Measurement of diaphragm loading during pressure support ventilation.
OBJECTIVE
The diaphragmatic pressure-time product (PTPdi) has been used to quantify the loading and unloading of the diaphragm. The validity of the relationship between PTPdi and diaphragm electrical activity (EMGdi) during pressure-support ventilation (PSV) is unclear. We examined this relationship.


METHODS
Physiological study in a physiology laboratory.


METHODS
Six healthy adults.


METHODS
Spontaneous breathing (SB) and two levels of PSV (6 and 12 cmH(2)O), breathing room air and incremental concentrations of carbon dioxide, sufficient to achieve an EMGdi signal of approximately 200% of baseline value.


RESULTS
We measured the electrical (EMGdi) and mechanical (PTPdi) activity of the diaphragm using oesophageal electrode and oesophageal and gastric balloon catheters. The relationship between EMGdi and PTPdi during SB was linear in five subjects and curvilinear in one. However, with PSV 12 cmH(2)O we observed that the relationship between EMGdi and PTPdi was 'left shifted'; specifically, for any given level of EMGdi the PTPdi was smaller with PSV 12 cmH(2)O than during SB. However, when PTPdi was converted to power (the product of pressure and flow) the tendency to left shift was largely reversed.


CONCLUSIONS
We conclude that when assessing of diaphragm unloading during PSV flow measurements are required. Where flow is constant, PTPdi is a valid measure of diaphragm unloading, but if not these data may be used to make an appropriate correction.
Fauroux
Brigitte
B
Respiratory Muscle Laboratory, Royal Brompton Hospital, Fulham Road, SW3 6NP, London, UK.
Hart
Nicholas
N
Luo
Yuan M
YM
MacNeill
Stephanie
S
Moxham
John
J
Lofaso
Frédéric
F
Polkey
Michael I
MI
D000403:Airway Resistance; D003964:Diaphragm / Q000502:physiology*; D004576:Electromyography / Q000379:methods* / Q000592:standards; D004947:Esophagus; D006801:Humans; D007441:Intubation, Gastrointestinal / Q000379:methods* / Q000592:standards; D016014:Linear Models; D008176:Lung Volume Measurements / Q000379:methods*; D008365:Manometry / Q000379:methods* / Q000592:standards; D011175:Positive-Pressure Respiration / Q000379:methods*; D012123:Pulmonary Ventilation; D015656:Respiratory Mechanics; D012146:Rest / Q000502:physiology; D012815:Signal Processing, Computer-Assisted; D013270:Stomach; D013990:Tidal Volume; D013997:Time Factors; D014939:Work of Breathing