Respiratory responses to sustained isometric muscle contractions in man: the effect of muscle mass

Vasopressin and Breathing: Review of Evidence for Respiratory Effects of the Antidiuretic Hormone

Vasopressin (AVP) is a key neurohormone involved in the regulation of body functions. Due to its urine-concentrating effect in the kidneys, it is often referred to as antidiuretic hormone. Besides its antidiuretic renal effects, AVP is a potent neurohormone involved in the regulation of arterial blood pressure, sympathetic activity, baroreflex sensitivity, glucose homeostasis, release of glucocorticoids and catecholamines, stress response, anxiety, memory, and behavior. Vasopressin is synthesized in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus and released into the circulation from the posterior lobe of the pituitary gland together with a C-terminal fragment of pro-vasopressin, known as copeptin. Additionally, vasopressinergic neurons project from the hypothalamus to the brainstem nuclei. Increased release of AVP into the circulation and elevated levels of its surrogate marker copeptin are found in pulmonary diseases, arterial hypertension, heart failure, obstructive sleep apnoea, severe infections, COVID-19 due to SARS-CoV-2 infection, and brain injuries. All these conditions are usually accompanied by respiratory disturbances. The main stimuli that trigger AVP release include hyperosmolality, hypovolemia, hypotension, hypoxia, hypoglycemia, strenuous exercise, and angiotensin II (Ang II) and the same stimuli are known to affect pulmonary ventilation. In this light, we hypothesize that increased AVP release and changes in ventilation are not coincidental, but that the neurohormone contributes to the regulation of the respiratory system by fine-tuning of breathing in order to restore homeostasis. We discuss evidence in support of this presumption. Specifically, vasopressinergic neurons innervate the brainstem nuclei involved in the control of respiration. Moreover, vasopressin V1a receptors (V1aRs) are expressed on neurons in the respiratory centers of the brainstem, in the circumventricular organs (CVOs) that lack a blood-brain barrier, and on the chemosensitive type I cells in the carotid bodies. Finally, peripheral and central administrations of AVP or antagonists of V1aRs increase/decrease phrenic nerve activity and pulmonary ventilation in a site-specific manner. Altogether, the findings discussed in this review strongly argue for the hypothesis that vasopressin affects ventilation both as a blood-borne neurohormone and as a neurotransmitter within the central nervous system.

Feedforward consequences of isometric contractions: effort and ventilation

The onset of voluntary muscle contractions causes rapid increases in ventilation and is accompanied by a sensation of effort. Both the ventilatory response and perception of effort are proportional to contraction intensity, but these behaviors have been generalized from contractions of a single muscle group. Our aim was to determine how these relationships are affected by simultaneous contractions of multiple muscle groups. We examined the ventilatory response and perceived effort of contraction during separate and simultaneous isometric contractions of the contralateral elbow flexors and of an ipsilateral elbow flexor and knee extensor. Subjects made 10-sec contractions at 25, 50, and 100% of maximum during normocapnia and hypercapnia. For simultaneous contractions, both muscle groups were activated at the same intensities. Ventilation was measured continuously and subjects rated the effort required to produce each contraction. As expected, ventilation and perceived effort increased proportionally with contraction intensity during individual contractions. However, during simultaneous contractions, neither ventilation nor effort reflected the combined muscle output. Rather, the ventilatory response was similar to when contractions were performed separately, and effort ratings showed a small but significant increase for simultaneous contractions. Hypercapnia at rest doubled baseline ventilation, but did not affect the difference in perceived effort between separate and simultaneous contractions. The ventilatory response and the sense of effort at the onset of muscle activity are not related to the total output of the motor pathways, or the working muscles, but arise from cortical regions upstream from the motor cortex.

Selective α1-adrenergic blockade disturbs the regional distribution of cerebral blood flow during static handgrip exercise

