airway narrowing in patients with obstructive sleep apnea (OSA) during sleep and in awake humans during respiratory muscles relaxation. This is compatible with the hypothesis that upper airway muscles are activated during expiration. We studied five healthy volunteers and four patients with OSA in a tank respirator (Emerson; Cambridge, Mass). Supraglottic pressure (Psg) was measured with a catheter with the tip at the retroepiglottic level, tidal volume with an inductance plethysmograph and airflow with a pneumotachograph at the mouth. Diaphragmatic electromyogram was recorded with an esophageal bipolar electrode. Measurements were done at -30 cm H2O. Subjects were asked to breathe in phase with the respirator and then asked to breathe in phase with the respirator and then to relax their muscles. During muscular relaxation, there was supraglottic obstruction and flow limitation. This was observed during both inspiration and expiration. Upper airway obstruction was more severe in patients with OSA than in healthy subjects. In two healthy volunteers, fiberoptic bronchoscopy showed a wide-open oropharyngeal isthmus during active breathing that narrowed during muscular relaxation. This was true during both inspiration and expiration. We conclude that muscular relaxation is associated with upper airway narrowing and flow limitation occurring during both inspiration and expiration. We suggest that to preserve an open upper airway, airway muscles have to be activated during both inspiration and expiration.

A Comparative Evaluation of Pressure-Triggering and Flow-Triggering in Pressure Support Ventilation (PSV) for Neonates Using an Animal Model

The triggering system in pressure support ventilation needs to respond rapidly, especially in neonates. The aim of this study was to compare the effects of flow-triggered and pressure-triggered pressure support ventilation on neonatal mechanical ventilation using an animal model. Respiratory flow, airway pressure, oesophageal pressure, and diaphragmatic electromyogram were measured during pressure support ventilation in five anaesthetized rabbits. The animals were connected to a VIPBIRD (Bird, U.S.A.) (CPAP mode, pressure support ventilation, 5 cm H2O and PEEP 0 cm H2O). Flow-triggering sensitivity was set at 0.2l/min, 0.5l/min, 1.01l/min, or 1.5l/min. Pressure-triggering sensitivity was set at -1.0 cm H2O. Shorter trigger delay and longer pressure support time were observed in flow-triggering. There was also less diaphragmatic activity in flow-triggering as evidenced by the amplitude of integrated diaphragmatic electromyogram and negative deflection of oesophageal pressure. The findings suggest that flow-triggering will prove superior to pressure-triggering in pressure support ventilation for neonates.

Activity of costal and crural diaphragm during progressive hypoxia or hypercapnia.

Because costal and crural diaphragm segments have different functional characteristics, ventilatory stimulation with hypoxia or hypercapnia may elicit differential segmental function. We report measurements of diaphragm segmental length, shortening, and electromyogram (EMG) activity from 11 canines that were chronically implanted with sonomicrometry transducers and EMG electrodes and then studied a mean of 18 days postimplantation while awake and breathing spontaneously during CO2 rebreathing and progressive isocapnic hypoxia. Ventilatory responses to hypercapnia and progressive hypoxia were moderate at 1.13 +/- 0.31 (SD) 1. min-1. mm-1 arterial Pco2 and -0.98 +/- 0.51 l. min-1.%arterial O2 saturation-1. When tidal values for breathing pattern and segmental function were compared at matching tidal volumes that correspond to mean CO2 of 49.4 arterial Pco2 and 77% arterial O2 saturation, there was no significant difference in resting length, tidal shortening, or tidal EMG of costal or crural segments. Intrabreath profiles of flow, shortening, and EMG activity at matched tidal volumes showed that 1) inspiratory flow during hypoxia was significantly greater during early inspiration, 2) crural EMG activity preceded costal EMG activity in early inspiration during both hypercapnia and hypoxia, 3) both segments showed increased postinspiratory inspiratory activity with stimulated ventilation, and 4) postinspiratory shortening and EMG were greatest for the crural segment during hypoxia. These results suggest that costal and crural diaphragm segments exhibit differential function during chemical stimulation, especially during postinspiration.

