Acute unloading of the work of breathing extends exercise duration in patients with heart failure

Acute respiratory muscle unloading improves time-to-exhaustion during moderate- and heavy-intensity cycling in obese adolescent males

Obesity significantly impairs breathing during exercise. The aim was to determine, in male obese adolescents (OB), the effects of acute respiratory muscle unloading, obtained by switching the inspired gas from ambient air (AIR) to a normoxic helium + oxygen gas mixture (HeO₂) (AIR → HeO₂) during moderate [below gas exchange threshold (GET)] and heavy [above GET] constant work rate cycling. Ten OB [age 16.0 ± 2.0 years (mean ± SD); body mass index (BMI) 38.9 ± 6.1 kg/m²] and ten normal-weight age-matched controls (CTRL) inspired AIR for the entire exercise task, or underwent AIR → HeO₂ when they were approaching volitional exhaustion. In OB time to exhaustion (TTE) significantly increased in AIR → HeO₂ vs. AIR during moderate [1524 ± 480 s vs. 1308 ± 408 (P = 0.024)] and during heavy [570 ± 306 s vs. 408 ± 150 (P = 0.0154)] exercise. During moderate exercise all CTRL completed the 40-min task. During heavy exercise no significant differences were observed in CTRL for TTE (582 ± 348 s [AIR → HeO₂] vs. 588 ± 252 [AIR]). In OB, but not in CTRL, acute unloading of respiratory muscles increased TTE during both moderate- and heavy-exercise. In OB, but not in CTRL, respiratory factors limit exercise tolerance during both moderate and heavy exercise.

The interactions between respiratory and cardiovascular systems in systolic heart failure

Heart failure (HF) is a complex and multifaceted disease. The disease affects multiple organ systems, including the respiratory system. This review provides three unique examples illustrating how the cardiovascular and respiratory systems interrelate because of the pathology of HF. Specifically, these examples outline the impact of HF pathophysiology on 1) respiratory mechanics and the mechanical "cost" of breathing; 2) mechanical interactions of the heart and lungs; and on 3) abnormalities of pulmonary gas exchange during exercise, and how this may be applied to treatment. The goal of this review is to, therefore, raise the awareness that HF, though primarily a disease of the heart, is accompanied by marked pathology of the respiratory system.

Work of breathing influences muscle sympathetic nerve activity during semi-recumbent cycle exercise

Reducing the work of breathing during exercise improves locomotor muscle blood flow and reduces diaphragm and locomotor muscle fatigue and is thought to be the result of a sympathetically mediated reflex.

AIM

The aim of this study was to assess muscle sympathetic nerve activity (MSNA) when the work of breathing is experimentally lowered during dynamic exercise.

METHODS

Healthy subjects (n = 12; age = 29 ± 9 years) performed semi-recumbent cycling trials at 40%, 60%, and 80% of peak workload. Exercise trials consisted of spontaneous breathing, reduced work of breathing (proportional assist ventilator), followed by further spontaneous breathing (post-ventilator). MSNA was recorded from the median nerve.

RESULTS

There was no difference in work of breathing between PAV and post-PAV at 40% peak work. At 60% peak work, the ventilator significantly (P < 0.05) reduced work of breathing (103 ± 39 vs 144 ± 47 J min^-1 ), sympathetic nerve activity (35 ± 5 vs 42 ± 8 burst min^-1 ), and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> (2.4 ± 0.5 vs 2.6 ± 0.5 L min^-1 ) without influencing ventilation (86 ± 9 vs 82 ± 10 L min^-1 ; P > 0.05), for PAV and post-PAV respectively. During 80% peak work (n = 8), the ventilator significantly (P < 0.05) reduced work of breathing (235 ± 110 vs. 361 ± 150 J min^-1 ), MSNA (48 ± 7 vs 54 ± 11 burst min^-1 ), and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> (2.9 ± 0.6 vs 3.2 ± 0.7 L min^-1 ) but not ventilation (121 ± 20 vs 123 ± 20 L min^-1 ; P > 0.05), for PAV and post-PAV respectively. There was a significant relationship between MSNA and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> (P < 0.0001) with a significant interaction due to the ventilator (P < 0.05).

CONCLUSION

Lowering the normally occurring work of breathing during exercise results in commensurate reductions in MSNA. Our findings provide evidence of a sympathetically mediated vasoconstrictor effect emanating from respiratory muscles during exercise.

Sex differences in large conducting airway anatomy

Airway luminal area is the major determinant of resistance to airflow in the tracheobronchial tree. Women may have smaller central conducting airways than men; however, previous evidence is confounded by an indirect assessment of airway geometry and by subjects with prior smoking history. The purpose of this study was to examine the effect of sex on airway size in healthy nonsmokers. Using low-dose high-resolution computed tomography, we retrospectively assessed airway luminal area in healthy men ( n = 51) and women ( n = 73) of varying ages (19-86 yr). Subjects with a positive smoking history, cardiopulmonary disease, or a body mass index > 40 kg/m² were excluded. Luminal areas of the trachea, right and left main bronchus, bronchus intermediate, left and right upper lobes, and the left lower lobe were analyzed at three discrete points. The luminal areas of the conducting airways were ~26%-35% smaller in women. The trachea had the largest differences in luminal area between men and women (298 ± 47 vs. 195 ± 28 mm² or 35% smaller for men and women, respectively), whereas the left lower lobe had the smallest differences (57 ± 15 vs. 42 ± 9 mm² or 26% smaller for men and women, respectively). When a subset of subjects was matched for height, the sex differences in airway luminal area persisted, with women being ~20%-30% smaller. With all subjects, there were modest relationships between height and airway luminal area ( r = 0.73-0.53, P < 0.05). Although there was considerable overlap between sexes, the luminal areas of the large conducting airways were smaller in healthy women than in men. NEW & NOTEWORTHY Previous evidence for sex differences in airway size has been confounded by indirect measures and/or cohorts with significant smoking histories or pathologies. We found that central airways in healthy women were significantly smaller (~26%-35%) than men. The significant sex-difference in airway size was attenuated (20%-30% smaller) but preserved in a subset of subjects matched for height. Over a range of ages, healthy women have smaller central airways than men.

