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

Quantifying Diaphragm Blood Flow With Contrast-Enhanced Ultrasound in Humans

BACKGROUND

Despite the known interplay between blood flow and function, to our knowledge, there is currently no minimally invasive method to monitor diaphragm hemodynamics. We used contrast-enhanced ultrasound to quantify relative diaphragm blood flow (Q˙DIA) in humans and assessed the technique's efficacy and reliability during graded inspiratory pressure threshold loading. We hypothesized that: (1) Q˙DIA would linearly increase with pressure generation, and (2) that there would be good test-retest reliability and interanalyzer reproducibility.

RESEARCH QUESTION

Can we validate what is, to our knowledge, the first minimally invasive method to measure relative diaphragm blood flow in humans?

STUDY DESIGN AND METHODS

Quantitative contrast-enhanced ultrasound of the costal diaphragm was performed in healthy participants (10 male participants, 6 female participants; mean age 28 ± 5 years; BMI 22.8 ± 2.0 kg/m) during unloaded breathing and three stages of loaded breathing on two separate days. Gastric and esophageal balloon catheters measured transdiaphragmatic pressure. Ultrasonography was performed during a constant-rate IV infusion of lipid-stabilized microbubbles following each stage. Ultrasound images were acquired after a destruction-replenishment sequence and diaphragm specific time-intensity data were used to determine Q˙DIA by two individuals.

RESULTS

Transdiaphragmatic pressure for unloaded and each loading stage were 15.2 ± 0.8, 26.1 ± 0.8, 34.6 ± 0.8, and 40.0 ± 0.8 percentage of the maximum, respectively. Q˙DIA increased with each stage of loading (3.1 ± 3.1, 6.9 ± 3.6, 11.0 ± 4.9, and 13.5 ± 5.4 acoustic units/s; P < .0001). The linear relationship between diaphragmatic flow and pressure was reproducible from day to day. Q˙DIA had good to excellent test-retest reliability (0.86 [0.77, 0.92]; P < .0001) and excellent interanalyzer reproducibility (0.93 [0.90, 0.95]; P < .0001) with minimal bias.

INTERPRETATION

Relative Q˙DIA measurements had valid physiological underpinnings, were reliable day-to-day, and were reproducible analyzer-to-analyzer. This study indicated that contrast-enhanced ultrasound is a viable, minimally invasive method for assessing costal Q˙DIA in humans and may provide a tool to monitor diaphragm hemodynamics in clinical settings.

Formulation and exploration of novel, intramuscular pressure based, muscle activation strategies in a spine model

Optimization models are often devised to assess spinal stability via estimating individual muscle forces. However, neglecting muscles' fluidic behavior remains an approximation due to the role of muscle pressure in force transmission. The purpose of this study was to leverage a validated Finite Element (FE) model of the spine, inclusive of Intra-Muscular Pressure (IMP), to explore muscle activation strategies towards maintaining equilibrium spinal stability. Three conventional strategies governing minimizing muscle effort, minimizing IVD compressive forces, and maintaining stability at all costs were first investigated to explore model's validity. Thereafter, two novel IMP-based strategies were devised and explored, specifically minimizing and maximizing IMP. The model was previously shown valid in light of in vivo and in silico observations with an average discrepancy of 6%. This being the case, the conventional strategies dictated efficacy in muscular activations whilst maintaining an equilibrium stable position, as quantified in the present paper, with a difference of 9.8% from documented data. In addition, the explored novel IMP-based strategies suggested the presence of a threshold individual muscles IMP, approximately 272 mmHg for the longissimus muscle for example, beyond which muscles potentially start to share radial loads with surrounding tissues, whilst limiting the contraction of the underlying muscles. In conclusion, this study theoretically supports the possibility of activation strategies based on muscular pressure, which the developed, verified, and validated FE spine model was leveraged to investigate. The explored novel IMP-based strategies may have significance in informing clinical applications such as motion analysis and functional electrical stimulation of muscles.