Stability of evoked parasternal intercostal muscle electromyogram at increased end-expiratory lung volume.

The diaphragmatic electromyogram has been measured as an index of the level of diaphragmatic activation. The diaphragmatic electromyogram, however, even when measured by intramuscular electrodes, can be artifactually altered by a change in lung volume (A. Brancatisano, S. M. Kelly, A. Tully, S. H. Loring, and L. A. Engel. J. Appl. Physiol. 66: 1699-1705, 1989) or by a change in body position. The parasternal intercostal muscle may be less subject to the mechanisms that are believed to produce this artifactual change. We asked whether the parasternal intercostal electromyographic activity could be reliable when lung volume changes. Six supine rabbits were anesthetized with ketamine and xylazine. Fine bipolar copper wires, with their tips exposed, were inserted into the left parasternal intercostal muscle in the third interspace. A stimulus that was three times maximal was applied to the corresponding intercostal nerve, and the resulting action potential (AP) was photographed. Parasternal intercostal muscle length was measured by sonomicrometry over the vital capacity range. There were small nonsignificant changes in the AP from functional residual capacity (FRC) to total lung capacity. From FRC to residual volume there was variation in the AP. The AP was also quite stable when regional conductivity was altered but showed variation when the parasternal intercostal muscle length change was accentuated by traction on the rib cage. We conclude that the parasternal intercostal electromyographic activity can be reliably used to measure inspiratory motoneuron output to it over the range of lung volumes from FRC to total lung capacity.

Comparative evaluation of diaphragmatic activity during pressure support ventilation and intermittent mandatory ventilation in animal model.

The aim of the present study is a comparative evaluation of the effects of pressure support ventilation (PSV) and intermittent mandatory ventilation (IMV) on diaphragmatic activity in rabbit model of neonate. The animals were divided into a PSV group and an IMV group. In the IMV group, spontaneous breathing and four kinds of IMV rate (5, 10, 15, and 20/min) were applied (Ventilator: Bear BP200, peak inspiratory pressure [PIP]: 12 cm H2O, inspiratory time: 0.6 s). In the PSV group, spontaneous breathing and four levels of PSV (3, 6, 9, and 12 cm H2O) were applied (Ventilator: VIP Bird, flow triggering). Airway pressure (Paw), flow (V), esophageal pressure (Pes), integrated diaphragmatic electromyogram (Edi), and arterial gas data were measured. Amplitudes of Pes and Edi were expressed as percentages (% Edi and % Pes) of the control value during spontaneous breathing to evaluate diaphragmatic activity. Lower IMV rates did not reduce diaphragmatic activity. Approximately half of diaphragmatic activity of control remained even at IMV 15/min. Diaphragmatic activity disappeared at IMV20/min. In contrast, PSV reduced Edi and Pes linearly according to support level. In conclusion, diaphragmatic activity could be reduced more gradually with PSV than IMV by altering ventilatory support level.

Effects of pentobarbital anesthesia on intercostal muscle activation and shortening.

Although pentobarbital (PB) is a commonly used anesthetic in animal studies examining respiratory motor control, there are virtually no studies that have examined the differential effects of deepening anesthesia on the activation of the various intercostal muscles. In dogs, anesthetized initially with 25 mg/kg of PB, the effects of additional doses of PB (20 mg) provided every 15 min on intercostal electromyogram (EMG) were monitored. In each animal, peak external intercostal (EI) and levator costae (LC) activation progressively decreased with additional doses of PB and were eventually abolished, at which point peak parasternal (PA) EMG had increased to 127 +/- 13% (SE) of control values; peak diaphragm EMG was unaffected. The reductions in EI activation were associated with progressive reductions in EI muscle shortening that, in turn, were associated with progressive reductions in lateral rib cage expansion. PA shortening was not significantly affected. Similar results were obtained in animals breathing supplemental oxygen. These results indicate that 1) activation of EI and LC compared with PA have divergent responses, with EI and LC decreasing and PA increasing; 2) the fall in EI activation results in decrements in EI shortening and lateral rib cage motion; and 3) anesthetic depth is an important variable that must be controlled in studies assessing intercostal muscle activation.