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Influence of Different Trigger Techniques on Twitch Mouth Pressure During Bilateral Anterior Magnetic Phrenic Nerve Stimulation^*

^* From the Department of Pneumology, University Hospital Freiburg, Freiberg, Germany.

Correspondence to: Wolfram Windisch, MD, Department of Pneumology, University Hospital Freiburg, Killianstrasse 5, D-79106 Freiburg, Germany; e-mail: windisch{at}med1.ukl.uni-freiburg.de

Background: The trigger has a key role when assessing the twitch mouth pressure (Tw Pmo), since a "gentle" inspiratory or expiratory effort is needed for triggering to ensure an open glottis during twitch, but which also guaranties only very mild changes of transdiaphragmatic pressure following changes in lung volume.

Study objectives: To test if different trigger mechanisms cause different Tw Pmo values, if the predefined trigger criteria were accomplished, and if the breathing maneuver during triggering can influence the Tw Pmo.

Design: Experimental study.

Setting: Respiratory muscle and lung function laboratory of a university hospital.

Participants: Twenty healthy men (mean age, 25.6 ± 1.2 years [± SD]; mean FEV₁, 105.9 ± 11.5% of predicted).

Measurements: Tw Pmo produced by bilateral anterior magnetic phrenic nerve stimulation was measured using an inspiratory flow trigger (40 mL/s), an inspiratory pressure trigger, and an expiratory pressure trigger (3.75 mm Hg). All trigger criteria were controlled.

Results: Unusable pressure-time curves occurred in 40% during expiratory triggering, but not during inspiratory triggering. For inspiratory pressure (flow) triggering, 10% (30%) of the predefined trigger criteria were exceeded by 50%, indicating that a "gentle" inspiratory effort was not warranted. The Tw Pmo was higher during inspiratory compared to expiratory triggering (analysis of variance, p < 0.05). The Tw Pmo during inspiratory pressure and flow triggering were comparable and significantly correlated (r = 0.70, p < 0.0001). The time between start of inspiration and trigger release, and the pressure-time product during that period ranged widely, but this could not predict the Tw Pmo (multiple linear regression).

Conclusions: The trigger technique influences the Tw Pmo with higher values during inspiratory compared to expiratory triggering. Expiratory triggering more often produced unusable pressure-time curves. Inspiratory flow and pressure triggering is comparably useful in healthy subjects, but this might be different in patients. The trigger criteria need to be controlled to warrant a gentle breathing effort.

Key Words: bilateral anterior magnetic phrenic nerve stimulation • diaphragm • inspiratory muscle strength • maximal inspiratory mouth pressure • phrenic nerve stimulation • respiratory muscle testing • sniff pressure • trigger • twitch pressure

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