Úrsula Pinelo Cavegn,  Luiz Fernando Rodrigues Junior  

¹Postgraduate course in Cardiovascular Physiotherapy, National Institute of Cardiology, Rio de Janeiro, RJ, Brazil

²Department of Physiological Sciences, Federal University of the State of Rio de Janeiro, Rio de Janeiro, RJ, Brazil

Abstract: Heart failure (HF) is one of the most prevalent diseases in the world and is  characterized as a complex clinical syndrome. It is a disease that affects both the  peripheral muscles and the respiratory muscles, causing damage such as reduced  strength and endurance. One of the main symptoms is fatigue and exercise  intolerance.

The aim of this review article is to assess whether the use of inspiratory  muscle training (IMT) associated with conventional training enhances inspiratory  muscle strength (PImax) and inspiratory muscle endurance (SPImax) and functional  capacity in patients with HF.

Methods: A literature review was carried out on the use  of IMT in patients, including 6 randomized clinical trials, searched in the PubMed,  PEDro and Lilacs databases.

Results: There is evidence showing an increase in  PImax and SPImax with the addition of IMT to training; in addition, an improvement in  VO2 peak, circulatory power and ventilatory efficiency.

Conclusion: The use of IMT  associated with conventional training is an interesting strategy for boosting gains in  strength and inspiratory muscle endurance, increasing exercise tolerance and  consequently improving patients' quality of life. However, more studies are needed in  order to more clearly define the selection criteria and methodology for applying IMT.     

Key words:  heart failure, inspiratory muscle training, respiratory metaborreflex,  functional capacity

Introduction

Heart failure (HF) is characterized as a complex clinical syndrome in which the  heart is unable to pump blood to meet the metabolic demands of the tissues or does  so under high filling pressures. It is one of the most prevalent diseases, with around 23 million cases worldwide and a 5-year survival rate of only 35% after diagnosis (1).  This condition can occur as a result of biochemical, structural and congenital alterations, as well as being determined by extra-cardiac conditions such as systemic arterial hypertension (2).  In this sense, at the beginning of the development of HF,  the body recruits compensatory mechanisms to minimize the overload imposed on  the myocardium, leading to cardiac remodeling, which will feed back into this  condition (3, 4).   

Cardio-sarcopenia is a condition that arises from the combination of two degenerative processes: sarcopenia, which is the gradual loss of skeletal muscle mass related to ageing, and cardiac dysfunction, characteristic of HF. Changes in  cardiac function and structure trigger a series of negative effects on skeletal muscle,  resulting in decreased strength, endurance and functional capacity.

This skeletal  muscle dysfunction can, in turn, contribute to a vicious cycle, where muscle  weakness resulting from HF leads to a sedentary lifestyle, further aggravating  sarcopenia (5).   Among the most common symptoms of HF are dyspnea, a feeling of fatigue and exercise intolerance (6). This can occur due to impaired muscle performance in both the peripheral and respiratory muscles, affecting exercise capacity and  consequently reducing quality of life (4, 7).

Factors such as hypoxia, oxidative stress,  systemic inflammation and disuse are strongly related to muscle alterations,  contributing to the appearance of these symptoms (3, 8).  

In turn, inspiratory muscle weakness is present in 50% of patients with HF, who will show changes in variables such as maximum inspiratory pressure  (MIP), which corresponds to inspiratory muscle strength, and maximum sustained  inspiratory pressure (MPSI), which corresponds to inspiratory muscle endurance.

A  decrease in the predicted values of these measurements is related to an increase in  the sensation of dyspnea and a worsening of functional class, implying a worse  prognosis (9).   In this context, several studies have used inspiratory muscle training (IMT) as  a form of intervention to specifically increase the strength and endurance of the  inspiratory muscles and thereby improve functional capacity. This is due to the  attenuation of the respiratory metaborreflex, which is a mechanism triggered by  intense respiratory effort and which leads to fatigue of the peripheral muscles. In this  way, there is a redistribution of flow from the peripheral muscles to the diaphragm  through sympathetic vasoconstriction, exacerbating the individual's perception of  effort and limiting performance in high-intensity exercise. With improved respiratory  performance, this effect is minimized, leading to greater exercise tolerance and a  better quality of life (10, 11).   

The general aim of this article is to review the findings related to the effects of  IMT in the cardiac rehabilitation of patients with HF. The specific objectives of this  article include evaluating the effect of IMT on: inspiratory muscle strength and   endurance, cardiopulmonary exercise test (CPET) variables, 6-minute walk test  (6MWT), quality of life (QoL) and change in NYHA functional class.     

