Histochemical characteristics of human expiratory and inspiratory intercostal muscles

The relative occurrence of slow-twitch (ST) and fast-twitch (FTa and FTb) fibers, fiber size, and capillary supply in internal (INT) and external intercostal muscles (EXT), the costal diaphragm (DIA), and vastus lateralis muscle (VAS) was examined post-mortem in eight healthy males. The relative occurrence of ST fibers in INT [64 +/- 3% (SE)] and EXT (62 +/- 3%) was similar but higher than in DIA (49 +/- 3%) and VAS (40 +/- 6%; P less than 0.05). The occurrence of FTa fibers in expiratory INT (35 +/- 3%) was higher than in inspiratory INT and EXT (17 +/- 1%; P less than 0.05) but similar to DIA (28 +/- 6%) and VAS (32 +/- 2%). Accordingly, expiratory INT had fewer FTb fibers (1 +/- 1%) than the others (P less than 0.05). Expiratory INT had a 60% larger fiber area than inspiratory INT and EXT and DIA (P less than 0.05), but the area was similar to that of VAS. The number of capillaries per fiber was higher in expiratory INT (2.3 +/- 0.1) than in inspiratory INT and EXT (1.6 +/- 0.1), DIA (1.9 +/- 0.1), and VAS (1.8 +/- 0.2; P less than 0.05). The results suggest that the occurrence of many large capillary-rich FTa fibers in expiratory INT is bound to function (expiratory vs. inspiratory) rather than to anatomy (INT vs. EXT).

Blood flow to the respiratory muscles during hypercapnic hyperpnoea in the newborn lamb

The blood flow to the diaphragm, external and internal intercostal muscles, abdominal oblique muscles, and other rib-cage and abdominal muscles was measured, using radio-labelled microspheres, in 6 newborn lambs quietly breathing in air and during 3 different levels of CO2 induced hypercapnic hyperpnoea (inspired gas containing 4%, 5.5%, or 7% CO2). We also calculated the oxygen uptake of the diaphragm (VO2di). While the lambs were breathing air diaphragmatic blood flow (Qdi, 38.2 +/- 4.0 SEM ml.min-1.100 g-1) was similar to external intercostal muscle blood flow (Qei, 37.1 +/- 8.1 ml.min-1.100 g-1), and both were greater than internal intercostal muscle blood flow (Qii, 24.8 +/- 6.1 ml.min-1.100 g-1; P less than 0.05). During hyperpnoea Qdi, Qei, and Qii were augmented with Qdi equal to 200.1 +/- 12.2 ml.min-1.100 g-1 in 7% CO2 and Qei equal to 88.4 +/- 14.1 ml.min-1.100 g-1 in 7% CO2 (Qdi was greater than Qei, P less than 0.01). Qii was 40.7 +/- 5.6 ml.min-1.100 g-1 in 7% CO2 being less than Qdi (P less than 0.01) and Qei (P less than 0.05). The abdominal oblique muscles also had augmented flow in response to hyperpnoea. The level of hypercapnia that resulted in an augmentation of Qdi (5.5% inspired CO2) was lower than that which augmented Qei and Qii (7% inspired CO2). VO2di was linearly related to Qdi (r = 0.98). Our results suggest that in the newborn lamb the diaphragm is the dominant respiratory muscle in response to hypercapnia.

Respiratory muscle blood flow

Respiratory muscle fatigue appears to be the cause of hypercapnic respiratory failure in many patients with lung disease. Recent studies have suggested that the rate of development of respiratory muscle fatigue largely depends on the balance between the level of respiratory muscle blood flow and the metabolic demands of these muscles. Physiological factors that alter muscle blood flow (for example, cardiogenic or septic shock, alterations in muscle length) or respiratory muscle metabolic demands (for example, increases in the work of breathing) may influence this balance, affecting the rate of development of respiratory muscle fatigue in these patients. Therapeutic measures that augment respiratory muscle blood flow (restoration of normal arterial pressure in patients in shock) or reduce the work of breathing (for example, mechanical ventilation) may prevent or reverse respiratory muscle fatigue.