Diaphragm abnormalities in heart failure and aging: mechanisms and integration of cardiovascular and respiratory pathophysiology

Inspiratory function is essential for alveolar ventilation and expulsive behaviors that promote airway clearance (e.g., coughing and sneezing). Current evidence demonstrates that inspiratory dysfunction occurs during healthy aging and is accentuated by chronic heart failure (CHF). This inspiratory dysfunction contributes to key aspects of CHF and aging cardiovascular and pulmonary pathophysiology including: (1) impaired airway clearance and predisposition to pneumonia; (2) inability to sustain ventilation during physical activity; (3) shallow breathing pattern that limits alveolar ventilation and gas exchange; and (4) sympathetic activation that causes cardiac arrhythmias and tissue vasoconstriction. The diaphragm is the primary inspiratory muscle; hence, its neuromuscular integrity is a main determinant of the adequacy of inspiratory function. Mechanistic work within animal and cellular models has revealed specific factors that may be responsible for diaphragm neuromuscular abnormalities in CHF and aging. These include phrenic nerve and neuromuscular junction alterations as well as intrinsic myocyte abnormalities, such as changes in the quantity and quality of contractile proteins, accelerated fiber atrophy, and shifts in fiber type distribution. CHF, aging, or CHF in the presence of aging disturbs the dynamics of circulating factors (e.g., cytokines and angiotensin II) and cell signaling involving sphingolipids, reactive oxygen species, and proteolytic pathways, thus leading to the previously listed abnormalities. Exercise-based rehabilitation combined with pharmacological therapies targeting the pathways reviewed herein hold promise to treat diaphragm abnormalities and inspiratory muscle dysfunction in CHF and aging.

Dietary nitrate supplementation opposes the elevated diaphragm blood flow in chronic heart failure during submaximal exercise

Chronic heart failure (CHF) results in a greater cost of breathing and necessitates an elevated diaphragm blood flow (BF). Dietary nitrate (NO₃‾) supplementation lowers the cost of exercise. We hypothesized that dietary NO₃‾ supplementation would attenuate the CHF-induced greater cost of breathing and thus the heightened diaphragm BF during exercise. CHF rats received either 5days of NO₃‾-rich beetroot (BR) juice (CHF+BR, n=10) or a placebo (CHF, n=10). Respiratory muscle BFs (radiolabeled microspheres) were measured at rest and during submaximal exercise (20m/min, 5% grade). Infarcted left ventricular area and normalized lung weight were not significantly different between groups. During submaximal exercise, diaphragm BF was markedly lower for CHF+BR than CHF (CHF+BR: 195±28; CHF: 309±71mL/min/100g, p=0.04). The change in diaphragm BF from rest to exercise was less (p=0.047) for CHF+BR than CHF. These findings demonstrate that dietary NO₃‾ supplementation reduces the elevated diaphragm BF during exercise in CHF rats thus providing additional support for this therapeutic intervention in CHF.

Effect of chronic heart failure in older rats on respiratory muscle and hindlimb blood flow during submaximal exercise

Submaximal exercise diaphragm blood flow (BF) is elevated in young chronic heart failure (CHF) rats, while it is unknown if this occurs in older animals. Respiratory and hindlimb muscle BFs (radiolabeled microspheres) were measured at rest and during submaximal exercise (20m/min, 5% grade) in older healthy (n=7) and CHF (n=6) Fischer 344X Brown Norway rats (27-29 mo old). Older CHF, compared to healthy, rats had greater (p<0.01) left ventricular end-diastolic pressure and right ventricle and lung weight (normalized to body weight). During submaximal exercise, respiratory and hindlimb muscle BFs increased (p<0.02) in both groups, while diaphragm BF was higher (CHF: 257±32; healthy: 121±9mL/min/100g, p<0.01) and hindlimb BF lower (CHF: 111±10; healthy: 133±12mL/min/100g, p=0.04) in older CHF compared to healthy rats. Submaximal exercise hindlimb BF was negatively related (r=-0.93; p=0.03) to diaphragm BF in older CHF rats. During submaximal exercise, diaphragm BF is elevated in older CHF compared to healthy rats in proportion to the compromised hindlimb BF.

Effect of exercise training on respiratory muscle oxygenation in children with congenital heart disease

BACKGROUND

Diminished aerobic capacity and weakness of both respiratory and peripheral muscles have been observed in cardiac patients and may contribute to exercise limitation. The aim of this study was to evaluate the effects of a home-based training programme on aerobic fitness and oxygenation of the respiratory muscles in children with congenital heart disease (CHD).

METHODS AND RESULTS

Eighteen patients with CHD aged 12-15 years participated in this study. Ten patients (training group, TG) underwent a training programme for 12 weeks and eight patients served as a non-training control group (CG). All subjects performed a cardiopulmonary exercise test before and after the study period. Oxygenation of the respiratory muscles was assessed using near-infrared spectroscopy. No significant differences were observed, at baseline and after the completion of the study, between the CG and TG in peak exercise workload, oxygen uptake (VO2), carbon dioxide output (VCO2), pulmonary ventilation (VE), and heart rate (HR). However, a significant improvement in exercise performance was found in the TG versus the CG when results were compared at the ventilatory threshold (Vth): workload (45.2+/-8.0 versus 58.5+/-7.4%; P<0.05), VO2 (62.3+/-7.5 versus 69.8+/-5.1%; P<0.05), VCO2 (49.8+/-5.7 versus 60.0+/-5.8%; P<0.05), VE (42.8+/-9.9 versus 50.1+/-9.5%; P<0.05), and HR (69.5+/-6.1 versus 76.0+/-3.5%; P<0.05). After training, an improvement in oxygenation of the respiratory muscles was found in the TG from 60% of VO2max until the end of exercise. At the Vth, the TG showed greater oxygenation after training (55.1+/-6.6 versus 43.0+/-6.9%, P<0.01, respectively). Furthermore, we showed a significant correlation of the change in respiratory muscle oxygenation and VO2 in the TG (r=0.90, P<0.01).

CONCLUSION

It is concluded that general physical training at submaximal intensity induces better aerobic fitness and improves respiratory muscle oxygenation in children with CHD.

Impaired skeletal muscle vasodilation during exercise in heart failure with preserved ejection fraction

BACKGROUND

Exercise intolerance is a hallmark symptom of heart failure patients with preserved ejection fraction (HFpEF), which may be related to an impaired ability to appropriately increase blood flow to the exercising muscle.

METHODS

We evaluated leg blood flow (LBF, ultrasound Doppler), heart rate (HR), stroke volume (SV), cardiac output (CO), and mean arterial blood pressure (MAP, photoplethysmography) during dynamic, single leg knee-extensor (KE) exercise in HFpEF patients (n=21; 68 ± 2 yrs) and healthy controls (n=20; 71 ± 2 yrs).

RESULTS

HFpEF patients exhibited a marked attrition during KE exercise, with only 60% able to complete the exercise protocol. In participants who completed all exercise intensities (0-5-10-15 W; HFpEF, n=13; Controls, n=16), LBF was not different at 0 W and 5 W, but was 15-25% lower in HFpEF compared to controls at 10 W and 15 W (P<0.001). Likewise, leg vascular conductance (LVC), an index of vasodilation, was not different at 0 W and 5 W, but was 15-20% lower in HFpEF compared to controls at 10 W and 15 W (P<0.05). In contrast to these peripheral deficits, exercise-induced changes in central variables (HR, SV, CO), as well as MAP, were similar between groups.