Methods

 A literature review was carried out to investigate whether combining IMT with  conventional training in patients with HF had a greater impact on respiratory muscle  performance and the functional capacity of these individuals. This review included  articles that addressed the proposed topic between 1996 and 2022.   The searches were carried out in the Pubmed, PEDro and Lilacs databases,  using the descriptors: inspiratory muscle training, exercise; and heart, failure. The  initial search was carried out using articles that contained these terms in the title or  abstract and were considered eligible for the review.   The exclusion criteria were duplicate articles, those that were not in line with  the research topic and studies that did not use IMT as a form of intervention. The  remaining studies were included in the review.  

Results

There were identified 187 possible relevant articles, with 15 being excluded as  duplicates and 154 by absence of association with the study topic, remaining six  studies included in the review (Fig. 1).  

Considering the interventions presented by the studies (Table 1), only one study did not  perform aerobic training (AT) as an intervention associated with IMT. One study  performed the aerobic interval training (AIT) modality in aerobic training and the  others used moderate continuous training. 3 of the 6 studies used resistance training  associated with IMT.

Effect of IMT on inspiratory muscle strength and endurance

In all the studies, PImax measurements were taken pre- and post-intervention,  and two studies did not assess SPImax. In the study by Winkelmann et al. (12),  PImax increased significantly (110% compared to 72%) when compared to the  control group, which only performed AT. Similarly, in all the other studies there was an improvement in PImax, both in the group that used IMT and in the  groups that used AT and/or resistance training (RT). However, the most significant improvement occurred in the groups that used combined training. In addition, SPImax, which corresponds to inspiratory muscle endurance, was  assessed in 4 studies (13-15, 17), and only showed a significant increase in the groups that underwent combined  training.   In the study by Kawauchi et al. (16), the low-intensity training group and the  moderate-intensity training group both increased PImax, but the greatest increase  was in the moderate-intensity group and occurred more quickly than in the low- intensity group. Another point worth highlighting in this study is that the patients in the  control group who did not undergo any type of intervention had an 8% reduction in  PImax. 

 Figure 1. Identification of studies via databases

Cardiopulmonary Exercise Test Variables 

In the study by Winkelmann et al. (12), interesting responses were seen in  terms of the variables derived from the CPET. With  the inclusion of CMT associated with aerobic training in the intervention group,  significant improvements were seen in VO2 peak, circulatory power, O2 recovery  kinetics and ventilatory efficiency (VE/VCO2 slope).  The 2013 study by Lautaris et al. (13) corroborates the findings of the study by  Winkelmann et al. (12). There was a significant improvement in VO2 peak; exercise  time; ventilatory threshold and reduction in VE/VCO2 slope in the ARIS group  (Aerobic Training (AT) + Resistance Training (RT) + IMT). The aerobic training group  had a significant increase in VO2 peak, exercise time and ventilatory threshold.  Circulatory power improved significantly in both groups.  

Heart, Vessels and Transplantation 2023; 7: doi: 10.24969/hvt.2023.451

Inspiratory muscle training in HF                                                                                                             Pinelo Cavegn, Rodrigues Jr 

In another study, Lautaris et al. conducted an intervention program with 4  groups: ARIS, who performed IMT associated with aerobic and resistance training;  AT + IMT, performed inspiratory muscle training associated with aerobic training; AT  + RT, performed aerobic and resistance training; AT who performed only aerobic  training. As a result, VO2 peak showed no significant difference between the groups,  but the ARIS group tended to increase. In addition, circulatory power and CPET time  were higher in the ARIS group when compared to the other groups. Parameters such as ventilatory efficiency, VE/VCO2 slope and O2 recovery kinetics did not have  significant differences between the groups (17). 

 The study by Adamapoulos et al. (14) found no significant differences in  circulatory power, ventilatory threshold and VE/VCO2 slope, like the other studies.  VO2 peak improved in both groups and the intervention group achieved a significant  improvement in exercise time, respiratory exchange ratio and ventilation.  

The studies by Kawauchi et al. (15) and Sadek et al. (16) did not perform  CPET.   

6 minute walk test 

In the study by Kawauchi et al. (15), the distance in the 6MWT improved significantly in the low- and moderate-intensity intervention group  when compared to the control group. The ARIS group in the most recent study by  Lautaris et al. (17) showed significant results for the distance covered in the 6MWT. 

Sadek et al. (16) also observed a significant improvement in the combined  group (HIIT + TMI) when compared to the control group. And in their study,  Winkelmann et al. (12), unlike the other studies, found that both groups showed  improvements in the distance covered in the 6MWT.  

