The Science Journal of the American Association for Respiratory Care
Background: Spontaneous Ventilatory Parameters (SVPs) are an accepted part of the decision making process in discontinuing ventilation. The original validation literature used a consistent method for measuring SVPs. The patient is removed from the ventilator and minute ventilation (MV) measured with a cumulative meter. Frequency (f) is counted for that one minute period and the average tidal volume (Vt) calculated. No CPAP or pressure support (PS) is utilized. With the advent of mechanical ventilator digital measurement, some institutions now use the ventilator to collect SVPs. This method uses the breath-by-breath Vt and f displays to ?approximate? the traditional parameters. MV is calculated from those estimates. This survey was conducted to determine what SVP methods are currently being utilized at university teaching hospitals and how this compares to the original validation literature. Method: We surveyed the 72 respiratory care departments of the University Health System Consortium using a written questionnaire with written and telephone follow-up. From November 1997 to January 1998, a twelve question survey was sent to each Department Head. The survey questions focused on demographics, weaning techniques, and SVP methodology.
Results: Of the 72 surveys, 48 (67%) were returned and complete. Hospital sizes ranged from 110-1100 (mean 491) total beds with 11-120 (mean 59) ICU beds. Therapist Driven Protocols were used for at least some weaning in 69% (33/48) of the hospitals. The most commonly measured parameters were Vt (99%), f (96%), negative inspiratory force (NIF) (83%), vital capacity (VC) (81%), and MV (73%). Other measurements used less frequently were f/Vt ratio (27%), occlusion pressure (P100) (6%), and maximum voluntary ventilation (MVV) (2%). When measuring SVPs; 25% (12/48) of the hospitals always disconnect patients from the ventilator and measure using a meter, 44% (21/48) always use the ventilator display, 29% (14/48) use either method, and 2% (1/48) never monitor SVPs. Therefore, 73% (35/48) of the hospitals use the ventilator to measure SVPs at least some of the time. Of these, 46% (16/35) use some level of added CPAP and 20% (7/35) use some level of added PS. In hospitals in which SVPs are measured off the ventilator, they are done with the patient breathing room air in 96% (25/26) of institutions. Lastly, 85% (41/48) of the hospitals have no set period of time to wait before measuring SVPs. Of the 15% (7/48) that wait a set period of time, 5/7 wait one minute. Discussion: Both using SVPs to predict successful spontaneous ventilation and the process to collect that information have been well documented in the literature. This survey suggests that most hospitals currently use techniques that differ markedly from these established methods, at least for some patients. Vt measurement using the ventilator display may vary with each breath. This coupled with a f that is calculated on a rolling 12 second average (depending on the ventilator), may skew the calculated MV. In addition, the incorporation of CPAP and PS into SVP measurement has not been studied. Whether measurement variation and differences in the SVP process are clinically relevant is unknown, but both should be evaluated more fully. The specific parameters used continue to be consistent with earlier literature but only 27% of the hospitals utilize the f/Vt ratio, which has been found to be the most reliable index to predict weaning failure. Also, 96% of the group that measures SVPs off the ventilator do so on room air which could increase the number of desaturation complications.