CONCLUSIONS

These data reveal a marked reduction in LBF and LVC in HFpEF patients during exercise that cannot be attributed to a disease-related alteration in central hemodynamics, suggesting that impaired vasodilation in the exercising skeletal muscle vasculature may play a key role in the exercise intolerance associated with this patient population.

Exercise training in chronic heart failure: improving skeletal muscle O2 transport and utilization

Chronic heart failure (CHF) impairs critical structural and functional components of the O2 transport pathway resulting in exercise intolerance and, consequently, reduced quality of life. In contrast, exercise training is capable of combating many of the CHF-induced impairments and enhancing the matching between skeletal muscle O2 delivery and utilization (Q̇mO2 and V̇mO2 , respectively). The Q̇mO2 /V̇mO2 ratio determines the microvascular O2 partial pressure (PmvO2 ), which represents the ultimate force driving blood-myocyte O2 flux (see Fig. 1). Improvements in perfusive and diffusive O2 conductances are essential to support faster rates of oxidative phosphorylation (reflected as faster V̇mO2 kinetics during transitions in metabolic demand) and reduce the reliance on anaerobic glycolysis and utilization of finite energy sources (thus lowering the magnitude of the O2 deficit) in trained CHF muscle. These adaptations contribute to attenuated muscle metabolic perturbations (e.g., changes in [PCr], [Cr], [ADP], and pH) and improved physical capacity (i.e., elevated critical power and maximal V̇mO2 ). Preservation of such plasticity in response to exercise training is crucial considering the dominant role of skeletal muscle dysfunction in the pathophysiology and increased morbidity/mortality of the CHF patient. This brief review focuses on the mechanistic bases for improved Q̇mO2 /V̇mO2 matching (and enhanced PmvO2 ) with exercise training in CHF with both preserved and reduced ejection fraction (HFpEF and HFrEF, respectively). Specifically, O2 convection within the skeletal muscle microcirculation, O2 diffusion from the red blood cell to the mitochondria, and muscle metabolic control are particularly susceptive to exercise training adaptations in CHF. Alternatives to traditional whole body endurance exercise training programs such as small muscle mass and inspiratory muscle training, pharmacological treatment (e.g., sildenafil and pentoxifylline), and dietary nitrate supplementation are also presented in light of their therapeutic potential. Adaptations within the skeletal muscle O2 transport and utilization system underlie improvements in physical capacity and quality of life in CHF and thus take center stage in the therapeutic management of these patients.

Effect of Expiratory Muscle Fatigue on the Respiratory Response during Exercise

[Purpose] The aim of this study was to reveal the effect of expiratory muscle fatigue (EMF) on respiratory response under two different exercise conditions: exercise (EX) with EMF (EMF-EX) and control EX without EMF (CON-EX). [Methods] Nine healthy adult men performed cycle exercise with a ramp load, and a spirometer was used to measure forced vital capacity (FVC), forced expiratory volume in one second, percent of forced expiratory volume, maximal expiratory mouth pressure, and maximal inspiratory mouth pressure (PI_max) to evaluate respiratory functions immediately and at 15 and 30 min after exercise. To assess the respiratory response during exercise, an exhaled gas analyzer was used to measure minute ventilation (V_E), respiratory frequency (f), tidal volume (V_T), oxygen uptake, and carbon dioxide output. In addition, the Borg Scale was used to evaluate dyspnea, while electrocardiography was used to measure heart rate. [Results] The results showed that compared with the CON-EX condition, no change in V_E, an increase in f, or a decrease in V_T was observed under the medium-intensity EMF-EX condition, while high-intensity exercise reduced V_E and f without changing V_T. [Conclusion] These results suggest that during medium-intensity exercise, EMF modulates the respiratory response by inducing shallow and fast breathing to increase ventilation volume.

Dynamic pulmonary hyperinflation occurs without expiratory flow limitation in chronic heart failure during exercise

To assess the occurrence of tidal expiratory flow limitation (EFL) and/or dynamic pulmonary hyperinflation (DH) in chronic heart failure (CHF) during exercise 15 patients with stable systolic CHF, aged 69 ± 6yr, underwent pulmonary function testing and incremental cardio-pulmonary exercise testing. They subsequently performed constant load exercise testing at 30, 60 and 90% of respective maximum workload. At each step the presence of EFL, by negative expiratory pressure technique, and changes in inspiratory capacity (IC) were assessed. Ejection fraction amounted to 36 ± 6% and VO₂, peak (77 ± 19% pred.) was reduced. EFL was absent at any step during constant load exercise. In 6 patients IC decreased more than 10% pred. at highest step. Only in these patients TLC, FRC, RV FEF(25-75%) and DL(CO) were decreased at rest. VO₂, peak correlated with DL(CO), TLC and IC at rest and with IC (r(2)=0.59; p<0.001) and decrease in IC (r(2)=0.44; p<0.001) at 90% of maximum workload. During exercise CHF patients do not exhibit EFL, but some of them develop DH that is associated with lower VO₂, peak.

Determination of blood volume by pulse CO-oximetry

The objective of this study was to determine whether changes in carboxyhaemoglobin (COHb) saturation following carbon monoxide (CO) rebreathing can be accurately detected by pulse CO-oximetry in order to determine blood volume. Noninvasive measurements of carboxyhaemoglobin saturation (SpCO) were continuously monitored by pulse CO-oximetry before, during and following 2 min of CO rebreathing. Reproducibility and accuracy of noninvasive blood volume measurements were determined in 16 healthy non-smoking individuals (15 males, age: 28 ± 2 years, body mass index: 25.4 ± 0.6 kg m(-2)) through comparison with blood volume measurements calculated from invasive measurements of COHb saturation. The coefficient of variation for noninvasive blood volume measurements performed on separate days was 15.1% which decreases to 9.1% when measurements were performed on the same day. Changes in COHb saturation and SpCO following CO rebreathing were strongly correlated (r = 0.90, p < 0.01), resulting in a significant correlation between invasive and noninvasive blood volume measurements (r = 0.83, p = 0.02). Changes in SpCO following CO rebreathing can be accurately detected by pulse CO-oximetry, which could potentially provide a simplified, convenient and reproducible method to rapidly determine blood volume in healthy individuals.