Effects of Microcurrent on Oxygen Saturation by Controlling Rectus Abdominis Activity in Preterm Infant With Desaturation During Feeding: A Pilot Study

Objective: To determine whether a portable microcurrent therapy device (PMTD) of the rectus abdominis muscles is effective for treating desaturation during feeding in preterm infants and to evaluate the association between initial electrical activity of respiratory muscle and long-term development delay. Methods: Twenty preterm infants with desaturation during feeding were recruited. Respiratory muscle activity was quantified by calculating the root mean square (RMS) of the electromyography. All preterm infants received a 30 min PMTD application to the rectus abdominis and diaphragm daily for 2 weeks. RMS of diaphragm and rectus abdominis, feeding volume, frequency of desaturation during feeding at baseline (pre-PMTD) and 1, 2 week post-PMTD were measured. The number of days it took to treat desaturation after PMTD was measured. A Denver developmental screening test was performed and infants were divided into 3 groups: (1) normal; (2) caution; and (3) delayed at 3months after PMTD. Results: The desaturation during feeding of all the preterm infants subsided after PMTD and the mean days took to treat desaturation was 25.4 ± 14.2 days. The RMS of diaphragm, rectus abdominis, and frequency of desaturation during feeding were significantly decreased and the feeding volume was significantly increased after PMTD (p < 0.01). The mean treatment duration for desaturation was negatively correlated with RMS of rectus abdominis at baseline and 1 week post-PMTD, respectively (Pearson's correlation coefficient = -0.461,-0.514, p-value = 0.047, 0.029). RMS of rectus abdominis of Group 3 is lower than that of group 1 and 2 (p < 0.01). Conclusions: This pilot study showed that the microcurrent therapy of rectus abdominis is an efficient therapy for the treatment of preterm infants with desaturation during feeding, especially preterm infants with higher activity of the rectus abdominis. In preterm infants with lower rectus abdominis activity, longer time is required to treat desaturation by microcurrent therapy and developmental delay is observed at months post-treatment.

Challenging the Cinderella Hypothesis: A New Model for the Role of the Motor Unit Recruitment Pattern in the Pathogenesis of Myofascial Pain Syndrome in Postural Muscles

BACKGROUND

The energy crisis hypothesis, which is a widely accepted model for the pathogenesis of myofascial pain, has been corroborated by experimental observations. However, the nature of the insult leading to the energy crisis remains elusive. A commonly cited model for this insult is the Cinderella hypothesis, suggesting that hierarchical recruitment of motor units leads to a disproportional load on small units, thus driving them towards an energy crisis. New findings cast doubt on this model, showing that in postural muscles motor units are recruited in rotation, rather than in a hierarchical order, precluding the formation of the so-called Cinderella units.

OBJECTIVE

To explore the influence of common myofascial predisposing factors such as muscle load and muscle strength on the relaxation time of postural muscle motor units, assuming they are recruited in rotation.

METHODS

A stochastic model of a postural skeletal muscle was developed which integrates the energy crisis model and motor unit rotation patterns observed in postural muscles. Postulating that adequate relaxation time is essential for the energetic replenishment of motor units, we explored the influence of different parameters on the relaxation time of individual motor units under varying conditions of muscle loads and muscle strengths.

RESULTS

The motor unit relaxation/contraction time ratio decreases with elevated muscle loads and with decreased total muscle strength.

CONCLUSIONS

In a model of a postural muscle, in which motor units are recruited in rotation, common predisposing factors of myofascial pain, such as increased muscle load and decreased muscle force, lead to shortened motor unit relaxation periods.

Use of a Poroelastic Model to Predict Intramuscular Pressure

Measurement of individual muscle tension in a clinical setting has yet to be achieved. Previous investigators have suggested that the tension in skeletal muscle, comprised of approximately 70% fluid, could be determined using interstitial muscle fluid pressure (IMP). A computational model is needed to aid in understanding IMP distribution in muscles of varying geometry and contractile states without exhaustive testing. The first aim of this study was to determine a set of transversely isotropic material properties (i.e., permeability, relaxed modulus, and drained Poisson's ratio) for excised skeletal muscle using inverse finite element analysis with a poroelastic constitutive formulation on tension data from either longitudinal or transverse uniaxial load-relaxation tests of skeletal muscle tissue. The second aim was to compare pore pressure estimated from a model to experimental pressure measurements to assess its ability to accurately predict IMP. Results of this study indicated that skeletal muscle was transversely isotropic under load-relaxation as demonstrated by significant differences in the drained Poisson's ratio. It was also noted that the drained Poisson's ratios under both longitudinal and transverse loading were negative in these tests of excised muscle tissue. Pore pressure calculated with this model provided a good prediction of the development of IMP. These results point to the benefit of using a poroelastic model of skeletal muscle to predict IMP.

