The effects of neuromuscular paralysis on systemic and splanchnic oxygen utilization in mechanically ventilated patients

OBJECTIVE

To evaluate the effect of neuromuscular paralysis on systemic and splanchnic oxygen utilization in patients in respiratory failure during controlled mechanical ventilation.

SETTING

A university-affiliated teaching hospital.

INTERVENTION

Mechanically ventilated patients, who were undergoing hemodynamics monitoring and who had a gastric intramucosal pH (pHi) of less than 7.35, were studied. Prior to paralysis, the patients were sedated with lorazepam and morphine to standard end points, and the cardiac output and oxygenation were optimized. The patients were then paralyzed with doxacurium and the ventilator rate adjusted to keep the PaCO2 at baseline value. The hemodynamic and oxygenation profile and pHi were determined prior to paralysis and repeated 2 to 2.5 h later.

RESULTS

Eight patients were studied; their mean age was 63 +/- 8 years and acute physiology and chronic health evaluation II score was 22 +/- 4. The mean fraction of inspired oxygen, positive end-expiratory pressure, and venous admixture ratio prior to the study was 0.7 +/- 0.14, 11.8 +/- 2.4 cm H2O, and 26 +/- 9%, respectively. Prior to paralysis, the mean set assist controlled ventilation rate was 15 +/- 2 breaths/min and the patient rate was 23 +/- 5 breaths/min. With neuromuscular paralysis, the cardiac index fell from 4.6 +/- 2.2 to 4.3 +/- 2.4 L/min/m2 (p=0.1), the oxygen delivery fell from 537 +/- 129 to 471 +/- 95 mL/min/m2 (p=0.03), and the oxygen consumption and extraction ratio fell from 200 +/ 77 to 149 +/- 35 mL/min/m2 (p=0.03) and 36 +/- 5 to 31 +/- 10, respectively (p=0.2). The pHi increased from 7.21 +/- 0.16 to 7.29 +/- 0.1 (p=0.02).

CONCLUSION

In critically ill patients in respiratory failure, neuromuscular paralysis decreased whole body oxygen consumption and increased pHi. Presumably, by eliminating the work of breathing, there is a redistribution of blood flow from the respiratory muscles to the splanchnic and other nonvital vascular beds.

Myoglobin saturation in free-diving Weddell seals

Although the consumption of myoglobin-bound O2 (MbO2) stores in seal muscles has been demonstrated in seal muscles during laboratory simulations of diving, this may not be a feature of normal field diving in which measurements of heart rate and lactate production show marked differences from the profound diving response induced by forced immersion. To evaluate the consumption of muscle MbO2 stores during unrestrained diving, we developed a submersible dual-wavelength laser near-infrared spectrophotometer capable of measuring MbO2 saturation in swimming muscle. The probe was implanted on the surface of the latissimus dorsi of five subadult male Weddell seals (Leptonychotes weddelli) released into a captive breathing hole near Ross Island, Antarctica. Four seals had a monotonic decline of muscle O2 saturation during free diving to depths up to 300 m with median slopes of -5.12 +/- 4.37 and -2.54 +/- 1.95%/min for dives lasting < 17 and > 17 min, respectively. There was no correlation between the power consumed by swimming and the desaturation rate. Two seals had occasional partial muscle resaturations late in dives, indicating transfer of O2 from circulating blood to muscle myoglobin. Weddell seals partially consume their MbO2 stores during unrestrained free diving.