The resistive and elastic work of breathing during exercise in patients with chronic heart failure

Patients with heart failure (HF) display numerous derangements in ventilatory function, which together serve to increase the work of breathing (W(b)) during exercise. However, the extent to which the resistive and elastic properties of the respiratory system contribute to the higher W(b) in these patients is unknown. We quantified the resistive and elastic W(b) in patients with stable HF (n = 9; New York Heart Association functional class I-II) and healthy control subjects (n = 9) at standardised levels of minute ventilation (V'(E)) during graded exercise. Dynamic lung compliance was systematically lower for a given level of V'(E) in HF patients than controls (p<0.05). HF patients displayed slightly higher levels of inspiratory elastic W(b) with greater amounts of ventilatory constraint and resistive W(b) than control subjects during exercise (p<0.05). Our data indicates that the higher W(b) in HF patients is primarily due to a greater resistive, rather than elastic, load to breathing. The greater resistive W(b) in these patients probably reflects an increased hysteresivity of the airways and lung tissues. The marginally higher inspiratory elastic W(b) observed in HF patients appears related to a combined decrease in the compliances of the lungs and chest wall. The clinical and physiological implications of our findings are discussed.

Effects of respiratory muscle work on blood flow distribution during exercise in heart failure

Heart failure (HF) patients have a reduced cardiac reserve and increased work of breathing. Increased locomotor muscle blood flow demand may result in competition between respiratory and locomotor vascular beds. We hypothesized that HF patients would demonstrate improved locomotor blood flow with respiratory muscle unloading during activity. Ten patients (ejection fraction = 31 +/- 3%) and 10 controls (CTL) underwent two cycling sessions (60% peak work). Session 1 (S1): 5 min of normal breathing (NB), 5 min respiratory muscle unloading with a ventilator, and 5 min of NB. Session 2 (S2): 5 min NB, 5 min of respiratory muscle loading with inspiratory resistance, and 5 min of NB. Measurements included: leg blood flow (LBF, thermodilution), cardiac output (Q), and oesophageal pressure (P(pl), index of pleural pressure). S1: P(pl) was reduced in both groups (HF: 73 +/- 8%; CTL: 60 +/- 13%, P < 0.01). HF: Q increased (9.6 +/- 0.4 vs. 11.3 +/- 0.8 l min(-1), P < 0.05) and LBF increased (4.8 +/- 0.8 vs. 7.3 +/- 1.1 l min(-1), P < 0.01); CTL: no changes in Q (14.7 +/- 1.0 vs. 14.8 +/- 1.6 l min(-1)) or LBF (10.9 +/- 1.8 vs. 10.3 +/- 1.7 l min(-1)). S2: P(pl) increased in both groups (HF: 172 +/- 16%, CTL: 220 +/- 40%, P < 0.01). HF: no change was observed in Q(10.0 +/- 0.4 vs. 10.3 +/- 0.8 l min(-1)) or LBF (5.0 +/- 0.6 vs. 4.7 +/- 0.5 l min(-1)); CTL: increased (15.4 +/- 1.4 vs. 16.9 +/- 1.5 l min(-1), P < 0.01) and LBF remained unchanged (10.7 +/- 1.5 vs. 10.3 +/- 1.8 l min(-1)). These data suggest HF patients preferentially steal blood flow from locomotor muscles to accommodate the work of breathing during activity. Further, HF patients are unable to vasoconstrict locomotor vascular beds beyond NB when presented with a respiratory load.

Effects of respiratory muscle unloading on leg muscle oxygenation and blood volume during high-intensity exercise in chronic heart failure

Blood flow requirements of the respiratory muscles (RM) increase markedly during exercise in chronic heart failure (CHF). We reasoned that if the RM could subtract a fraction of the limited cardiac output (QT) from the peripheral muscles, RM unloading would improve locomotor muscle perfusion. Nine patients with CHF (left ventricle ejection fraction = 26 ± 7%) undertook constant-work rate tests (70-80% peak) receiving proportional assisted ventilation (PAV) or sham ventilation. Relative changes (Δ%) in deoxy-hemoglobyn, oxi-Hb ([O2Hb]), tissue oxygenation index, and total Hb ([HbTOT], an index of local blood volume) in the vastus lateralis were measured by near infrared spectroscopy. In addition, QT was monitored by impedance cardiography and arterial O2 saturation by pulse oximetry (SpO2). There were significant improvements in exercise tolerance (Tlim) with PAV. Blood lactate, leg effort/Tlim and dyspnea/Tlim were lower with PAV compared with sham ventilation ( P < 0.05). There were no significant effects of RM unloading on systemic O2 delivery as QT and SpO2 at submaximal exercise and at Tlim did not differ between PAV and sham ventilation ( P > 0.05). Unloaded breathing, however, was related to enhanced leg muscle oxygenation and local blood volume compared with sham, i.e., higher Δ[O2Hb]% and Δ[HbTOT]%, respectively ( P < 0.05). We conclude that RM unloading had beneficial effects on the oxygenation status and blood volume of the exercising muscles at similar systemic O2 delivery in patients with advanced CHF. These data suggest that blood flow was redistributed from respiratory to locomotor muscles during unloaded breathing.

Exercise-induced respiratory muscle fatigue: implications for performance

It is commonly held that the respiratory system has ample capacity relative to the demand for maximal O2and CO2transport in healthy humans exercising near sea level. However, this situation may not apply during heavy-intensity, sustained exercise where exercise may encroach on the capacity of the respiratory system. Nerve stimulation techniques have provided objective evidence that the diaphragm and abdominal muscles are susceptible to fatigue with heavy, sustained exercise. The fatigue appears to be due to elevated levels of respiratory muscle work combined with an increased competition for blood flow with limb locomotor muscles. When respiratory muscles are prefatigued using voluntary respiratory maneuvers, time to exhaustion during subsequent exercise is decreased. Partially unloading the respiratory muscles during heavy exercise using low-density gas mixtures or mechanical ventilation can prevent exercise-induced diaphragm fatigue and increase exercise time to exhaustion. Collectively, these findings suggest that respiratory muscle fatigue may be involved in limiting exercise tolerance or that other factors, including alterations in the sensation of dyspnea or mechanical load, may be important. The major consequence of respiratory muscle fatigue is an increased sympathetic vasoconstrictor outflow to working skeletal muscle through a respiratory muscle metaboreflex, thereby reducing limb blood flow and increasing the severity of exercise-induced locomotor muscle fatigue. An increase in limb locomotor muscle fatigue may play a pivotal role in determining exercise tolerance through a direct effect on muscle force output and a feedback effect on effort perception, causing reduced motor output to the working limb muscles.

Immune response to inspiratory muscle training in patients with chronic heart failure

BACKGROUND

The effects of inspiratory muscle training on plasma cytokines, C-reactive protein and the soluble apoptosis mediators Fas and Fas ligand in chronic heart failure are unknown.