Neurovascular proximity in the diaphragm muscle of adult mice

OBJECTIVE

Regional blood flow to the diaphragm muscle varies with the workload of inspiration. To provide anatomical insight into coupling between muscle fiber recruitment and oxygen supply, we tested whether arterioles are physically associated with motor nerve branches of the diaphragm.

METHODS

Following vascular casting, intact diaphragm muscles of C57BL/6 and CD-1 mice were stained for motor innervation. Arteriolar networks and nerve networks were mapped (~2 μm resolution) to evaluate their physical proximity.

RESULTS

Neurovascular proximity was similar between muscle regions and mouse strains. Of total mapped nerve lengths (C57BL/6, 70 ± 15 mm; CD-1, 87 ± 13 mm), 80 ± 14% and 67 ± 10% were ≤250 μm from the nearest arteriole and associated predominantly with arterioles ≤45 μm in diameter. Distances to the nearest arteriole encompassing 50% of total nerve length (D(50)) were consistently within 200 μm. With nerve networks repositioned randomly within muscle borders, D(50) values nearly doubled (p < 0.05). Reference lines within anatomical boundaries reduced proximity to arterioles (p < 0.05) as they deviated from the original location of motor nerves.

CONCLUSION

Across two strains of mice, motor nerves and arterioles of the diaphragm muscle are more closely associated than can be explained by chance. We hypothesize that neurovascular proximity facilitates local perfusion upon muscle fiber recruitment.

Human muscle length-dependent changes in blood flow

Although a multitude of factors that influence skeletal muscle blood flow have been extensively investigated, the influence of muscle length on limb blood flow has received little attention. Thus the purpose of this investigation was to determine if cyclic changes in muscle length influence resting blood flow. Nine healthy men (28 ± 4 yr of age) underwent a passive knee extension protocol during which the subjects' knee joint was passively extended and flexed through 100-180° knee joint angle at a rate of 1 cycle per 30 s. Femoral blood flow, cardiac output (CO), heart rate (HR), stroke volume (SV), and mean arterial pressure (MAP) were continuously recorded during the entire protocol. These measurements revealed that slow passive changes in knee joint angle did not have a significant influence on HR, SV, MAP, or CO; however, net femoral blood flow demonstrated a curvilinear increase with knee joint angle (r(2) = 0.98) such that blood flow increased by ∼90% (125 ml/min) across the 80° range of motion. This net change in blood flow was due to a constant antegrade blood flow across knee joint angle and negative relationship between retrograde blood flow and knee joint angle (r(2) = 0.98). Thus, despite the absence of central hemodynamic changes and local metabolic factors, blood flow to the leg was altered by changes in muscle length. Therefore, when designing research protocols, researchers need to be cognizant of the fact that joint angle, and ultimately muscle length, influence limb blood flow.

Bench-to-bedside review: ventilatory abnormalities in sepsis

In septic patients increased central drive and increased metabolic demands combine to increase energy demands on the ventilatory muscles. This occurs at a time when energy supplies are limited and energy production hindered, and it leads to an energy supply-demand imbalance and often ventilatory failure. Problems related to contractile function of the ventilatory muscles also contribute, especially when the clinical course is prolonged. The increased ventilatory activity increases interactions between the ventilatory and cardiovascular systems, and when ventilatory muscles fail and mechanical ventilatory support is required a new set of problems emerges. In this review I discuss factors related to ventilatory muscle failure, giving emphasis to mechanical and supply demand aspects. I also review the implications of changes in ventilatory patterns for heart-lung interactions.