Hyperinflation with intrinsic PEEP and respiratory muscle blood flow

Increased end-expiratory lung volume and intrinsic positive end-expiratory pressure (PEEP) are common in obstructive lung disease, especially during exacerbations or exercise. This loads the respiratory muscles and may also stress the circulatory system, causing a reduction or redistribution of cardiac output. We measured the blood flow to respiratory muscles and systemic organs using colored microspheres in 10 spontaneously breathing anesthetized tracheotomized dogs. Flows during baseline breathing (BL) were compared with those during hyperinflation (HI) induced by a mechanical analogue of airway closure and with those during an inspiratory resistive load (IR) that produced an equivalent increase in inspiratory work and time-integrated transdiaphragmatic pressure. Cardiac output was unchanged during IR (3.19 +/- 0.27 l/min at BL, 3.09 +/- 0.34 l/min during IR) but was reduced during HI (2.14 +/- 0.29 l/min; P < 0.01). Among the organs studied, flow was unaltered by IR but decreased to the liver and pancreas and increased to the brain during HI. For the respiratory muscles, flow to the diaphragm increased during IR. However, despite a 1.9-fold increase in inspiratory work per minute and a 2.5-fold increase in integrated transdiaphragmatic pressure during HI, blood flow to the diaphragm was unchanged and flow to the scalenes and sternomastoid fell. The only respiratory muscle to which flow increased during HI was the transversus abdominis, an expiratory muscle. We conclude that the circulatory effects of hyperinflation in this model impair inspiratory muscle perfusion and speculate that this may contribute to respiratory muscle dysfunction in hyperinflated states.

Theophylline alters distribution of blood flow to respiratory muscles

This study was designed to examine the effects of theophylline on respiratory muscle blood flow in 11 lightly anesthetized and spontaneously breathing dogs using the radioactive microsphere tracer technique. During quiet breathing, blood flow to the costal diaphragm (25.1 +/- 13.9 ml/100 g/min) exceeded blood flow to the parasternal intercostals (18.0 +/- 10.2 ml/100 g/min, p < 0.05). Inspiratory resistive loading abolished these differences by increasing blood flow to the parasternal intercostals more than to the diaphragm. Aminophylline (40 mg/kg) significantly increased minute ventilation and tidal transdiaphragmatic pressure (Pdi) swing during quiet breathing but not during inspiratory resistive loading. Theophylline did not affect diaphragmatic blood flow during inspiratory resistive loading while the same Pdi swing and tension-time index (TTdi) were reached. During quiet breathing, however, theophylline significantly (p < 0.05) increased blood flow to the triangularis sterni from 7.9 +/- 5.6 to 18.1 +/- 25.6 ml/100 g/min and to the transversus abdominis from 10.8 +/- 8.4 to 14.6 +/- 10.5 ml/100 g/min and tended to increase blood flow to the costal diaphragm and the parasternals. We conclude that (1) during quiet breathing, but not during inspiratory resistive loading, blood flow to the costal diaphragm exceeded flow to the parasternal intercostals; (2) during quiet breathing, theophylline increased blood flow to the expiratory muscles as it promoted recruitment of expiratory muscles; and (3) theophylline did not affect diaphragmatic blood flow for a given TTdi.

Respiratory muscle blood flow in the fetal lamb during apnoea and breathing

We measured blood flow to the respiratory muscles of the fetal lamb using the radioactively-labelled microsphere technique in order to assess whether fetal breathing is an energetically costly activity as has been reported. Diaphragm flow ranged from 6.4-35.2 ml.min-1.100 g-1 during fetal apnoea and rose to 21.1-615 ml.min-1.100 g-1 during fetal breathing (P < 0.02; n = 7). Parasternal muscle flow also increased significantly (P < 0.02) between fetal apnoea and breathing while external and internal intercostal flows did not change. Expressed as a percentage of cardiac output the diaphragm received 0.08-0.28% during apnoea and 0.22-2.2% during fetal breathing. Neither placental blood flow nor fetal O2 consumption increased significantly between fetal apnoea and breathing. We conclude that the levels of perfusion required by the respiratory muscles for breathing in the fetus are inconsistent with fetal breathing costing a large proportion of the fetal O2 budget.

The clinical use of upper extremity exercise

There has been a revival of interest in the interaction between arm exercise and ventilation. Although arm ergometry continues to be the gold standard for the testing and training of upper extremities, an increasingly larger body of evidence indicates a more important role for the testing and training of upper extremities in forms that more closely resemble their physiologic adaptation in humans. As our knowledge of the functional anatomy of shoulder girdle muscles improves, so will our capacity to apply this knowledge in more rational, effective exercise regimens.