Handgrip-induced increases in blood flow through the contralateral artery that supplies the cortical representation of the arm have been hypothesized as a consequence of neurovascular coupling and a resultant metabolic attenuation of sympathetic cerebral vasoconstriction. In contrast, sympathetic restraint, in theory, inhibits changes in perfusion of the cerebral ipsilateral blood vessels. To confirm whether sympathetic nerve activity modulates cerebral blood flow distribution during static handgrip (SHG) exercise, beat-to-beat contra- and ipsilateral internal carotid artery blood flow (ICA; Doppler) and mean arterial pressure (MAP; Finometer) were simultaneously assessed in nine healthy men (27 ± 5 yr), both at rest and during a 2-min SHG bout (30% maximal voluntary contraction), under two experimental conditions: 1) control and 2) α1-adrenergic receptor blockade. End-tidal carbon dioxide (rebreathing system) was clamped throughout the study. SHG induced increases in MAP (+31.4 ± 10.7 mmHg, P < 0.05) and contralateral ICA blood flow (+80.9 ± 62.5 ml/min, P < 0.05), while no changes were observed in the ipsilateral vessel (−9.8 ± 39.3 ml/min, P > 0.05). The reduction in ipsilateral ICA vascular conductance (VC) was greater compared with contralateral ICA (contralateral: −0.8 ± 0.8 vs. ipsilateral: −2.6 ± 1.3 ml·min−1·mmHg−1, P < 0.05). Prazosin was effective to induce α1-blockade since phenylephrine-induced increases in MAP were greatly reduced ( P < 0.05). Under α1-adrenergic receptor blockade, SHG evoked smaller MAP responses (+19.4 ± 9.2, P < 0.05) but similar increases in ICAs blood flow (contralateral: +58.4 ± 21.5 vs. ipsilateral: +54.3 ± 46.2 ml/min, P > 0.05) and decreases in VC (contralateral: −0.4 ± 0.7 vs. ipsilateral: −0.4 ± 1.0 ml·min−1·mmHg−1, P > 0.05). These findings indicate a role of sympathetic nerve activity in the regulation of cerebral blood flow distribution during SHG.

Influence of muscle metaboreceptor stimulation on middle cerebral artery blood velocity in humans

Regional anaesthesia to attenuate skeletal muscle afferent feedback abolishes the exercise-induced increase in middle cerebral artery mean blood velocity (MCA Vmean). However, such exercise-related increases in cerebral perfusion are not preserved during post exercise muscle ischaemia (PEMI) where the activation of metabolically sensitive muscle afferents is isolated. We tested the hypothesis that a hyperventilation-mediated decrease in the arterial partial pressure of CO2, hence cerebral vasoconstriction, masks the influence of muscle metaboreceptor stimulation on MCA Vmean during PEMI. Ten healthy men (20 ± 1 years old) performed two trials of fatiguing isometric hand-grip exercise followed by PEMI, in control conditions and with end-tidal CO2 (P ET ,CO2) clamped at ∼1 mmHg above the resting partial pressure. In the control trial, P ET ,CO2 decreased from rest during hand-grip exercise and PEMI, while MCA Vmean was unchanged from rest. By design, P ET ,CO2 remained unchanged from rest throughout the clamp trial, while MCA Vmean increased during hand-grip (+10.6 ±1.8 cm s(-1)) and PEMI (+9.2 ± 1.6 cm s(-1); P < 0.05 versus rest and control trial). Increases in minute ventilation and mean arterial pressure during hand-grip and PEMI were not different in the control and P ET ,CO2 clamp trials (P > 0.05). These findings indicate that metabolically sensitive skeletal muscle afferents play an important role in the regional increase in cerebral perfusion observed in exercise, but that influence can be masked by a decrease in P ET ,CO2 when they are activated in isolation during PEMI.

Tracking pulmonary gas exchange by breathing control during exercise: role of muscle blood flow

Populations of group III and IV muscle afferent fibres located in the adventitia of the small vessels appear to respond to the level of venular distension and to recruitment of the vascular bed within the skeletal muscles. The CNS could thus be informed on the level of muscle hyperaemia when the metabolic rate varies. As a result, the magnitude and kinetics of the change in peripheral gas exchange – which translates into pulmonary gas exchange – can be sensed. We present the view that the respiratory control system uses these sources of information of vascular origin, among the numerous inputs produced by exercise, as a marker of the metabolic strain imposed on the circulatory and the ventilatory systems, resulting in an apparent matching between pulmonary gas exchange and alveolar ventilation.

Hypocapnia and its relation to fear of falling

OBJECTIVE

To determine if hypocapnia occurs in patients with fear of falling and to explore potential causes of hypocapnia.

DESIGN

Observational study in patients who fall with and without fear of falling.

SETTING

Rehabilitation wards of an elderly care unit.

PATIENTS

Consecutive fallers with (n = 20) and without (n = 10) fear of falling.

MAIN OUTCOME MEASURES

End-tidal CO2 (PETCO2) and respiratory rate (RR) responses were measured during sustained isometric muscle contraction (SIMC) (40% of maximum voluntary contraction of quadriceps for 2 min) and during a 5-meter walk. Falls efficacy scale (FES) and Hospital anxiety and depression scale (HAD).

RESULTS

Patients with fear of falling had significantly higher FES and HAD scores (p < .01). During SIMC, baseline and nadir PETCO2 levels were significantly lower in patients with a fear of falling (p < .01). During the 5-meter walk, PETCO2 was lower at baseline, at nadir, and at the end of the walk in the fear of falling group than in controls (p < .01). RR was higher at nadir and end of the walk in the fear of falling group than in controls (p < .02).

