Technique and Validity of Sniff Airway Pressure and Sniff Diaphragm Thickening to Assess Diaphragm Function
CCCF ePoster library. Vorona S. Oct 28, 2015; 117368; P98 Disclosure(s): n/a
Stefannie Vorona
Stefannie Vorona
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Abstract
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Introduction: Maximal transdiaphragmatic pressure measurement is challenging to assess in mechanically ventilated patients due in part to the variability in maximal volitional effort. Sniff maneuvers have been employed to measure respiratory muscle strength; the utility of this approach has not been tested in ventilated patients. It is unknown whether sniff pressures generated at the airway are similar to sniff nasal and esophageal pressures. Bedside sonography is a non-invasive, feasible method for visualizing the diaphragm during a sniff inspiratory effort; it is unclear whether diaphragm thickening or excursion should be preferred to measure diaphragm function in mechanically ventilated patients. It is also unknown how such measurements are affected by inspiratory pattern.
Aim: To define the optimal technique for evaluating sniff airway pressure and the most valid sonographic method for assessing diaphragm function during maximal sniff maneuvers.
Methods: Airway (Paw), nasal (Pnas), esophageal (Pes) and gastric pressures (Pga) were measured along with surface electromyogram of the diaphragm and sternocleidomastoid muscles while healthy volunteers performed a series of different maximal inspiratory maneuvers (Müller and sniff maneuvers) under varying airway resistance loads (none, marked, complete occlusion) and elastance loads (semirecumbent and supine positions) while varying the relative contributions of rib cage and abdomen to inspiratory motion. Diaphragm thickening fraction was measured using a 13 MHz transducer placed on the right chest wall in the zone of apposition and diaphragm excursion was measured using a phased array transducer, 3-5 MHz, below the costal margin at the right mid-clavicular line.
Results: Ten healthy subjects were enrolled. Sniff airway pressure closely approximated sniff nasal pressure (bias 5.6 cm H2O, limits of agreement -8.4 – 19.6 cm H2O, mean value -66 cm H2O) and sniff esophageal pressure (bias 3 cm H2O, limits of agreement -6 – 13 cm H2O) (Figure 1). Agreement was maximal when marked resistance and complete occlusion conditions were applied. The relationship between diaphragm thickening and esophageal pressure generation was similar across resistance and elastance conditions (R2=0.34, p=0.42 for interaction). By contrast, the relationship between diaphragm excursion and esophageal pressure generation varied significantly with resistance and elastance conditions (R2=0.40, p=0.003 for interaction). Thickening fraction measurement was unaffected by variations in position and applied resistance (p>0.05 for all comparisons), while excursion was reduced in the supine position and with increased airway resistance (p<0.001 for both effects). Diaphragm thickening fraction was significantly greater when abdominal motion predominated over rib cage motion during inspiration (p<0.001).
Conclusion: Pressure measured at the airway during an inspiratory sniff maneuver under high airway resistance is a valid measure of respiratory muscle function. Mechanical loading conditions do not affect diaphragm thickening fraction and its relationship to diaphragm force generation. By contrast, mechanical loading conditions significantly affect diaphragm excursion measurements. Sniff airway pressure and diaphragm thickening fraction measurements may enhance the assessment of diaphragm function in invasively mechanically ventilated patients, but variation in inspiratory pattern may confound these measurements.
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