The effect of muscle contraction velocity on cardiorespiratory responses to repetitive isokinetic exercise in humans

We investigated the effect of muscle contraction velocity on cardiorespiratory responses during exercise. Eight males (23 +/- 2 years, 175 +/- 5 cm, 64 +/- 6 kg, mean +/- SD) performed 3-min repetitive one-leg extension exercises at various angular velocities (30, 60, 120, and 240 deg/s) with a controlled relaxation interval, relatively constant (duty cycle = 1:1, A trial) and absolutely constant (relaxation time = 0.75 s, B trial) at a total work of 2,100-2,400 J in an isokinetic mode, using a Cybex II dynamometer. We measured heart rate (HR), mean blood pressure (MAP), minute ventilation (Vdot;E), and oxygen uptake (Vdot;O(2)) during the exercise. The angular velocity significantly affected the increase in HR, MAP, Vdot;E, and Vdot;O(2) at the end of exercise from resting in both A and B trials (e.g., MAP: 12 +/- 2, 10 +/- 2, 11 +/- 2, and 18 +/- 2 mmHg in the A trial). The result suggests that muscle contraction velocity affects cardiorespiratory responses during repetitive isokinetic exercise.

Effect of contraction frequency on the contractile and noncontractile phases of muscle venous blood flow

The purpose of this study was to test the hypothesis that increasing muscle contraction frequency, which alters the duty cycle and metabolic rate, would increase the contribution of the contractile phase to mean venous blood flow in isolated skeletal muscle during rhythmic contractions. Canine gastrocnemius muscle (n = 5) was isolated, and 3-min stimulation periods of isometric, tetanic contractions were elicited sequentially at rates of 0.25, 0.33, and 0.5 contractions/s. The O2 uptake, tension-time integral, and mean venous blood flow increased significantly (P < 0.05) with each contraction frequency. Venous blood flow during both the contractile (106 +/- 6, 139 +/- 8, and 145 +/- 8 ml x 100 g-1 x min-1) and noncontractile phases (64 +/- 3, 78 +/- 4, and 91 +/- 5 ml x 100 g-1 x min-1) increased with contraction frequency. Although developed force and duration of the contractile phase were never significantly different for a single contraction during the three contraction frequencies, the amount of blood expelled from the muscle during an individual contraction increased significantly with contraction frequency (0.24 +/- 0.03, 0.32 +/- 0.02, and 0.36 +/- 0.03 ml x N-1 x min-1, respectively). This increased blood expulsion per contraction, coupled with the decreased time in the noncontractile phase as contraction frequency increased, resulted in the contractile phase contribution to mean venous blood flow becoming significantly greater (21 +/- 4, 30 +/- 4, and 38 +/- 6%) as contraction frequency increased. These results demonstrate that the percent contribution of the muscle contractile phase to mean venous blood flow becomes significantly greater as contraction frequency (and thereby duty cycle and metabolic rate) increases and that this is in part due to increased blood expulsion per contraction.

Sarcomere length-induced alterations of capillary hemodynamics in rat spinotrapezius muscle: vasoactive vs passive control