DESIGN AND METHODS

Thirty-eight patients with chronic heart failure, age 57+/-2 years, New York Heart Association classification II-III, were assigned to either a high intensity training group (n=15, age 53+/-2 years) exercised at 60% of sustained maximal inspiratory pressure, or a low intensity training group (n=23, age 59+/-2 years), exercised at 15% of sustained maximal inspiratory pressure, three times per week for 10 weeks. Patients in the high intensity training group and low intensity training group were matched for age, sex and New York Heart Association functional class. Plasma levels of tumor necrosis factor (TNF)-alpha, soluble TNF receptor I, interleukin-6, C-reactive protein, soluble apoptosis mediators Fas and Fas ligand were measured at baseline and at post-inspiratory muscle training. Pulmonary function was assessed by spirometry, exercise capacity by a cardiopulmonary exercise test and the 6 min walk test, whereas dyspnea by the Borg scale after the 6 min walk test.

RESULTS

High intensity training group improved inspiratory muscle strength (105.1+/-4.9 vs. 79.8+/-4.7 cmH2O, P<0.001), sustained maximal inspiratory pressure (504.5+/-39.7 vs. 312.5+/-26.5 cmH2O/s/10, P<0.001), forced vital capacity (98.9+/-3.9 vs. 96+/-3.3%, P<0.05), peak VO2 (19.4+/-1.2 vs. 17.3+/-0.9 ml/kg per min, P<0.01), 6 min walk test distance (404.3+/-11.9 vs. 378.2+/-10.4 m, P<0.01) and dyspnea (8.0+/-0.4 vs. 9.2+/-0.4, P<0.01). Circulating TNF-alpha, soluble TNF receptor I, interleukin-6, C-reactive protein, soluble apoptosis mediators Fas and Fas ligand were not significantly altered. Low intensity training group increased only the inspiratory muscle strength (90.3+/-5.9 vs. 80.2+/-5 cmH2O, P<0.01). Comparison between groups was significant for soluble TNF receptor I change (high intensity training group, 5.8+/-0.49 vs. 6.1+/-0.42; low intensity training group, 8.4+/-0.6 vs. 7.8+/-0.6, P<0.01).

CONCLUSION

A high intensity inspiratory muscle training program resulted in improvement in functional status of chronic heart failure patients compared with low intensity inspiratory muscle training. Improvement in exercise capacity was not associated with an anti-inflammatory effect, although a beneficial influence on soluble TNF receptor I was recorded. Possible reasons include inadequate level of muscle mass exercise and the low pretraining New York Heart Association class.

Inspiratory muscle training attenuates the human respiratory muscle metaboreflex

We hypothesized that inspiratory muscle training (IMT) would attenuate the sympathetically mediated heart rate (HR) and mean arterial pressure (MAP) increases normally observed during fatiguing inspiratory muscle work. An experimental group (Exp, n = 8) performed IMT 6 days per week for 5 weeks at 50% of maximal inspiratory pressure (MIP), while a control group (Sham, n = 8) performed IMT at 10% MIP. Pre- and post-training, subjects underwent a eucapnic resistive breathing task (RBT) (breathing frequency = 15 breaths min(-1), duty cycle = 0.70) while HR and MAP were continuously monitored. Following IMT, MIP increased significantly (P < 0.05) in the Exp group (-125 +/- 10 to -146 +/- 12 cmH(2)O; mean +/- s.e.m.) but not in the Sham group (-141 +/- 11 to -148 +/- 11 cmH(2)O). Prior to IMT, the RBT resulted in significant increases in HR (Sham: 59 +/- 2 to 83 +/- 4 beats min(-1); Exp: 62 +/- 3 to 83 +/- 4 beats min(-1)) and MAP (Sham: 88 +/- 2 to 106 +/- 3 mmHg; Exp: 84 +/- 1 to 99 +/- 3 mmHg) in both groups relative to rest. Following IMT, the Sham group observed similar HR and MAP responses to the RBT while the Exp group failed to increase HR and MAP to the same extent as before (HR: 59 +/- 3 to 74 +/- 2 beats min(-1); MAP: 84 +/- 1 to 89 +/- 2 mmHg). This attenuated cardiovascular response suggests a blunted sympatho-excitation to resistive inspiratory work. We attribute our findings to a reduced activity of chemosensitive afferents within the inspiratory muscles and may provide a mechanism for some of the whole-body exercise endurance improvements associated with IMT.

Exercise-Disordered Breathing in Chronic Heart Failure

Patients with heart failure develop various ventilatory abnormalities, including reduced ventilatory reserves, increased ventilatory demands, and high work and cost of breathing. They tend to breathe with a rapid-shallow pattern, have low end-expiratory lung volumes, and may develop rhythmic oscillations in ventilation and tidal volume. These abnormalities likely contribute to exercise limitations and are associated with a worsening prognosis.

The effects of inspiratory intrathoracic pressure production on the cardiovascular response to submaximal exercise in health and chronic heart failure

We sought to determine whether the normal inspiratory intrathoracic pressures (P(ITP)) produced during exercise contribute to the blunted cardiac output and locomotor limb blood flow responses observed in chronic heart failure (CHF). Five chronically instrumented dogs exercised on a treadmill at 2.5 mile/h at 5% grade while healthy or after the induction of tachycardia-induced CHF. We observed several key differences in the cardiovascular responses to changes in the inspiratory P(ITP) excursion between health and CHF; namely, 1) removing approximately 70% of the normally produced inspiratory P(ITP) excursion during exercise (with 15 cmH(2)O inspiratory positive pressure ventilation) significantly reduced stroke volume (SV) in healthy animals by 5 +/- 2% (P < 0.05) but significantly increased SV and cardiac output (Q(TOT)) in animals with CHF by 5 +/- 1% (P < 0.05); 2) doubling the magnitude of the inspiratory P(ITP) excursion had no effect on SV or Q(TOT) in healthy animals but significantly reduced steady-state Q(TOT) and SV in animals with CHF by -4 +/- 3% and -10 +/- 3%, respectively; 3) removing the majority of the normally produced inspiratory P(ITP) excursion had no effect on blood flow distribution in healthy animals but increased hindlimb blood flow (9 +/- 3%, P < 0.05) out of proportion to the increases in Q(TOT); and 4) the only similarity between healthy and CHF animals was that increasing the inspiratory P(ITP) excursion significantly reduced steady-state locomotor limb blood flow by 5 +/- 2% and 6 +/- 3%, respectively (P < 0.05 for both). We conclude that 1) the normally produced inspiratory P(ITP) excursions are required for a maximal SV response to submaximal exercise in healthy animals but detrimental to the SV and Q(TOT) responses to submaximal exercise in CHF, 2) the respiratory muscle ergoreflex tonically restrains locomotor limb blood flow during submaximal exercise in CHF, and 3) excessive inspiratory muscle work further compromises cardiac function and blood flow distribution in both health and CHF.

