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Inspiratory muscle load and capacity in chronic heart failure

¹ Respiratory Muscle Laboratory, Royal Brompton Hospital, London, UK
² Department of Cardiology, Guy’s, King’s and St Thomas’ School of Medicine King’s College Hospital, London, UK
³ Department of Clinical Physiology, Raymond Poincaré Hospital, Garches, France
⁴ Department of Respiratory Medicine & Allergy, Guy’s, King’s and St Thomas’ School of Medicine, King’s College Hospital, London, UK

Correspondence to:
Correspondence to:
Dr N Hart
Respiratory Muscle Laboratory, Royal Brompton Hospital, London SW3 6NP, UK; drnhart{at}aol.com

Background: Although breathlessness is common in chronic heart failure (CHF), the role of inspiratory muscle dysfunction remains unclear. We hypothesised that inspiratory muscle endurance, expressed as a function of endurance time (Tlim) adjusted for inspiratory muscle load and inspiratory muscle capacity, would be reduced in CHF.

Methods: Endurance was measured in 10 healthy controls and 10 patients with CHF using threshold loading at 40% maximal oesophageal pressure (Poes_max). Oesophageal pressure-time product (PTPoes per cycle) and Poes_max were used as indices of inspiratory muscle load and capacity, respectively.

Results: Although Poes_max was slightly less in the CHF group (–117.7 (23.6) v –100.0 (18.3) cm H₂O; 95% CI –37.5 to 2.2 cm H₂O, p = 0.1), Tlim was greatly reduced (1800 v 306 (190) s; 95% CI 1368 to 1620 s, p<0.0001) and the observed PTPoes per cycle/Poes_max was increased (0.13 (0.05) v 0.21 (0.04); 95% CI –0.11 to –0.03, p = 0.001). Most of this increased inspiratory muscle load was due to a maladaptive breathing pattern, with a reduction in expiratory time (3.0 (5.8) v 1.1 (0.3) s; 95% CI 0.3 to 3.5 s, p = 0.03) accompanied by an increased inspiratory time relative to total respiratory cycle (Ti/Ttot) (0.43 (0.14) v 0.62 (0.07); 95% CI –0.3 to –0.1, p = 0.001). However, log Tlim, which incorporates the higher inspiratory muscle load to capacity ratio caused by this altered breathing pattern, was 85% predicted in seven of 10 patients.

Conclusions: Although a marked reduction in endurance time was observed in CHF, much of this reduction was explained by the increased inspiratory muscle load to capacity ratio, suggesting that the major contributor to task failure was a maladaptive breathing pattern rather than impaired inspiratory muscle endurance.

Keywords: inspiratory muscle endurance; respiratory muscles; chronic heart failure

Abbreviations: BMI, body mass index; CHF, chronic heart failure; CLdyn, dynamic lung compliance; FEV₁, forced expiratory volume in 1 second; FRC, functional residual capacity; FVC, forced vital capacity; LVEDD, left ventricular end diastolic dimension; LVEF, left ventricular ejection fraction; NYHA, New York Heart Association functional class; Poes, oesophageal pressure; Poes_exp, peak expiratory oesophageal pressure; Poes_max, maximum negative oesophageal pressure during an inspiratory manoeuvre; Poes_peak/TTPoes_peak, inspiratory muscle contraction rate; PTPoes per cycle, inspiratory oesophageal pressure time product per breath; PTPoes per cycle/Poes_max, inspiratory muscle load to capacity ratio; RR, respiratory rate; RV, residual volume; TLC, total lung capacity; Te, expiratory time; Ti, inspiratory time; Ti/Ttot, duty cycle; Tlim, endurance time; VE, minute ventilation; VT, tidal volume

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