Skeletal muscle blood flow is reduced as fibers are stretched longitudinally. Neither the underlying cause(s) of this decrement in blood flow nor the consequences in terms of capillary red blood cell (rbc) hemodynamics has been established clearly within the physiological range of muscle sarcomere length. Using intravital microscopy, this investigation determined arteriolar diameter and capillary rbc velocity (Vrbc), flux (Frbc), and hematocrit (Hct(t)) in the rat spinotrapezius muscle at shortened/resting (2.6 microm) and physiological extended (3.2 microm) sarcomere lengths under control (c) and local maximally vasodilated (v, phentolamine, 1 micromol/L; prazosin, 0.1 micromol/L; nitroprusside, 10 micromol/L) conditions. The hypothesis tested was that muscle stretch would reduce Vrbc and Frbc proportionally such that Hct(t) would remain unchanged and that these reductions in Vrbc and Frbc would be attenuated following maximal vasodilation. Vrbc and Frbc were increased significantly following maximal vasodilation at 2.6-microm (59 and 84%) and 3.2-microm (64 and 104%) sarcomere lengths, respectively. Irrespective of sarcomere length, Hct(t) was elevated significantly following vasodilation (c, 0.20 +/- 0.01; v, 0.27 +/- 0.01). At 3.2 microm compared with the 2.6-microm sarcomere length, Vrbc and Frbc were both reduced significantly under control and vasodilated conditions as expected. However, the percent reduction in either Vrbc (c, 27%, and v, 29%) or Frbc (c, 26%, and v, 33%) was not significantly different between the 2.6- and 3.2-microm sarcomere lengths. In addition, arteriolar diameter was not altered discernably as sarcomere length was increased from 2.7 microm (c, 29.0 +/- 4.5; v, 37.9 +/- 6.7 microm) to 3.2 microm (c, 29.4 +/- 4.5; v, 37.3 +/- 6.2 microm). These data suggest that increasing sarcomere length from resting to the upper extreme of the physiological range in the rat spinotrapezius muscle reduces Vrbc and Frbc (at constant hematocrit) by a mechanism that is independent of stretch-activated arteriolar vasoconstriction.

Oxygenation and perfusion of rabbit tibialis anterior muscle subjected to different patterns of electrical stimulation

Dual amperometric microelectrodes were used to measure local pO2 and perfusion at multiple sites in the fast-twitch tibialis anterior muscles of anaesthetized rabbits. Six muscles were stimulated continuously at 10, 5, or 2.5 Hz. For all three frequencies, perfusion declined to about 50% of resting levels and recovered after stimulation. These changes corresponded to a rise followed by a fall in extracellular pO2. The highest levels of pO2 were reached during stimulation at 10 Hz. Eight muscles were stimulated tetanically at 100 Hz for 200 ms with duty cycles that were varied between 1.3 and 20.0%. Perfusion rose to 8.7 +/- 2.0 ml s(-1) 100 g(-1) at a duty cycle of 5% and declined with further increases in duty cycle. pO2 was depressed for duty cycles less than 10% but rose above resting levels at higher duty cycles. It is suggested that the paradoxical combination of elevated pO2 and depressed perfusion is attributable to stimulation conditions that exceed the oxygen transport capacity of a fast muscle.

Relaxation of diaphragm muscle

Relaxation is the process by which, after contraction, the muscle actively returns to its initial conditions of length and load. In rhythmically active muscles such as diaphragm, relaxation is of physiological importance because diaphragm must return to a relatively constant resting position at the end of each contraction-relaxation cycle. Rapid and complete relaxation of the diaphragm is likely to play an important role in adaptation to changes in respiratory load and breathing frequency. Regulation of diaphragm relaxation at the molecular and cellular levels involves Ca(2+) removal from the myofilaments, active Ca(2+) pumping by the sarcoplasmic reticulum (SR), and decrease in the number of working cross bridges. The relative contribution of these mechanisms mainly depends on sarcomere length, muscle tension, and the intrinsic contractile function. Increased capacity of SR to take up Ca(2+) can arise from increased density of active SR pumping sites or in slow-twitch fibers from phosphorylation of phospholamban, whereas impaired coupling between ATP hydrolysis and Ca(2+) transport into the SR or intracellular acidosis reduces SR Ca(2+) pump activity. In experimental conditions of decreased contractile performance, slowed, enhanced, or unchanged relaxation rates have been reported in vitro. In vivo, a slowing in the rate of decline of the respiratory pressure is generally considered an early reliable index of respiratory muscle fatigue. Impaired relaxation rate may, in turn, favor mismatch between blood flow and metabolic demand, especially at high breathing frequencies.