EMG changes in respiratory and skeletal muscles during isometric contraction under normoxic, hypoxemic, or ischemic conditions

The consequences of general hypoxemia (PaO2 = 51 mmHg) on two muscle groups (adductor pollicis and diaphragm) sustaining 80% maximal isometric voluntary contraction were studied in healthy individuals. For adductor pollicis, contractions were also executed after 10-s or 3-min rest ischemia. Compared to control, i.e., normoxic, sustained isometric workloads, significant shortening of endurance time occurred only when adductor pollicis contracted under hypoxemic conditions. In both muscle groups, a 3-min ischemia test as well as hypoxemia reduced the rate of changes in integrated surface EMG in a low frequency band and lowered, or did not modify, the rate of change in the high above low frequency ratio. Recovery of normal patterns of EMG changes was prolonged only after the adductor pollicis contracted under hypoxemic conditions. The present data show that both hypoxemia and prolonged rest ischemia reduced the rate of changes in quantitative EMG activity, with the more significant effects being measured under hypoxemia.

Effects of potassium channel blockers on basal vascular tone and reactive hyperemia of canine diaphragm

Glibenclamide, iberiotoxin, and apamin (blockers of ATP-sensitive, large-conductance, and small-conductance Ca(2+)-activated potassium channels, respectively) were infused into the diaphragmatic vasculature of anesthetized dogs to assess the contribution of these channels in the regulation of basal tone and the response to brief occlusions of the left phrenic artery (reactive hyperemia). Baseline phrenic flow (Qphr), peak postocclusive flow, and reactive hyperemia duration in response to 10-, 30-, 60-, and 120-s arterial occlusions were measured before (control) and after the infusion of K+ channel blockers in three groups of animals. Glibenclamide at 5 x 10(-6), 1 x 10(-5), and 8 x 10(-5) M increased baseline phrenic resistance to 140, 204, and 192% of control values, respectively. Peak postocclusive Qphr and duration of hyperemia in response to all occlusion durations were significantly attenuated after glibenclamide infusion. Iberiotoxin infusion at 1 x 10(-8), 3 x 10(-8), and 1 x 10(-7) M increased phrenic resistance to 141, 133, and 146% of control values, respectively. By comparison, baseline phrenic resistance rose to 159 and 145% of control in response to 1 x 10(-7) and 1 x 10(-6) M apamin, respectively. Iberiotoxin and apamin reduced peak postocclusive flow and duration of hyperemia only in response to 10- and 30-s occlusions. We infused K+ channel blockers along with lemakalim into the diaphragm during constant flow perfusion in separate groups of animals. When infused alone, lemakalim reduced phrenic resistance by 60-70%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of selective diaphragmatic paralysis on the inspiratory motor drive

Using alpha-chloralose-anesthetized mechanically ventilated vagotomized dogs, we assessed the effects of selective diaphragmatic paralysis on the inspiratory motor drive. Diaphragmatic paralysis was accomplished by a bolus injection of vecuronium, a neuromuscular junction blocker, into the left phrenic artery of an in situ vascularly isolated and innervated left diaphragm. The inspiratory motor drive during spontaneous breathing attempts was assessed by measuring peak integrated electromyographic (EMG) activities of the left and right diaphragms and parasternal and alae nasi muscles. Respiratory timing parameters were measured from the integrated EMG signals of the diaphragm. Three groups of dogs were studied. In group 1 (n = 9), vecuronium was injected into the phrenic artery with the left diaphragmatic length adjusted at the functional residual capacity. Vecuronium injection (0.2 mg) resulted in a significant decline in left diaphragmatic tension and integrated EMG. Breathing frequency increased by 24% of the baseline value, whereas right diaphragm, parasternal, and alae nasi EMG activities rose to 136, 227, and 165% of their respective baseline values a few seconds after the vecuronium injection. In group 2 (n = 6), vecuronium injection in left phrenectomized animals did not alter the EMG activities of the inspiratory muscles (left EMG signal was abolished) nor did it alter respiratory timing. In group 3 (n = 4), the left diaphragm was placed in a flaccid position. Vecuronium injection in this group did not produce any changes in the EMG activities or respiratory timing. We conclude that selective diaphragmatic paralysis elicits a significant rise in the inspiratory motor drive. This effect is likely to be mediated through the inhibition of diaphragmatic Golgi tendon organ activity.