CONCLUSIONS

Hypocapnia may occur in patients with a fear of falling during SIMC and walking. Anxiety seems to be the main cause, but muscle weakness may contribute. Breathing or relaxation techniques and reconditioning may have a role in treating fear of falling in the rehabilitation setting.

Role of muscular factors in cardiorespiratory responses to static exercise: contribution of reflex mechanisms

We investigated the effects of muscle mass and contraction intensity on the cardiorespiratory responses to static exercise and on the contribution afforded by muscle metaboreflex and arterial baroreflex mechanisms. Ten subjects performed static handgrip at 30% maximal voluntary contraction (MVC) (SHG-30) and one-leg extension at 15% (SLE-15) and 30% (SLE-30) MVC, followed by postexercise circulatory occlusion (PECO). Mean arterial pressure (MAP) and heart rate (HR) responses were greater during SLE-30 than during SHG-30. The difference in MAP was maintained by PECO, and the part of the pressor response maintained by PECO was greater after SLE-30 than after SHG-30 (88.3 +/- 10.6 and 67.8 +/- 12.7%, respectively, P = 0. 02). There were no differences in MAP and HR responses between SHG-30 and SLE-15 trials. Baroreflex sensitivity was maintained during SHG-30 and SLE-15, whereas it was significantly reduced during SLE-30 and recovered back to the resting level during PECO. Minute ventilation and oxygen uptake increased more during SLE-30 than during both SHG-30 and SLE-15 trials. Minute ventilation remained significantly elevated above rest only during PECO following SLE-30. These data suggest that during static exercise the muscle mass and contraction intensity affect 1) the magnitude of the cardiorespiratory responses, 2) the contribution of muscle metaboreflex to the cardiorespiratory responses, and 3) the arterial baroreflex contribution to HR control.

Breathing when chest wall muscle are tonically contracted for isometric, non-respiratory tasks

We have previously shown that regional chest wall impedance increases when the chest wall muscles are tonically contracted to perform isometric, non-respiratory tasks. To test how this affects breathing, we measured respiratory frequency, tidal volume, end-tidal PCO2, electromyographic activity (EMG) at four points on the chest wall surface, and regional displacements across six planes of the chest wall during maintenance of three different postures that necessitated strong tonic respiratory muscle contraction. These postures included a static push-up, a bilateral leg-lift and a partial sit-up. The subjects (n = 8) were able to maintain the postures for 1.5-2.5 min, and strong tonic EMG activity was observed in each posture at all points measured. The rate and depth of breathing and pattern of regional chest wall displacements were variable within the group of subjects and among the three postures. However, minute ventilation increased and end-tidal PCO2 decreased in each subject during each posture (P < 0.05). In six of the eight subjects, transdiaphragmatic pressure (Pdi) was measured during 1 min of the same exercises. The ratio of the breathing fluctuation in Pdi to tidal volume was at least twice as high compared with rest, except for two subjects during the leg-lifts. We conclude that strong tonic contraction of the chest wall muscles impedes, but does not limit, breathing, and that there is no single breathing strategy used during such conditions.

Effect of muscle mass on the pressor response in man during isometric contractions

1. Changes in blood pressure and heart rate were measured in six healthy male subjects during voluntary isometric contractions of the forearm and quadriceps muscles. Arterial pressure was measured directly via a catheter inserted into the radial artery of the non-contracting arm. Each subject exerted two types of contractions: (a) sustained contractions at 70% of the maximum voluntary contraction (MVC) until fatigue occurred and (b) sustained contractions starting at maximum tension (100% MVC), held for a total duration of 1 min. 2. During fatiguing contractions at 70% MVC, there was a progressive increase in blood pressure, reaching a peak level at fatigue. The same level of mean arterial pressure was achieved during contractions of the same relative tension, regardless of the muscle mass. The same trend was observed for the changes in heart rate. 3. During contractions which started with the maximum tension, where tension fell continuously during the 60 s of maximal effort, mean arterial pressure rapidly increased to high levels within a few seconds, and then increased further by 20-30 mmHg during the sustained maximal effort. There was no difference in the initial rapid increases in mean arterial pressure, nor in the final mean arterial pressures reached between contractions of the forearm or quadriceps muscles. There were no differences in the heart rates achieved during these contractions either. 4. There was no significant difference between the mean arterial pressures observed at fatigue of a 70% MVC contraction or at the end of the 60 s maximum effort during handgrip contractions, or during contractions with the quadriceps muscles. 5. These results support the view that muscle mass is not a determinant of the magnitude of the cardiovascular reflexes during fatiguing isometric contractions in man.