Regional intramuscular pressure development and fatigue in the canine gastrocnemius muscle in situ

Intramuscular pressure (PIM) was measured simultaneously in zones of the medial head of the gastrocnemius-plantaris muscle group (zone I, popliteal origin; zone II, central; zone III, near calcaneus tendon) to determine regional muscle mechanics during isometric tetanic contractions. Peak PIM averages were 586, 1,676, and 993 mmHg deep in zones I, II, and III and 170, 371, and 351 mmHg superficially in zones I, II, and III, respectively. During fatigue, loss of PIM across zones was greatest in zone III (-81%) and least in zone I (-60%) when whole muscle tension loss was -49%. Recovery of PIM was greatest in zone III and least in zone II, achieving 86% and 67% of initial PIM, respectively, when tension recovered to 89%. These data demonstrate that 1) regional mechanical performance can be measured as PIM within a whole muscle, 2) PIM is nonuniform within the canine gastrocnemius-plantaris muscle, being greatest in the deep central zone, and 3) fatigue and recovery of PIM are dissimilar across regions. These differences suggest distinct local effects that integrate to determine whole muscle mechanical capacity during and after intense exercise.

Regulation of ventilatory muscle blood flow

The ventilatory muscles perform various functions such as ventilation of the lungs, postural stabilization, and expulsive maneuvers (e.g., coughing). They are classified in functional terms as inspiratory muscles, which include the diaphragm, parasternal intercostal, external intercostal, scalene, and sternocleidomastoid muscles; and expiratory muscles, which include the abdominal muscles, internal intercostal, and triangularis sterni. The ventilatory muscles require high-energy phosphate compounds such as ATP to fuel the biochemical and physical processes of contraction and relaxation. Maintaining adequate intracellular concentrations of these compounds depends on adequate intracellular substrate levels and delivery of these substrates by arterial blood flow. In addition to the delivery of substrates, blood flow influences muscle function through the removal of metabolic by-products, which, if accumulated, could exert negative effects on several excitatory and contractile processes. Skeletal muscle substrate utilization is also dependent on the ability to extract substrates from arterial blood, which, in turn, is accomplished by increasing the total number of perfused capillaries. It follows that matching perfusion to metabolic demands is critical for the maintenance of normal muscle contractile function. In this article, I review the factors that influence ventilatory muscle blood flow. Major emphasis is placed on the diaphragm because a large number of published reports deal with diaphragmatic blood flow. The second reason for focusing on the diaphragm is because it is the largest and most important inspiratory muscle.

Muscle length directs sympathetic nerve activity and vasomotor tone in resistance vessels of hamster retractor

Increased resistance to blood flow with muscle extension has been explained by the deformation of vessels within the muscle. In the present study, we developed a novel preparation of the hamster retractor muscle to investigate whether passive changes in skeletal muscle length elicit active vasomotor responses through a range of motion (85% to 130% of in vivo length; sarcomere length, 2.69 +/- 0.02 to 4.05 +/- 0.01 microns) encompassing the classic length-tension relationship. Arterioles (diameter, 32 +/- 3 microns) and feed arteries (diameter, 75 +/- 4 microns) were observed to progressively constrict (by 8 +/- 1 and 17 +/- 2 microns, respectively) with muscle lengthening, reducing blood flow by > 50%; reciprocal changes occurred with passive shortening. Sodium nitroprusside (10 mumol/L) dilated vessels (to 47 +/- 2 and 98 +/- 4 microns, respectively) and abolished vasomotor responses to changing muscle length. The coordination of vasomotor responses between arterioles and feed arteries maintained wall shear rate (control, 1764 +/- 200 s-1) and perfusion pressure (60 +/- 5 mm Hg) into the arteriolar network. Tetrodotoxin (TTX, 1 mumol/L), phentolamine (1 mumol/L), prazosin (0.1 mumol/L), or 6-hydroxydopamine (1 mmol/L) inhibited vasoconstrictor responses, indicating that action potentials initiated by muscle lengthening give rise to norepinephrine release from sympathetic nerves. As shown with glyoxylic acid staining, sympathetic nerves formed a plexus encompassing arterioles and feed arteries. To test for a reflexive response initiated by intramuscular mechanoreceptors, TTX was applied with micropipettes to proximal segments of feed arteries, thereby neurally "isolating" the muscle from the hamster. Whereas lengthening-induced vasoconstriction persisted in arterioles and in feed artery segments distal to TTX, there was no vasomotor response central to the block. We conclude that passive lengthening stimulates the activity of periarteriolar sympathetic nerves; this activity propagates antidromically along nerve fibers into the feed arteries. These findings identify a mechanotransduction sequence by which the length of skeletal muscle actively governs vasomotor tone and the supply of oxygen to muscle fibers.