Blood flow distribution to upper airway muscles

Radiolabeled (15-microns) microspheres were used to measure blood flow to upper airway muscles [alae nasi (AN), intrinsic laryngeal, tongue, cervical strap, and hyoid musculature], diaphragm (DI), and parasternals (PS) during spontaneous breathing in 24 anesthetized tracheotomized supine dogs. Six dogs were also studied while -28 +/- 3 (SE) cmH2O tracheal airway pressure was generated against an inspiratory resistance (IR) (upper airway bypassed). Blood flow to posterior cricoarytenoid muscle (PCA) [24.0 +/- 2.1 (SE) ml.min-1.100 g-1] was greater than that to DI (18.0 +/- 2.3 ml.min-1.100 g-1) and comparable to that to PS (21.4 +/- 2.9 ml.min-1.100 g-1). Blood flow per unit weight did not differ between AN, tongue muscles, laryngeal adductors, cervical strap muscles, and cricothyroid (CT). Average blood flow to these muscles was only 8.0 +/- 0.8 ml.min-1.100 g-1. With the exception of CT, blood flow to these upper airway muscles was less than that to DI and PCA. Relative to blood flow during spontaneous breathing, IR loading increased blood flow to AN by a factor of 7.5, to PCA by 3.4, to DI by 3.2 and to PS by 1.9. There was no change in blood flow in the other muscles during loading. Our results show that at rest blood flow to main glottic dilator (PCA) is similar to that to main inspiratory muscles. Furthermore, in response to an IR load, blood flow to PCA and AN increased by an equivalent or greater amount than that to DI.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of oxygen consumption and cardiac output to work of breathing

This study examined the relationship between work of breathing and estimated blood flow to and oxygen consumption by the respiratory muscles. Five subjects performed inspiratory loaded breathing and voluntary hyperpnea while ventilatory work, cardiac output, and oxygen consumption were measured. Blood flow to and oxygen consumption by the respiratory muscles were estimated by subtracting the resting from the working values of cardiac output the oxygen consumption, respectively. Loaded breathing increased cardiac output, but there was no significant correlation with work of breathing, while oxygen consumption was significantly correlated with work of breathing. During hyperpnea both cardiac output and oxygen consumption were correlated with work of breathing. Our results indicate that blood flow and oxygen consumption are increased in a regular pattern with increases in work of breathing. These results may be significant in estimating the demand of the respiratory muscles in disease and exercise.

Blood flow to respiratory muscles and major organs during inspiratory flow resistive loads

To determine whether diaphragmatic fatigue in the intact animal subjected to loaded breathing is associated with a decrease in diaphragmatic blood flow, seven unanesthetized sheep were subjected to severe inspiratory flow resistive (IFR) loads that led to a decrease in transdiaphragmatic pressure (Pdi) and a rise in arterial PCO2 (PaCO2). Blood flow to the diaphragm, other respiratory muscles, limb muscles, and major organs was measured using the radionuclide-labeled microsphere method. With these loads blood flow increased to the diaphragm (621 +/- 242%) and all the other inspiratory and expiratory diaphragm (621 +/- 242%) and all the other inspiratory and expiratory muscles; there was no statistically significant change in blood flow to these muscles at the time when Pdi decreased and PaCO2 rose. Blood flow also increased to the heart (103 +/- 34%), brain (212 +/- 39%), and adrenals (76 +/- 9%), whereas pancreatic flow decreased (-66 +/- 14%). Limb muscle blood flow remained unchanged. We conclude that in unanesthetized sheep subjected to IFR loads 1) we did not demonstrate a decrease in respiratory muscle blood flow associated with diaphragmatic fatigue and ventilatory failure, and 2) there is a redistribution of blood flow among major organs.