Expiratory threshold loading impairs cardiovascular function in health and chronic heart failure during submaximal exercise

We determined the effects of augmented expiratory intrathoracic pressure (PITP) production on cardiac output (QTOT) and blood flow distribution in healthy dogs and dogs with chronic heart failure (CHF). From a control expiratory PITP excursion of 7 ± 2 cmH2O, the application of 5, 10, or 15 cmH2O expiratory threshold loads increased the expiratory PITP excursion by 47 ± 23, 67 ± 32, and 118 ± 18% ( P < 0.05 for all). Stroke volume (SV) rapidly decreased (onset <10 s) with increases in the expiratory PITP excursion (−2.1 ± 0.5%, −2.4 ± 0.9%, and −3.6 ± 0.7%, P < 0.05), with slightly smaller reductions in QTOT (0.8 ± 0.6, 1.0 ± 1.1, and 1.8 ± 0.8%, P < 0.05) owing to small increases in heart rate. Both QTOT and SV were restored to control levels when the inspiratory PITP excursion was augmented by the addition of an inspiratory resistive load during 15 cmH2O expiratory threshold loading. The highest level of expiratory loading significantly reduced hindlimb blood flow by −5 ± 2% owing to significant reductions in vascular conductance (−7 ± 2%). After the induction of CHF by 6 wk of rapid cardiac pacing at 210 beats/min, the expiratory PITP excursions during nonloaded breathing were not significantly changed (8 ± 2 cmH2O), and the application of 5, 10, and 15 cmH2O expiratory threshold loads increased the expiratory PITP excursion by 15 ± 7, 23 ± 7, and 31 ± 7%, respectively ( P < 0.05 for all). Both 10 and 15 cmH2O expiratory threshold loads significantly reduced SV (−3.5 ± 0.7 and −4.2 ± 0.7%, respectively) and QTOT (−1.7 ± 0.4 and −2.5 ± 0.4%, P < 0.05) after the induction of CHF, with the reductions in SV predominantly occurring during inspiration. However, the augmentation of the inspiratory PITP excursion now elicited further decreases in SV and QTOT. Only the highest level of expiratory loading significantly reduced hindlimb blood flow (−4 ± 2%) as a result of significant reductions in vascular conductance (−5 ± 2%). We conclude that increases in expiratory PITP production-similar to those observed during severe expiratory flow limitation-reduce cardiac output and hindlimb blood flow during submaximal exercise in health and CHF.

Respiratory muscle energetics during exercise in healthy subjects and patients with COPD

The energy expenditure required by the respiratory muscles during exercise is a function of their work rate, cost of breathing, and efficiency. During exercise, ventilatory requirements increase further exacerbating the potential imbalance between inspiratory muscle load and capacity. High level of exercise intensity in conjunction with contracting respiratory muscles is the reason for respiratory muscle fatigue in healthy subjects. Available evidence would suggest that fatigue of the diaphragm and other respiratory muscles is an important mechanism involved in redistribution of blood flow. Reflex mechanisms of sympathoexcitation are triggered in fatigued diaphragm during heavy exercise when cardiac output is not sufficient to adequately meet the high metabolic requirements of both respiratory and limb musculature. It is very likely that local changes in locomotor muscle blood flow may occur during exhaustive endurance exercise and that changes may have important effect on O2 transport to the working locomotor muscles and, therefore, on their fatigability. In a condition when the respiratory muscles receive their share of blood flow at the expense of limb locomotor muscles, minimizing mechanical work of breathing and therefore its metabolic cost allows a greater amount of cardiac output to be available to be delivered to working limb muscles. Malfunction in any of the multiple components responsible for circulatory flow and O2 delivery will limit the blood supply therefore inhibiting the supply of O2 and the energy substrate to the contracting muscles. Studies are needed to overcome these limitations.

Does symptom-limited cycle exercise cause low frequency diaphragm fatigue in patients with heart failure?

BACKGROUND

Reduced diaphragm contractility occurs in some healthy subjects when they exercise to exhaustion. This indicates low frequency fatigue, which may contribute to task failure. We hypothesised that patients with congestive heart failure (CHF) might be especially vulnerable to the development of low frequency diaphragm fatigue after exhaustive exercise.

AIMS

To study the effect of exhaustive incremental cycle exercise on diaphragm contractility in patients with CHF.

METHODS

12 patients with CHF with an ejection fraction of 36.5 +/- 7.3% and 12 healthy age-matched control subjects performed an incremental cycle test to exhaustion. The unpotentiated twitch transdiaphragmatic pressure (twitch Pdi) in response to bilateral anterolateral magnetic phrenic nerve stimulation (BAMPS) was measured before and after exercise.

RESULTS

Twitch Pdi at baseline was 20.2 +/- 6.7 cm H2O in the CHF group and 20.3 +/- .3 cm H2O in the controls (p = 0.957). 25 and 35 min post exercise the values were 19.9+/-5.4 and 20.0+/-5.1 cm H2O in the CHF group and 20.6 +/- 4.3 and 21.2 +/- 3.4 cm H2O in the control group; neither change was significant (F(2,27) = 0.007, p = 0.993; F(2,33) = 0.144, p = 0.866, respectively).

CONCLUSION

When patients with CHF cycle to exhaustion, low frequency fatigue of the diaphragm does not occur, and this is unlikely to be an important factor limiting exercise capacity of such patients.

Disorders of the respiratory muscles

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Exercise training as a therapy for chronic heart failure: can older people benefit?

Despite recent advances in pharmacological therapy, chronic heart failure remains a major cause of morbidity and mortality in older people. Studies of exercise training in younger, carefully selected patients with heart failure have shown improvements in symptoms and exercise capacity and in many pathophysiological aspects of heart failure, including skeletal myopathy, ergoreceptor function, heart rate variability, endothelial function, and cytokine expression. Data on mortality and hospitalization are lacking, and effects on everyday activity, depression, and quality of life are unclear. Exercise therapy for patients with heart failure appears to be safe and has the potential to improve function and quality of life in older people with heart failure. To realize these potential benefits, exercise programs that are suitable for older, frail people need to be established and tested in an older, frail, unselected population with comorbidities.

Airflow limitation and breathing strategy in congestive heart failure patients during exercise

BACKGROUND

Congestive heart failure (CHF) patients experience dyspnea on exertion and therefore have decreased exercise tolerance.

OBJECTIVE

This study explores the hypothesis that stable New York Heart Association (NYHA) class III CHF patients without a history of pulmonary disease exhibit airflow limitation with increasing exercise.

METHODS

We characterized flow limitations and breathing reserves at baseline, during exercise before anaerobic threshold (pre-AT), and after anaerobic threshold (post-AT) in CHF patients and normal subjects. Data were collected in the form of maximal flow volume loops and subsequent tidal flow volume loops at baseline and during exercise. Expiratory flow limitation was expressed as percent of tidal volume that corresponded with overlap of the tidal flow volume loops and maximal flow volume loops during expiration. The area directly between the maximum flow volume loops and the tidal flow volume loops during the expiratory phase is expressed as expiratory flow volume reserve (EFVR).