Origins and regional distribution of blood flow to the respiratory muscles in conscious sheep

The blood flow to the portions of the diaphragm (sternal, costal, and crural) and to the external and internal intercostal muscles was measured, using radio-labelled microspheres, in 12 young sheep breathing quietly. We calculated total blood flow and oxygen uptake of the diaphragm (VO2di). We also determined the proportion of diaphragmatic and intercostal muscle blood flow that arose from the descending aorta. Diaphragmatic blood flow (Qdi) was 0.62 +/- 0.04% of cardiac output (mean +/- SEM). VO2di was 1.53 +/- 0.15 ml X min-1 X 100 g-1, and was linearly related to Qdi. When standardized for weight Qdi was 28.3 +/- 1.5 ml X min-1 X 100 g-1, significantly greater than both external intercostal muscle blood flow (18.9 +/- 2.6 ml X min-1 X 100 g-1; P less than 0.05), and internal intercostal muscle blood flow (11.4 +/- 1.4 ml X min-1 X 100 g-1; P less than 0.01). Blood flow to the costal portion of the diaphragm was significantly less than blood flow to the crural portions. There was no difference between flow to these portions and to the sternal portion of the diaphragm. The blood supplying the dorsal-most regions of the diaphragm arose predominantly from the descending aorta. The results suggest that the crural portions work at a higher rate than the costal portions under resting conditions.

Respiratory muscle function

The functional anatomy of the respiratory muscles has been reviewed. The diaphragm has been emphasized, since this is the most important inspiratory muscle, but the view has been presented that the intercostal, scaleni, and other accessory inspiratory muscles become increasingly important as airflow obstruction leads to hyperinflation. As work increases, the demand for energy and hence blood flow to those muscles has to increase. In spite of a large reserve there are situations in which demands may outstrip supply. This leads to local metabolic changes that result in muscle fatigue. We are now capable of detecting this change as alterations in EMG or in the ability to generate pressures. The latter leads to a decrease in the capacity of the respiratory pump to exchange gas, ultimately resulting in hypercapnia and hypoxemia. The true importance of respiratory muscle fatigue and its differentiation from weakness in patients with severe CAO requires more analysis. It is intuitively appropriate to address the overall decrement in the ability to maintain adequate ventilatory work at low energy cost with the different therapeutic modalities thought to be beneficial. A combination of a decrease in the load imposed on the respiratory muscles, an improvement in the contractility of those muscles, and, when there is absolute need, the resting of the fatigued muscles should result in a better chance to lead a meaningful life and perhaps to improve survival in these patients.

Blood flow to the respiratory and limb muscles and to abdominal organs during maximal exertion in ponies

Using radionuclide-labelled microspheres, 15 micron in diameter, we studied blood flow in the respiratory muscles (diaphragm and intercostal muscles), abdominal organs (adrenal glands, kidneys, pancreas, spleen and the small and large intestines), muscles of propulsion (gluteus medius and biceps femoris), and other working (triceps brachii and longissimus dorsi lumborum) and non-working (temporal and masseter) muscles of ponies at rest and during maximal exercise performed on a treadmill. During maximal exercise heart rate, whole body O2 consumption, cardiac output and mean aortic pressure increased 4.4-fold, 38-fold, 8-fold and 1.5-fold of their resting values, respectively. During maximal exertion arterial CO2 tension and arterial pH decreased while arterial O2 content increased by 58% due to a 59.6% rise in haemoglobin concentration. Arterial O2 tension decreased somewhat and the calculated alveolar to arterial O2 tension gradient widened during exertion. During maximal exertion blood flow in the adrenal glands increased while that in the kidneys, spleen, pancreas, small intestine and colon decreased precipitously. Thus ponies exhibited intense vasoconstriction in the renal and splanchnic vascular beds, similar to that reported in man but not in exercising dogs. During maximal exertion stride (and hence respiratory) frequency of galloping ponies was 138 +/- 3 min-1, and the blood flow and O2 delivery in the diaphragm were not different from those in other strenuously working muscles, namely gluteus medius, biceps femoris (muscles of propulsion) and triceps brachii. Blood flow in the intercostal muscles was only 54% of that in the diaphragm at rest, but with maximal exercise it registered a marked increment and the perfusion became similar to that in the longissimus dorsi lumborum, a powerful extensor of the back and loins.