Respiratory muscle blood flow in exercising dogs after pneumonectomy

In three foxhounds after left pneumonectomy, the relationships of ventilatory work and respiratory muscle (RM) blood flow to ventilation (VE) during steady-state exercise were examined. VE was measured using a specially constructed respiratory mask and a pneumotach; work of breathing was measured by the esophageal balloon technique. Blood flow to RM was measured by the radionuclide-labeled microsphere technique. Lung compliance after pneumonectomy was 55% of that before pneumonectomy; compliance of the thorax was unchanged. O2 uptake (VO2) of RM comprised only 5% of total body VO2 at exercise. At rest, inspiratory muscles received 62% and expiratory muscles 38% of the total O2 delivered to the RM (QO2RM). During exercise, inspiratory muscles received 59% and expiratory muscles 41% of total QO2RM. Blood flow per gram of muscle to the costal diaphragm was significantly higher than that to the crural diaphragm. The diaphragm, parasternals, and posterior cricoarytenoids were the most important inspiratory muscles, and internal intercostals and external obliques were the most important expiratory muscles for exercise. Up to a VE of 120 l/min through one lung, QO2RM constituted only a small fraction of total body VO2 during exercise and maximal vasodilation in the diaphragm was never approached.

Capillary and fiber geometry in rat diaphragm perfusion fixed in situ at different sarcomere lengths

To determine the potential range of diaphragm sarcomere lengths in situ and the effect of changes in sarcomere length on capillary and fiber geometry, rat diaphragms were perfusion fixed in situ with glutaraldehyde at different airway pressures and during electrical stimulation. The lengths of thick (1.517 +/- 0.007 microns) and thin (1.194 +/- 0.048 microns) filaments were not different from those established for rat limb muscle. Morphometric techniques were used to determine fiber cross-sectional area, sarcomere length, capillary orientation, and capillary length and surface area per fiber volume. All measurements were referenced to sarcomere length, which averaged 2.88 +/- 0.08 microns at -20 to -25 cmH2O airway pressure (residual volume) and 2.32 +/- 0.05 microns at +20 to +26 cmH2O airway pressure (total lung capacity). The contribution of capillary tortuosity and branching to total capillary length was dependent on sarcomere length and varied from 5 to 22%, consistent with that shown previously for mammalian limb muscles over this range of sarcomere lengths. Capillary length per fiber volume [Jv(c,f)] was significantly greater at residual volume (3,761 +/- 193 mm-2) than at total lung capacity (3,142 +/- 118 mm-2) and correlated with sarcomere length [l; r = 0.628, Jv(c,f) = 876l + 1,156, P less than 0.01; n = 18]. We conclude that the diaphragm is unusual in that the apparent in situ minimal sarcomere length is greater than 2.0 microns.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiratory muscle perfusion in ponies during prolonged submaximal exercise in thermoneutral environment