RESULTS

CHF patients experienced expiratory flow limitation during exercise (pre-AT and post-AT) that was significantly increased compared to baseline and to normal subjects at similar exercise levels (CHF, baseline 8.5 +/- 7, pre-AT 37 +/- 10, post-AT 38 +/- 8%, n = 9, p < 0.05). Both CHF patients and normal subjects increased EFVR during exercise, but only the normal subjects increased EFVR to a significantly different value at post-AT exercise levels (normal subjects, 9.5 +/- 2, 11 +/- 2, 32 +/- 4%, n = 7, p < 0.05). Both CHF patients and normal subjects increased end inspiratory lung volume (EILV) during exercise, but only the normal subjects significantly increased EILV at post-AT exercise levels (normal subjects, 49 +/- 4, 55 +/- 5, 76 +/- 4%, p < 0.05). Inspiratory capacity (IC)/forced vital capacity (FVC) ratios were increased in CHF patients compared to normal subjects. However, IC/FVC values did not change during exercise in either group.

CONCLUSIONS

CHF patients cannot utilize their full respiratory capacity during exercise secondary to expiratory flow limitation and an inability to increase EILV and EFVR.

Modulatory effects of respiration

Respiration is a powerful modulator of heart rate variability, and of baro- and chemoreflex sensitivity. Abnormal respiratory modulation of heart rate is often an early sign of autonomic dysfunction in a number of diseases. In addition, increase in venous return due to respiration may help in maintaining blood pressure during standing in critical situations. This review examines the possibility that manipulation of breathing pattern may provide beneficial effects in terms not only of ventilatory efficiency, but also of cardiovascular and respiratory control in physiologic and pathologic conditions, such as chronic heart failure. This opens a new area of future research in the better management of patients with cardiovascular autonomic dysfunction.

Inspiratory Muscle Relaxation Rate Slows during Exhaustive Treadmill Walking in Patients with Chronic Heart Failure

Exercise intolerance is a feature of chronic heart failure (CHF). We hypothesized that excessive loading of the respiratory muscle pump might contribute to exertional breathlessness. One marker of excessive muscle-loading is slowing of maximum relaxation rate (MRR) and, therefore, to test our hypothesis, we investigated the effect of exhaustive treadmill walking on inspiratory muscle MRR in patients with CHF. We studied eight stable patients with mild-moderate CHF walking on a treadmill until termination because of severe dyspnea. Inspiratory muscle MRR was determined from esophageal pressure (Pes) change during submaximal sniffs (Sn) before and immediately after exercise to a mean (SD) minute ventilation of 77 () L/min. For comparison, nine healthy subjects performed a similar protocol; exercise was terminated either by severe dyspnea or when minute ventilation reached 100 L/min. There were no significant differences in terms of heart rate, respiratory rate, tidal volume, or inspiratory duty cycle at cessation of exercise. The mean slowing of Sn Pes MRR in the first minute after termination of exercise in the CHF group was 22.4% and in the normal control group it was 2.8% (p < 0.01). Our data show that slowing of inspiratory muscle relaxation rate occurs in patients with CHF walking to severe breathlessness. We conclude that severe loading of the inspiratory muscles is a feature of exertional dyspnea in CHF.

Validating facemask use for gas exchange analysis in patients with congestive heart failure

PURPOSE

Functional limitation, often associated with congestive heart failure (CHF), is most accurately assessed with measurement of peak VO2 (VO2peak) during exercise testing. VO2peak strongly predicts survival in CHF patients and is used to triage patients for cardiac transplantation. However, the traditional mouthpiece/noseclip equipment used for gas exchange analysis is often cumbersome and uncomfortable. Several alternative facemask designs tested previously in healthy cohorts were inferior to the mouthpiece for measuring VO2peak. In this study, a series of facemasks designed specifically for VO2peak testing (Hans-Rudolph) was employed. Although previously validated in healthy young adults, this apparatus has not been validated in patients with severe CHF.

METHODS

The Hans-Rudolph facemasks were compared with the mouthpiece apparatus in 30 patients with severe CHF. Each subject completed a cardiopulmonary exercise test with each device. Twelve subjects were randomized to evaluate a possible training effect. To enhance recruitment, 18 additional subjects were recruited after they had completed a test with the mouthpiece.

RESULTS

There was no evidence of a training effect and no differences in gas exchange measurements were found between the two devices. The weight-adjusted VO2peak measurements were 17.1 +/- 5.3 ml O2/kg/min for the mouthpiece group and 17.3 +/- 5.6 ml O2/kg/min for the mask group (P = 0.54).

CONCLUSIONS

The facemask was shown to be equivalent to the mouthpiece for measuring VO2peak in CHF patients. Concerns of hypoventilation and decreased VO2peak associated with previous facemask designs were not substantiated. Since successful exercise testing depends on maximal exertion, providing a choice of equipment may facilitate cooperation without sacrificing accuracy.

Effects of respiratory muscle work on exercise performance

The normal respiratory muscle effort at maximal exercise requires a significant fraction of cardiac output and causes leg blood flow to fall. We questioned whether the high levels of respiratory muscle work experienced in heavy exercise would affect performance. Seven male cyclists [maximal O(2) consumption (VO(2)) 63 +/- 5 ml. kg(-1). min(-1)] each completed 11 randomized trials on a cycle ergometer at a workload requiring 90% maximal VO(2). Respiratory muscle work was either decreased (unloading), increased (loading), or unchanged (control). Time to exhaustion was increased with unloading in 76% of the trials by an average of 1.3 +/- 0.4 min or 14 +/- 5% and decreased with loading in 83% of the trials by an average of 1.0 +/- 0.6 min or 15 +/- 3% compared with control (P < 0.05). Respiratory muscle unloading during exercise reduced VO(2), caused hyperventilation, and reduced the rate of change in perceptions of respiratory and limb discomfort throughout the duration of exercise. These findings demonstrate that the work of breathing normally incurred during sustained, heavy-intensity exercise (90% VO(2)) has a significant influence on exercise performance. We speculate that this effect of the normal respiratory muscle load on performance in trained male cyclists is due to the associated reduction in leg blood flow, which enhances both the onset of leg fatigue and the intensity with which both leg and respiratory muscle efforts are perceived.

Role of respiratory function in exercise limitation in chronic heart failure

OBJECTIVE

To test the hypothesis that respiratory function contributes to limit maximal exercise performance in patients with chronic heart failure by using the technique of dead space loading during exercise.

DESIGN

Blinded subjects underwent two maximal incremental exercise tests in random order on an upright bicycle ergometer: one with and one without added dead space.

