The Science Journal of the American Association for Respiratory Care

2001 OPEN FORUM Abstracts

Determining if thelower inflection point on a pressure volume curve can be accurately estimatedby using a simple volume maneuver

John Boynton Jr. RRT, DeanHolland RRT, Kenneth S. Hawkins RRT, Grant O?Keefe MD MPH, Departmentsof Respiratory Care and Surgery, Parkland Health and Hospital System and UTSWMC,Dallas, Texas

Introduction: This work investigatesthe accuracy of a simple volume maneuver to estimate the lower inflection pointon a pressure volume curve. Recent literature suggests that the lower inflectionpoint indicates the first significant recruitment of new lung units. However,capturing pressure volume curves in the clinical arena is time-consuming, andrequires specialized equipment and skill set. Our experience shows that thelower inflection point corresponds with a reproducible narrow volume range.

Methods: Static P-V curveswere generated for seven patients with P/F ratio <200, average lung injuryscore of 3.0, and all having risk factors for ARDS. Patients with a historyof chronic obstructive pulmonary disease, heart failure, or the presence ofan air leak via any thoracostomy tubes were excluded. Static P-V curves werecaptured using two commercially available ventilators and the valve method describedby Estetter, et al (respiratory rate 60, tidal volume 50cc, I: E ratio 1:5,and FIO2 80-100%). Lower Pflex was calculated for each study condition usinga fitted logistic curve and commercially available statistical software (STATA).Plateau pressures were measured in each study participant using a two-secondinspiratory hold at set volumes of 1, 2, 3 and 4 ml/kg ideal body weight. Plateaupressure measurements were compared with the Pflex estimates from the staticpressure volume curve.

Results: The lower inflectionpoints ranged from 4 ? 23 cmH2O for the 7 patients. The plateau pressure measuredat the 2ml/kg inspiratory volume most closely approximated the lower inflectionpoint as estimated by the statistical software. The adjacent figure arrangesthe data for each study subject in graphical and tabular format.

Discussion: Our data suggeststhat in this patient population PEEP can be estimated by performing a staticP/V maneuver at 2 ml/kg ideal body weight. We believe this method may providea reproducible alternative to Pflex estimation as a method of identifying idealPEEP. This technique may allieviate the need to generate, record and interpretstatic pressure volume curves. Additional research is needed to establish the,practicality, reproducibility, safety, and clinical efficacy of the approach.


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