Distribution of blood flow among various respiratory muscles was examined in 8 healthy ponies during submaximal exercise lasting 30 minutes, using radionuclide labeled 15-microns diameter microspheres injected into the left ventricle. From the resting values (40 +/- 2 beats/min; 37.3 +/- 0.2 C), heart rate and pulmonary arterial blood temperature increased significantly at 5 (152 +/- 8 beats/min; 38.6 +/- 0.2 C), 15 (169 +/- 6 beats/min; 39.8 +/- 0.2 C), and 26 (186 +/- 8 beats/min; 40.8 +/- 0.2 C) minutes of exertion, and the ponies sweated profusely. Mean aortic pressure also increased progressively as exercise duration increased. Blood flow increased significantly with exercise in all respiratory muscles. Among inspiratory muscles, perfusion was greatest in the diaphragm and ventral serratus, compared with external intercostal, dorsal serratus, and scalenus muscles. Among expiratory muscles, blood flow in the internal abdominal oblique muscle was greatest, followed by that in internal intercostal and transverse thoracic muscles, in which the flow values remained similar. The remaining 3 abdominal muscles had similar blood flow, but these values were less than that in the internal intercostal, transverse thoracic, and internal abdominal oblique muscles. Blood flow values for all inspiratory and expiratory muscles remained similar for the 5 and 15 minutes of exertion. However, at 26 minutes, blood flow had increased further in the diaphragm, external intercostal, internal intercostal, transverse thoracic, and the external abdominal oblique muscle as vascular resistance decreased. On the basis of our findings, all respiratory muscles were activated during submaximal exercise and their perfusion had marked heterogeneity.(ABSTRACT TRUNCATED AT 250 WORDS)

Nailfold capillary density as a possible indicator of pulmonary capillary loss in systemic lupus erythematosus but not in mixed connective tissue disease

Nailfold capillary density was measured in 24 patients with systemic lupus erythematosus (SLE), 14 with mixed connective tissue disease (MCTD) and 21 healthy subjects. Pulmonary function tests were performed on all subjects and needle muscle biopsies on 12 patients with SLE and 9 with MCTD. A significant correlation was documented between nailfold capillary density and pulmonary gas transfer (KCO) in patients with SLE (p less than 0.001) but not in patients with MCTD. This suggests that in SLE poor gas transfer may be dependent on alveolar capillary loss and that nailfold capillary density may be a good indicator of alveolar capillary density. There was no significant correlation between skeletal muscle fiber atrophy and nailfold capillary density in SLE or MCTD. Additional studies to optimize the nailfold capillary counting method are described.

Regional distribution of blood flow within the diaphragm

We investigated the regional distribution of blood flow (Q) within the costal and crural portions of the diaphragm in a total of eight anesthetized supine mongrel dogs. Q was measured with 15-microns microspheres, radiolabeled with three different isotopes, injected into the left ventricle during spontaneous breathing (SB), inspiratory resistive loading (IR), and mechanical ventilation after paralysis (P). At necropsy, the costal and crural portions of each hemidiaphragm were arbitrarily subdivided along a sagittal plane into five to seven and three sections, respectively. During P, there was a dorsoventral Q gradient within the costal part of the diaphragm. During SB there was a fourfold increase in the gradient of Q. Furthermore, during IR, in which mouth pressures of -16 +/- 4 cmH2O were generated, there was a further increase in the gradient of Q. During both SB and IR, Q to the most ventral portion of the costal diaphragm was 26 +/- 6% less than the peak value. In two dogs, studied prone and supine, there was no difference in the Q gradients between the two postures. Over the dorsal 80% of the costal diaphragm there was also a dorsoventral gradient of muscle thickness, such that the most dorsal part was 54 +/- 2% (n = 5) that of the ventral portion. In contrast, there was no consistent gradient of Q or muscle thickness within the crural diaphragm. Our results demonstrate a topographical gravity-independent distribution of Q in the costal, but not the crural, diaphragm.(ABSTRACT TRUNCATED AT 250 WORDS)

Diaphragmatic intramuscular pressure in relation to tension, shortening, and blood flow