SETTING

: Tertiary-care university teaching hospital.

SUBJECTS

Seven patients with stable chronic heart failure (mean +/- SEM left ventricular ejection fraction, 27 +/- 3%).

RESULTS

Subjects were able to significantly increase their peak minute ventilation during exercise with added dead space when compared with control exercise (57.4 +/- 5.9 vs 50.0 +/- 5.6 L/min; p < 0.05). Peak oxygen uptake, workload, heart rate, and exercise duration were not significantly different between the added dead space and control tests. Breathing pattern was significantly deeper and slower at matched levels of ventilation during exercise with added dead space.

CONCLUSION

Because patients with chronic heart failure had significant ventilatory reserve at the end of exercise and were able to further increase their maximal minute ventilation, we conclude that respiratory function does not contribute to limitation of exercise in patients with chronic heart failure.

Ventilatory constraints during exercise in patients with chronic heart failure

We examined the degree of ventilatory constraint in patients with a history of chronic heart failure (CHF; n = 11; mean +/- SE age, 62 +/- 4 years; cardiac index [CI], 2.0 +/- 0.1; and ejection fraction [EF], 24 +/- 2%) and in control subjects (CTLS; n = 8; age, 61 +/- 5 years; CI, 2.6 +/- 0.3) by plotting the tidal flow-volume responses to graded exercise in relationship to the maximal flow-volume envelope (MFVL). Inspiratory capacity (IC) maneuvers were performed to follow changes in end-expiratory lung volume (EELV) during exercise, and the degree of expiratory flow limitation was assessed as the percent of the tidal volume (VT) that met or exceeded the expiratory boundary of the MFVL. CHF patients had significantly (p < 0.05) reduced baseline pulmonary function (FVC, 76 +/- 4%; FEV(1), 78 +/- 4% predicted) relative to CTLS (FVC, 99 +/- 4%; FEV(1), 102 +/- 4% predicted). At peak exercise, oxygen consumption (VO(2)) and minute ventilation (V(E)) were lower in CHF patients than in CTLS (VO(2), 17 +/- 2 vs 32 +/- 2 mL/kg/min; VE, 56 +/- 4 vs 82 +/- 6 L/min, respectively), whereas VE/carbon dioxide output was higher (42 +/- 4 vs 29 +/- 5). In CTLS, EELV initially decreased with light exercise, but increased as VE and expiratory flow limitation increased. In contrast, the EELV in patients with CHF remained near residual volume (RV) throughout exercise, despite increasing flow limitation. At peak exercise, IC averaged 91 +/- 3% and 79 +/- 4% (p < 0.05) of the FVC in CHF patients and CTLS, respectively, and flow limitation was present over > 45% of the VT in CHF patients vs < 25% in CTLS (despite the higher VE in CTLS). The least fit and most symptomatic CHF patients demonstrated the lowest EELV, the greatest degree of flow limitation, and a limited response to increased inspired carbon dioxide during exercise, all consistent with VE constraint. We conclude that patients with CHF commonly breathe near RV during exertion and experience expiratory flow limitation. This results in VE constraint and may contribute to exertional intolerance.

Diaphragm strength in chronic heart failure

Reduced respiratory muscle strength has been reported in chronic heart failure (CHF) in several studies. The data supporting this conclusion come almost exclusively from static inspiratory and expiratory mouth pressure maneuvers (MIP, MEP), which many subjects find difficult to perform. We therefore performed a study using measurements that are less dependent on patient aptitude and also provide specific data on diaphragm strength. In 20 male patients and 15 control subjects we measured MIP and MEP as well as esophageal and transdiaphragmatic pressure during maximal sniffs (Sn Pes, Sn Pdi) and cervical magnetic phrenic nerve stimulation (Tw Pdi). In a subgroup the response to paired phrenic nerve stimulation (pTw Pdi) at interpulse intervals from 10 to 200 ms (5 to 100 Hz) was also determined. As expected, MIP was significantly reduced in the CHF group (CHF, 69.5 cm H(2)O; control, 96.7 cm H(2)O; p = 0.01), but differences were much less marked for Sn Pes (CHF, 95.2 cm H(2)O; control, 104.8 cm H(2)O; p = 0.20) and MEP (CHF, 109.1 cm H(2)O; control, 135.7 cm H(2)O; p = 0.09). Diaphragm strength was significantly reduced (Sn Pdi: CHF, 123.8 cm H(2)O; control 143.5 cm H(2)O; p = 0.04. Tw Pdi: CHF, 21.4 cm H(2)O; control, 28.5 cm H(2)O; p = 0.0005). Paired phrenic nerve stimulation suggested a trend to increased twitch summation at 5 to 20 Hz in CHF, although this did not reach significance. We conclude that mild reduction in diaphragm strength occurs in CHF, possibly because of an increased proportion of slow fibers, but overall strength of the respiratory muscles remains well preserved.

Effect of breathing rate on oxygen saturation and exercise performance in chronic heart failure

BACKGROUND

In chronic heart failure (CHF), impaired pulmonary function can independently contribute to oxygen desaturation and reduced physical activity. We investigated the effect of breathing rate on oxygen saturation and other respiratory indices.

METHODS

Arterial oxygen saturation (SaO2) and respiratory indices were recorded during spontaneous breathing (baseline) and during controlled breathing at 15, six, and three breaths per min in 50 patients with CHF and in 11 healthy volunteers (controls). 15 patients with CHF were randomly allocated 1 month of respiratory training to decrease their respiratory rate to six breaths per min. Respiratory indices were recorded before training, at the end of training, and 1 month after training.

FINDINGS

During spontaneous breathing, mean SaO2 was lower in CHF patients than in controls (91-4% [SD 0.4] vs 95.4% [0.2], p<0.001). Controlled breathing increased SaO2 at all breathing rates in patients with CHF. Compared with baseline, minute ventilation increased at 15 breaths per min (+45.9% [9.8], p<0.01), did not change at six breaths per min, and decreased at three breaths per min (-40.3% [4.8], p<0.001). In the nine CHF patients who had 1 month of respiratory training, resting SaO2 increased from 92.5% (0.3) at baseline to 93.2% (0.4) (p<0.05), their breathing rate per min decreased from 13.4 (1.5) to 7.6 (1.9) (p<0.001), peak oxygen consumption increased from 1157 (83) to 1368 (110) L/min (p<0.05), exercise time increased from 583 (29) to 615 (23) min/s (p<0.05), and perception of dyspnoea reduced from a score of 19.0 (0.4) to 17.3 (0.9) on the Borg scale (p<0.05). There were no changes in the respiratory indices in the patients who did not have respiratory training.

INTERPRETATION

Slowing respiratory rate reduces dyspnoea and improves both resting pulmonary gas exchange and exercise performance in patients with CHF.