NYHA rating and quality of life 

The study by Winkelmann et al. (12) saw similar improvements in both groups.  The study by Lautaris et al. (13) showed that QoL and NYHA  functional class improved only in the intervention group, which underwent combined  training.

Two studies found that QoL improved more in the combined training group,  but NYHA improved in both training groups (14,16). Kawauchi et al (15), in turn,  obtained improved QoL in all their groups, but the moderate intensity training group  significantly improved functional class and NYHA symptoms. 

Finally, the most recent study by Lautaris et al. (17) showed a more significant  increase in QoL in the ARIS group. NYHA class tended to improve in the ARIS group when  compared to AT alone. The AT + IMT group also achieved significant improvements  in QoL and NYHA when compared to AT alone. 

Discussion

Inspiratory muscle training has been widely studied as a form of intervention in  patients with HF, with the aim of improving their functional capacity and QoL  by increasing the strength and endurance of the inspiratory muscles. Most of the  studies included in this review showed that patients who underwent IMT associated  with conventional training showed a significant increase in inspiratory muscle  performance. These results suggest that the use of IMT is beneficial for patients with  HF, attenuating the respiratory metaborreflex, improving peripheral blood flow and  increasing exercise tolerance (17, 18).  

The study by Meyer et al. (7) highlights that inspiratory muscle strength is an  important predictor of prognosis in HF and that a reduction in PImax is related to  increased mortality in these patients. When analyzing the study by Kawauchi et al.  (15), we observed that patients in the control group, who did not use IMT as an  intervention, reduced their PImax by 8%. These results raise the hypothesis of  whether only patients with reduced PImax should be eligible for IMT or whether all  HF patients should use this resource to prevent respiratory function decline and thus  reduce mortality.  On the other hand, studies such as those by Winkelmann et al.(12) and Sadek  et al. (16), recruited patients with PImax lower than predicted for their age,  categorizing inspiratory muscle weakness and showed that there was a significant  improvement in PImax in the groups that used IMT as an intervention force. This  raises the hypothesis that the patients who benefit most from IMT in terms of gaining  inspiratory muscle strength are those with inspiratory muscle weakness.

Heart, Vessels and Transplantation 2023; 7: doi: 10.24969/hvt.2023.451

Inspiratory muscle training in HF                                                                                                             Pinelo Cavegn, Rodrigues Jr 

However, other studies have also shown that even patients without inspiratory muscle weakness can benefit from IMT. More in-depth research into this issue is therefore needed in order to define more effectively which patients meet the selection  criteria for IMT. 

Another relevant study for reducing mortality in HF patients is HF-ACTION  (18). This study showed that a 6% increase in maximum oxygen consumption (VO2  peak) was associated with a 5% reduction in all-cause mortality and hospitalization rates. Most of the  studies selected for this review showed a significant increase in VO2 peak in the  combined and conventional training groups. Several studies suggest (19-21) that the most significant improvement in VO2 peak occurred in patients with  reduced PImax, which raises the hypothesis that the most effective indication for IMT  would be for these patients.  

Although there is promising evidence of the benefits of IMT in HF patients,  more studies are needed in order to clarify the best way to prescribe it, the ideal  duration of treatment and whether all HF patients would benefit from using IMT as a  form of intervention.  

Study limitations

The heterogeneity of the included studies in terms of methodology, intervention and outcomes made it difficult to directly compare the results. In addition, most of the studies had a small sample size and limited duration, making it difficult to assess significant effects. In addition, the lack of standardization in IMT protocols and assessment measures may also have introduced bias into the results.

In terms of future prospects, it is important to carry out well-designed studies, with larger samples and longer follow-up periods, in order to provide more robust evidence on the benefits of IMT in HF patients. These issues are important for

directing the clinical application of IMT and optimizing outcomes for HF patients.

Conclusion

According to the studies analyzed, we can conclude that the use of IMT associated with other conventional training modalities in HF patients is beneficial for increasing not only inspiratory muscle strength, but also inspiratory muscle endurance. As well as exercise tolerance time, distance covered in the 6MWT, ventilatory efficiency and circulatory power. As for quality of life and NYHA classification, the results are still controversial, but exercise alone showed a

significant improvement in these parameters.

 Peer-review:  External and internal

Conflict of interest: None to declare

Authorship: U.P.C. and L.F.R.Jr  equally contributed  to the preparation of manuscript

Acknowledgment and funding: None to declare