Change in End-Expiratory Lung Volume During Sleep in Patients at Risk for Obstructive Sleep Apnea.

STUDY OBJECTIVES: As lung volume decreases radial traction on the upper airway is reduced, making it more collapsible. The purpose of this study was to measure change in end-expiratory lung volume (EELV) following sleep onset and to evaluate the relationship between change in EELV and sleep-disordered breathing. METHODS: Twenty subjects underwent overnight polysomnography, of whom 14 (70%) had obstructive sleep apnea (OSA). Change in EELV was measured throughout the night using magnetometry. Sleep was staged and respiratory events scored using American Academy of Sleep Medicine criteria. An additional 10 subjects had change in EELV measured simultaneously by magnetometer and spirometer while awake. RESULTS: In the subjects studied while awake, change in EELV calculated from magnetometer data correlated very strongly (r = 0.89, P < .001) with that obtained by spirometry. In the 20 subjects who underwent polysomnography, there was a decline in EELV for sleep stages N1, N2, N3, and R (REM sleep); 17.9 ± 121.0 mL (mean ± standard deviation), 228.5 ± 151.8 mL, 198.1 ± 122.1 mL, and 316.7 ± 131.9 mL, respectively. Mean EELV reduction during stage R sleep doubled that noted during non-stage R sleep (316.7 ± 131.9 mL versus 150.9 ± 89.7 mL, respectively) (P < .001). The difference in EELV between non-stage R and stage R sleep inversely correlated with mean oxygen saturation (r = -0.56, P = .06). EELV reduction in individuals with moderate and severe OSA was greater than in those with mild SDB but did not reach statistical significance. CONCLUSIONS: Magnetometry provides a precise, unobtrusive, and continuous means to study lung volume changes during sleep. EELV declines from sleep onset, reaching its nadir during stage R sleep. The reduction in EELV in stage R sleep was associated with lower mean oxygen saturation but was not associated with greater sleep-disordered breathing.

A method for determining optimal mean airway pressure in high-frequency oscillatory ventilation.

BACKGROUND: "Optimal" mean airway pressure (MAP) during high-frequency oscillatory ventilation (HFOV) can be defined as the pressure that allows for maximal alveolar recruitment while minimizing alveolar overdistension. Choosing a MAP near or just below the point of maximal curvature (PMC) of the volume-pressure characteristics of the lung can serve as a guide to avoid overdistention during HFOV, while simultaneously preventing derecruitment. The purpose of this study was to assess whether optimal MAP at the PMC can be determined by using measures of PaO(2) in patients with acute respiratory distress syndrome (ARDS) undergoing HFOV. METHODS: We prospectively studied seven patients with ARDS who underwent HFOV after failed conventional ventilation. In addition, 11 healthy subjects were studied to validate measurements of changes in end-expiratory lung volume (∆EELV) using magnetometers. Using this validated method, plots of ∆EELV and MAP were constructed during decremental changes in MAP following a recruitment maneuver in seven ventilated patients with ARDS. The PMC was defined as the point where the slope of the ∆EELV versus MAP curve acutely changed. The MAP at the PMC was compared to that determined from plots of PaO(2) versus MAP. RESULTS: In the healthy cohort, measurements of ∆EELV obtained by magnetometry approximated the line of identity when compared to those obtained by spirometry. The MAP determined using either the ∆EELV or PaO(2) techniques were identical in all seven HFOV ventilated patients. Additionally, there was a significant correlation between the MAP associated changes in PaO2 and the MAP associated changes in ∆EELV (p < 0.001). CONCLUSIONS: The finding that MAP at the PMC is the same whether determined by measures of ∆EELV or PaO(2) suggest that bedside measures PaO(2) may provide an acceptable surrogate for measures of EELV when determining "optimal" MAP during HFOV.

MICU care delivered by PAs versus residents: do PAs measure up?

BACKGROUND: Data on the perfomance of physician assistants in a medical intensive care unit are scarce. OBJECTIVE: To compare clinical outcomes between patients admitted to a resident-run MICU and a PA-run MICU. METHODS: Retrospective analysis of prospectively collected MICU data was performed for 5,346 patients admitted to an MICU from January 2004 through January 2007; 3,971 patients were admitted to a resident-run MICU (resident group) and 1,375 to a PA-run MICU (PA group). RESULTS: The groups were relatively similar, though the resident group had a higher rate of renal insufficiency (25% vs. 22%, P = .05) and the PA group had a higher rate of cerebrovascular accidents (5.6% vs. 4%, P = .02). Hospital length of stay (LOS) was similar, with a median of 9 days in the PA group and 8 days in the resident group (P = .59). MICU LOS was slightly longer for the PA group: a median of 2.58 days (1.55-4.86) vs. 2.33 days (1.39-4.16) in the resident group. After matching by propensity score, we could not confirm this increased LOS. There was no difference in hospital mortality or in ICU mortality between the two groups either in uncontrolled or controlled analyses. Survival analyses showed no difference in 28-day survival between the two groups. CONCLUSION: A PA-run MICU has similar outcomes when compared to a resident-run MICU.