April 08, 2014

Ambulatory Mechanical Ventilation - the future is now!

Dr. William Sexauer is a Clinical Professor of Medicine in Thomas Jefferson University's Division of Pulmonary and Critical Care Medicine and he serves as the Director of Critical Care Services at Methodist's Hospital campus.

Reference:

Porszasz J, Cao R, Morishige R, van Eykern LA, Stenzler A, Casaburi R.  Physiologic Effects of an Ambulatory Ventilation System in Chronic Obstructive Pulmonary Disease. 2013;188(3):334-342.

Experienced pulmonologists have all faced the frustrating quandary of what more to offer their most severe COPD patients.  Pharmacologic therapies are maximized, supplemental oxygen is already prescribed, pulmonary rehabilitation (if the patient enrolled and completed the program, which may or may not be available locally) completed, yet the patient remains markedly impaired in their ability to carry out activities of daily living.  “If only the patient could wear their BIPAP all the time…” you think. BIPAP, of course, is too big, heavy, uncomfortable, and impractical for everyday ambulatory use, even though it has been shown to improve exercise tolerance for very severe COPD patients in experimental settings. 

A recent study published in the American Journal of Respiratory and Critical Care Medicine by Porszasz and colleagues, however, introduces a novel assist device that appears more practical for use with ambulatory COPD patients. This noninvasive open ventilation system (NIOV), the BT-V2S system from Breathe Technologies Inc., Irvine, CA, is a battery powered device that weighs about one lb, clips onto the belt, and provides tidal volume assist driven by compressed gas from a typical portable oxygen cylinder.  The non-sealing nasal pillow interface uses the Venturi principle to augment tidal volume by entraining room air along with an inspiration-triggered oxygen “bolus”.  The actual TV assist with each breath varies based on a number of factors but can augment TV up to 250 ml; the average TV assist in this study was 142 ml.

These investigators  assessed  endurance time in 15 male subjects with severe COPD (mean FEV1 = 32 % predicted) and known exercise-induced oxygen desaturation  during constant-work-rate exercise studies performed at 80% of maximum work capacity under 4 conditions:  1) breathing room air; 2) breathing oxygen via nasal cannula; 3) NIOV + compressed room air; and 4) NIOV + compressed oxygen.   Endurance time with NIOV + compressed O2 (17.6 minutes) was superior to any of the other 3 conditions: room air (5.6 mins), O2 via nasal cannula (11.4 mins), and NIOV + compressed air (6.3 mins).  Respiratory muscle EMG and oxygen saturation were superior for NIOV + O2 compared to the other 3 conditions.  Borg-scale assessed dyspnea was also reduced, though not significantly for the O2 via nasal cannula comparison. Transcutaneous Pco2 measures did not significantly change across the 4 conditions.

The physiologic mechanism of benefit seems to be primarily improved oxygenation, even when compared to oxygen via NC alone.  Interestingly, improved oxygenation occurred despite lack of evidence of increased ventilation or decrease in CO2.  Higher oxygen saturations have been shown to improve exercise tolerance even in subjects without oxygen desaturation.  Improved oxygenation may decrease carotid body chemoreceptor activation, slow respiratory rate, and thereby decrease dynamic hyperinflation.  This would improve respiratory muscle mechanical efficiency and decrease their activation, as seen in this study.  Whether other mechanisms contribute to improved exercise tolerance are speculative and require further study.

Though the precise mechanism(s) underlying improvement may not be clear, the benefits appear unequivocal.  Among currently available interventions for patients with advanced COPD and exercise intolerance, this degree of improvement is rivaled only by pulmonary rehabilitation, with the latter requiring 6-12 weeks of progressive exercise that some subjects cannot or will not complete.    This device has the potential to greatly improve the daily activity and functioning levels of the most impaired COPD patients.  Practicality will be patient specific—it is estimated that assistance duration will be between 1 and 12 hours depending on factors such as patient’s respiratory rate and the volume of oxygen provided.   In fact, the greatest benefit of this device may prove to be within the setting of pulmonary rehabilitation programs; patients who are able to exercise longer and/or at a higher work level may achieve a greater  training effect, further augmenting independent ambulation.  Whether this device has the potential for similar benefit in pulmonary diseases other than COPD remains to be seen.

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Jefferson