ASSISTED VENTILATION

Mechanisms of improvement of respiratory failure in patients with restrictive thoracic disease treated with non-invasive ventilation

A H Nickol¹, N Hart¹, N S Hopkinson¹, J Moxham², A Simonds¹, M I Polkey¹

¹ Respiratory Muscle Laboratory, Royal Brompton Hospital, London SW3 6NP, UK
² Respiratory Muscle Laboratory, Guy’s King’s and St Thomas’ School of Medicine, King’s College Hospital, London SE5 9PJ, UK

Correspondence to:
Dr A Nickol
Osler Chest Unit, Churchill Hospital, Headington, Oxford OX4 7LJ, UK; annabel{at}medex.org.uk

Background: Nocturnal non-invasive ventilation (NIV) is an effective treatment for hypercapnic respiratory failure in patients with restrictive thoracic disease. We hypothesised that NIV may reverse respiratory failure by increasing the ventilatory response to carbon dioxide, reducing inspiratory muscle fatigue, or enhancing pulmonary mechanics.

Methods: Twenty patients with restrictive disease were studied at baseline (D0) and at 5–8 days (D5) and 3 months (3M).

Results: Mean (SD) daytime arterial carbon dioxide tension (PaCO₂) was reduced from 7.1 (0.9) kPa to 6.6 (0.8) kPa at D5 and 6.3 (0.9) kPa at 3M (p = 0.004), with the mean (SD) hypercapnic ventilatory response increasing from 2.8 (2.3) l/min/kPa to 3.6 (2.4) l/min/kPa at D5 and 4.3 (3.3) l/min/kPa at 3M (p = 0.044). No increase was observed in measures of inspiratory muscle strength including twitch transdiaphragmatic pressure, nor in lung function or respiratory system compliance.

Conclusions: These findings suggest that increased ventilatory response to carbon dioxide is the principal mechanism underlying the long term improvement in gas exchange following NIV in patients with restrictive thoracic disease. Increases in respiratory muscle strength (sniff oesophageal pressure and sniff nasal pressure) correlated with reductions in the Epworth sleepiness score, possibly indicating an increase in the ability of patients to activate inspiratory muscles rather than an improvement in contractility.

Abbreviations: ESS, Epworth sleepiness score; FEV₁, forced expiratory volume in 1 second; FRC, functional residual capacity; FVC, forced vital capacity; HCVR, hypercapnic ventilatory response; NIV, non-invasive ventilation; PaCO₂, arterial carbon dioxide tension; PaO₂, arterial oxygen tension; PeMax, maximum expiratory mouth pressure; Pga, cough gastric pressure; PiMax, maximum inspiratory mouth pressure; PtcCO₂, transcutaneous CO_2­ pressure; SNIP, sniff nasal inspiratory pressure; Sniff Pdi, sniff transdiaphragmatic pressure; Sniff Poes, sniff oesophageal pressure; TwPdi, twitch transdiaphragmatic pressure

Keywords: hypercapnic respiratory failure; neuromuscular weakness; kyphoscoliosis; non-invasive ventilation

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