The mechanical effects of contractions on blood flow to the muscle

To determine whether muscle contractions can increase muscle blood flow independently from metabolic factors, we isolated the vasculature of the left diaphragm or gastrocnemius muscle of anesthetized and mechanically ventilated dogs. Arterial blood flow was controlled with a constant pressure source and the arterial pressure (Pa) was decreased in steps to obtain pressure-flow relationships (P-Q). The local vasculatures were maximally dilated with nitroprusside [mean (SD) 114.0 (32.0) micrograms.min-1], adenosine [1.43 (0.41) mmol.l-1.min-1], and acetylcholine [1.43 (0.41) mmol.l-1.min-1] and the P-Q with and without spontaneous contractions (n = 6), stimulated twitches (n = 12, 2-4 Hz), or tetanic trains (n = 7, 25 Hz) in the diaphragm and stimulated twitches (n = 6, 2-4 Hz), or tetanic contractions (n = 6, 12-16 trains) in the gastrocnemius were compared. The pressure axis intercept decreased (P < 0.5) with spontaneous contractions in the diaphragm and the slope did not change. At Pa of 13.3 kPa, flow increased from 36.2 (34.9) to 43.9 (38.2) ml.min-1.100 g-1 (P < 0.05). During twitch contractions, the slope and intercept of the P-Q were not significantly different from vasodilatation alone, but the flow at a pressure of 13.3 kPa increased slightly. In the gastrocnemius (n = 6), continuous and intermittent tetanic contractions did not affect P-Q or flow at Pa of 100 mmHg (n = 6). Furthermore, increasing venous pressure to 6.7 kPa did not affect flow in this muscle. We conclude that the muscle pump has only a small direct effect on muscle blood flow and its main effect is to reduce venous pressures.

Effect of inhibition of nitric oxide release on the diaphragmatic oxygen delivery-consumption relationship

PURPOSE

In the vascularly isolated resting and contracting (3 Hz) canine hemidiaphragm, the hypothesis that nitric oxide (NO) is an important regulator of diaphragmatic O2 extraction was tested.

METHODS

The effect of an intra-arterial infusion of an NO-synthase inhibitor NG-nitro-L-arginine (L-NA) on the critical O2 delivery (QO2c), below which O2 consumption becomes dependent on O2 supply, was assessed in two groups of animals in which either saline or L-NA (6 x 10(-4) mol/L) was infused into the phrenic artery over 20 minutes. The diaphragm was then perfused either by left femoral arterial blood (autoperfusion) or by pump perfusion with blood from the femoral artery. QO2 was reduced by stepwise hemorrhage in the autoperfusion groups and by reducing the pump rate in the pump perfusion groups.

RESULTS

During autoperfusion, QO2c in the saline- and L-NA-treated groups was not different (0.88 +/- 0.15 and 0.98 +/- 0.12 mL/min/100 g, respectively) for the resting diaphragm. Critical O2 extraction ratios were not different (64.5% +/- 9.9% and 67.8% +/- 6.4%, respectively). In the saline group, QO2c during 3-Hz stimulation was 5.03 +/- 0.9 mL/min/100 g. In the L-NA group, diaphragm flow was lower than the saline group, and no QO2c was found. In the pump-perfused contracting diaphragm, QO2c in both groups did not differ (3.1 +/- 0.5 and 4.05 +/- 0.65 mL/min/100 g, respectively). O2 extraction ratios at these O2 deliveries were different (63.3% +/- 5.2% and 77.4% +/- 4.3%, respectively). However, NO-synthase inhibiton had no effect on maximum diaphragmatic O2 extraction ratio.

CONCLUSIONS

These results indicate that NO release is an important modulator of the tone of diaphragmatic resistance vessels, but it does not appear to regulate the processes by which O2 extraction is enhanced to compensate for decreased O2 delivery.