We used an in situ isolated diaphragmatic preparation in anesthetized dogs to relate intramuscular pressure (IMP) to the blood flow, tension, and shortening of the diaphragm. In this preparation, the diaphragm shortens in a fashion similar to the intact diaphragm. Tension was measured by transducers attached to the left costal margin, which was detached from the rib cage and abdomen; IMP was measured by a miniature transducer placed between muscle fibers; length was measured by sonomicrometry; and diaphragmatic blood flow was monitored by measuring left phrenic arterial flow. In protocol 1, the relationships between tension, shortening, and IMP were assessed by stimulating the diaphragm for 2 s at various frequencies. Tension and shortening increased with increasing stimulation frequency up to 50 Hz with no change thereafter. Tension was linearly related to IMP. Similarly, there was a linear relationship between the degree of shortening and IMP; however, the slopes varied considerably between dogs. In protocol 2, the diaphragm was paced intermittently (12 trains/min, duty cycle of 0.5) with a gradual increase in stimulation frequency. Blood flow during contraction phase rose slightly at low tension and then declined significantly when tension exceeded 30% of maximum, whereas relaxation-phase flow increased with the increase in tension. IMP rose linearly with the increase in tension, and the IMP, at the point where contraction-phase flow became severely limited, was 50 +/- 14 mmHg (mean +/- SE). We conclude the following. 1) IMP is linearly related to tension and shortening; however, because tension and shortening changed simultaneously during contractions, the independent relationship of either tension or shortening and IMP remained untested.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood flow distribution within the rib cage muscles

We used 15-microns radiolabeled microspheres to study the regional distribution of blood flow (Q) among parasternal (PS), transversus thoracis, and external (EI) and internal intercostal muscles (II) in nine anesthetized supine mongrel dogs. We measured Q (ml.min-1.100 g-1) in each intercostal space (ICS) during spontaneous breathing, inspiratory resistive loading, and mechanical ventilation following paralysis. At necropsy the EI, II, and PS were excised and sampled separately for each ICS. During paralysis there was no consistent gradient in Q among the PS, II, and EI muscles. During spontaneous breathing, Q to PS increased linearly by 125% between the first and fourth to sixth ICS, Q to EI decreased progressively from the first/second ICS to the fifth/sixth ICS, whereas Q to the II was uniform. During inspiratory resistive loading, in which mouth pressures of -16 +/- 4 cmH2O were generated, the PS gradient was similar to that during spontaneous breathing. Also, Q to the EI increased in the cranial interspaces (P less than 0.02), whereas Q to the II of the seventh/eighth ICS was greater than that of the first/second ICS (P less than 0.001). Furthermore, with loading, ventrodorsal gradients in Q appeared within both EI and II interspaces. There was no consistent gradient in Q within the transversus thoracis muscle during any of the interventions. Our results demonstrate nonuniform Q within PS, EI, and II during both spontaneous and inspiratory resistive loaded breathing. On the assumption that changes in Q reflect changes in activation, our results suggest systematic topographical patterns of recruitment of rib cage respiratory muscles.

Human respiratory muscles: fibre morphology and capillary supply

In man the diaphragm (DIA) and abdominal muscles comprise approximately 50% slow-twitch (ST) fibres, whereas a higher proportion (60%) is found in intercostal muscles and the scalenes. All respiratory muscles show an equal distribution of fast-twitch (FTa and b) fibres with the exception of the expiratory intercostal muscles which have few FTb fibres. The inspiratory muscles have a uniformly small fibre size, in contrast to the expiratory intercostal muscle fibres which are large. The fibre size of the inspiratory muscles is maintained with ageing, whereas that of the expiratory intercostal muscles appears to be reduced after the age of 50 yrs. Capillary supply is most abundant in the expiratory muscles followed by DIA and the inspiratory intercostal muscles. In patients with chronic obstructive pulmonary disease (COPD) it is unknown whether a reduction in fibre size of the thoracic respiratory muscles is caused by extreme use due to increased ventilatory work, or by disuse due to an increased involvement of the extrathoracic respiratory muscles. Histochemical characteristics suggest that, in normal humans, the load on the inspiratory muscles is relatively small during contractions, whereas the expiratory intercostal muscles are exposed to severe continuous activity with a heavy load.

