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Decelerating flow shape for volume‐targeted mouthpiece ventilation

To the Editor: There is an increasing interest in the use of open-circuit mouthpiece ventilation (MPV) for the respiratory management of end-stage neuromuscular patients. However, recommendations and practical tips that could assist the clinician in the application of MPV are generally scarce. We report the case of a 73-year old gentleman who was diagnosed with nonbulbar amyotrophic lateral sclerosis 2 years earlier and has been using bilevel mask ventilation (BMV) during sleep ever since. During the previous 6 months, he extended the use of BMV into the daytime for a total of 16–18 hours per day. Although the patient was breathing comfortably while using BMV, he would quickly develop respiratory distress and recruit his accessory respiratory muscles during his off-ventilator time. At the time of evaluation, his vital capacity was 0.93 L (30%) and his sniff nasal inspiratory pressure 216 cm H2O. The patient was amenable to consider MPV for full time noninvasive respiratory support. We used a home ventilator (Astral 150, Resmed) with a single-limb circuit, an exhalation valve and an angled 15 mm mouthpiece. For initial settings, we chose a tidal volume (Vt) of 0.9 L with an inspiratory time (Ti) of 1.3 seconds, which were similar to the respective values attained when the patient was using BMV. PEEP and back-up rate were set to zero, inspiratory trigger to “high” and a rectangular flow shape was used. However, the patient developed respiratory distress immediately on being switched to MPV and started making use of his accessory respiratory muscles. These findings persisted even after Vt was gradually increased to 1.3 L. This trial ended prematurely as the patient became quickly fatigued and demanded to be switched back to BMV. For the second trial, we used the same initial settings, however this time we employed a decelerating flow shape, set to reach 25% of the peak flow at the end of the breath. Impressively, the patient responded immediately well this time, showing no signs of overt respiratory distress and making minimal use of his accessory respiratory muscles. During the following days, he continued using MPV for an increasing number of hours. Traditional volume-targeted ventilation delivers the chosen volume using a square flow shape. However, this mode is not always well tolerated by spontaneous breathing patients with high flow demands at the start of the breath. Nevertheless, new generation ventilators allow the practitioner to choose between various flow shapes in volume-targeted modes. When a decelerating flow pattern is selected, the same volume is delivered using a higher peak inspiratory flow to compensate for the decreasing flow during the rest of the breath. In our case, a Vt of 0.9 L, delivered over 1.3 seconds using a square flow shape, produced a constant flow of 40 L/min. Even when Vt was increased to 1.3 L, a square flow shape produced a flow of only 60 L/min. On the contrary, when a Vt of 0.9 L was delivered over the same inspiratory time with a decelerating flow shape, peak inspiratory flow was adjusted upward to a value of 80 L/min (based on ventilator data) with significant symptom improvement and palliation of breathlessness. An alternative strategy for flow augmentation would have been to decrease Ti, however this could lead to a steep rise in the airway pressure which is not usually well tolerated by neuromuscular patients. Nevertheless, according to some authorities, Ti should be initially set at least as high as 1 second [4]. Importantly, a decelerating flow shape has also been associated with decreases in respiratory system resistance, peak airway pressure and tidal volume-to-dead space ratio, an increase in static and dynamic compliance and a decrease in the work of breathing in critically ill patients; however direct extrapolation of these results to neuromuscular patients on MPV should be done cautiously. Although we cannot exclude the potential impact of a learning effect, we believe that this is a lesser possibility. Indeed, the first trial lasted only for a few minutes as the patient became profoundly distressed and demanded to be readily returned to BMV. Conversely, when the decelerating flow shape was tested, the patient performed immediately well without an intervening period of acquaintance. To conclude, we suggest that the use of a decelerating flow shape in neuromuscular patients on volume-targeted MPV might contribute to improved tolerance and decreased breathlessness, especially when flow demands are high. The use of this strategy might enhance the practitioner’s versatility in the ventilatory management of these patients.