Acute thoracoabdominal and hemodynamic responses to tapered flow resistive loading in healthy adults

Highlights

• TFRL is associated with increased minute ventilation, cardiac output, and respiratory muscle oxygen delivery

• TFRL activates both inspiratory and expiratory muscles, making it an important tool for global respiratory muscle training

• Adverse effects of TFRL include hypocapnia and increased dyspnoea scores with increasing intensity

• The optimal intensity for TFRL training at a fixed breathing frequency of 10 breaths/min is 50 % PI_max.

Abstract

We investigated the acute physiological responses of tapered flow resistive loading (TFRL) at 30, 50 and 70 % maximal inspiratory pressure (PI_max) in 12 healthy adults to determine an optimal resistive load. Increased end-inspiratory rib cage and decreased end-expiratory abdominal volumes equally contributed to the expansion of thoracoabdominal tidal volume (captured by optoelectronic plethysmography). A significant decrease in end-expiratory thoracoabdominal volume was observed from 30 to 50 % PI_max, from 30 to 70 % PI_max, and from 50 to 70 % PImax. Cardiac output (recorded by cardio-impedance) increased from rest by 30 % across the three loading trials. Borg dyspnoea increased from 2.36 ± 0.20 at 30 % PI_max, to 3.45 ± 0.21 at 50 % PI_max, and 4.91 ± 0.25 at 70 % PI_max. End-tidal CO₂ decreased from rest during 30, 50 and 70 %PI_max (26.23 ± 0.59, 25.87 ± 1.02 and 24.30 ± 0.82 mmHg, respectively). Optimal intensity for TFRL is at 50 % PI_max to maximise global respiratory muscle and cardiovascular loading whilst minimising hyperventilation and breathlessness.

Introduction

Inspiratory muscle training (IMT) aims to functionally overload the inspiratory muscles and has been shown to improve inspiratory muscle strength (reflected by increased maximal inspiratory pressure; PI_max), endurance and exercise tolerance in healthy individuals (McConnell and Romer, 2004), as well as in various respiratory (Enright et al., 2004; Gosselink et al., 2011) and cardiovascular disorders (Dall’Ago et al., 2006). IMT devices are usually small and portable allowing for training to be performed at home, with the most common training programmes consisting of 30 breaths performed twice daily (4−5 minute sessions) above 30 % PI_max, 7 days a week for 8–12 weeks (Gosselink et al., 2011; Langer et al., 2018; McConnell, 2013).

The mechanisms of improved PI_max following IMT has been attributed to structural and functional adaptations to the training stimulus, including strength, speed of respiratory muscle shortening and power output (Romer and McConnell, 2003) secondary to hypertrophy of inspiratory muscles (Enright et al., 2006). In terms of improving exercise capacity, previous research has suggested significant IMT-induced physiological adaptations including the attenuation of the respiratory muscle metaboreflex (Witt et al., 2007) and improved respiratory muscle efficiency, associated with reduced oxygen requirement for the respiratory musculature (Turner et al., 2012). Lower dyspnoea levels following IMT have also been reported during exercise (Ramsook et al., 2017; Turner et al., 2011) however, research into the mechanisms behind this needs to be further explored (Ramsook et al., 2017).

The majority of studies investigating the acute effects of inspiratory muscle loading have typically used mechanical threshold loading devices, which provide a constant pressure throughout inspiration. A more recently developed tapered flow resistive loading (TFRL) device, however, provides a tapered resistance via an electronic, dynamically adjusted valve allowing pressure to be volume-dependently tapered once the initial threshold has been overcome (Langer et al., 2015). Use of this device in an IMT programme in patients with chronic obstructive pulmonary disease (COPD) has resulted in greater improvements in inspiratory muscle function (i.e. strength, endurance, power, and shortening velocity) as well as a greater tolerance of higher training loads compared to those patients who trained with the mechanical threshold loading devices (Langer et al., 2015).

The acute effects of inspiratory muscle loading, via TFRL and mechanical threshold loading, on thoracoabdominal volumes in healthy individuals have shown significant increases in tidal volume, end-inspiratory volume and minute ventilation, along with decreases in end-expiratory volumes, albeit at moderate intensities (40 % PI_max) (da Fonsêca et al., 2019; de Souza et al., 2016). An increase in the fractional contribution of the pulmonary rib cage compartment to tidal volume expansion during moderate inspiratory resistance (40 % PI_max) and during a single PI_max manoeuvre compared to quiet breathing has previously been reported (de Souza et al., 2016). Furthermore, healthy individuals increased abdominal rib cage contribution to tidal volume expansion during moderate intensity loading only and decreased abdominal contribution during both moderate (40 % PI_max) and maximal intensities (100 % PI_max) (de Souza et al., 2016).

It is unclear, however, what the optimal training load is when using TFRL devices in terms of maximising respiratory muscle recruitment and mobilising central haemodynamic and local respiratory muscle oxygenation requirements. By combining these physiological measurements, we aimed to develop a greater understanding of the acute effects of separate short bouts of TFRL at low, moderate, and high intensities (corresponding to 30, 50, and 70 % PI_max, respectively) performed at a fixed breathing frequency, and ultimately determine an optimal inspiratory muscle training load to elicit beneficial physiological responses, whilst minimising potential adverse physiological effects. It was reasoned that due to the device’s inherent operational requirements (i.e. an initial threshold load that must be overcome before dynamical adjustments can arise) there would progressively be greater expiratory muscle recruitment at 50 and 70 % PI_max compared to 30 % PI_max to allow for the further expansion of tidal volume. This in turn was expected to maximise loading to both inspiratory and expiratory muscles making TFRL a suitable tool for both inspiratory and expiratory muscle training.