Inspiratory and expiratory muscle perfusion in maximally exercised ponies

The present study was carried out on seven healthy ponies to examine the extent of blood flow in various inspiratory and expiratory muscles at rest and during maximal exertion as well as to determine the proportion of cardiac output needed to perfuse respiratory muscles during these conditions. Tissue blood flow was studied with 15 micron-diameter radionuclide-labeled microspheres injected into the left ventricle during steady conditions. The inspiratory and expiratory muscles comprised 2.41 and 3.05% of body weight, respectively, and received 6.17 and 3.75% of the cardiac output at rest. With maximal exercise, heart rate (from 55 +/- 3 to 218 +/- 4 beats/min), mean aortic pressure (from 125 +/- 5 to 170 +/- 6 mmHg), and cardiac output (from 96 +/- 11 to 730 +/- 78 ml.min-1.kg-1) increased markedly. During exercise blood flow increased significantly in all respiratory muscles (P less than 0.0001) as vascular resistance decreased precipitously. Marked heterogeneity of perfusion existed among various inspiratory as well as expiratory muscles during exercise. Among the inspiratory muscles, the highest perfusion occurred in the diaphragm followed by serratus ventralis, and among the expiratory muscles, the highest perfusion occurred in the internal oblique abdominis and the transverse thoracis (triangularis sterni). Collectively, the inspiratory (8.44%) and expiratory (6.35%) muscle blood flow comprised 14.8 +/- 1.2% of the cardiac output during maximal exercise, a significant increase above resting value, whereas renal fraction of cardiac output decreased from 21% (at rest) to 0.72%.

Oxidative capacity and capillary density of diaphragm motor units

Motor units in the cat diaphragm (DIA) were isolated in situ by microdissection and stimulation of C5 ventral root filaments. Motor units were classified based on their isometric contractile force responses and fatigue indexes (FI). The muscle fibers belonging to individual units (i.e., the muscle unit) were identified using the glycogen-depletion method. Fibers were classified as type I or II based on histochemical staining for myofibrillar adenosine triphosphatase (ATPase) after alkaline preincubation. The rate of succinate dehydrogenase (SDH) activity of each fiber was determined using a microphotometric procedure. The location of capillaries was determined from muscle cross sections stained for ATPase after acid (pH = 4.2) preincubation. The capillarity of muscle unit fibers was determined by counting the number of capillaries surrounding fibers and by calculating the number of capillaries per fiber area. A significant correlation was found between the fatigue resistance of DIA units and the mean SDH activity of muscle unit fibers. A significant correlation was also observed between DIA unit fatigue resistance and both indexes of muscle unit fiber capillarity. The mean SDH activity and mean capillary density of muscle unit fibers were also correlated. We conclude that DIA motor unit fatigue resistance depends, at least in part, on the oxidative capacity and capillary density of muscle unit fibers.

Alterations in respiratory muscle activation in the ischemic fatigued canine diaphragm

The purpose of the present study was to examine the respiratory motor response to diaphragm fatigue. Studies were performed using in situ diaphragm muscle strips dissected from the left costal diaphragm in anesthetized dogs. The left inferior phrenic artery was isolated, and diaphragmatic strip fatigue was elicited by occluding this vessel. Strip tension, strip electromyographic activity, parasternal electromyographic activity, and the electromyogram of the right hemidiaphragm were recorded during spontaneous breathing efforts before, during, and after periods of phrenic arterial occlusion. In separate trials, we examined the neuromuscular responses to phrenic arterial occlusion at arterial PCO2 (PaCO2) of 40, 55, and 75 Torr. No fatigue and no alteration in electromyographic activities were observed in trials at PaCO2 of 40 Torr. During trials at PaCO2 of 55 and 75 Torr, however, diaphragm tension fell, the peak height of the diaphragm strip electromyogram decreased, and the peak heights of the parasternal and right hemidiaphragm electromyograms increased. Relief of phrenic arterial occlusion resulted in a return of strip tension and all electromyograms toward base-line values. In additional experiments, the left phrenic nerve was sectioned in the chest after producing fatigue. Phrenic section was followed by an increase in the peak height of the left phrenic neurogram (recorded above the site of section). This latter finding suggests that diaphragm strip motor drive may be reflexly inhibited during the development of fatigue by neural traffic carried along phrenic afferents.