1997 OPEN FORUM Abstracts
PRESSURE CONTROL VENTILATION TO STABILIZE MINUTE VENTILATION WITH AN AIRWAY LEAK: A CASE STUDY
Teresa Volsko, BS, RRT, Jim Martin, BS, RRT, CPFT, Joe Spyak, RRT, Todd Angle, RRT, Joel R. Peerless, M.D. Departments of Pulmonary Services and Critical Care Medicine, MetroHealth Medical Center, Cleveland, Ohio
INTRODUCTION: Leaks associated with artificial airways can complicate mechanical ventilation by altering minute ventilation (VE), contributing to instability of gas exchange. Variable leaks are problematic when they can not be corrected by the reinsertion of a larger artificial airway or head/body repositioning. Data from a model study (Respir Care 1996;41(8):728-735) suggest that pressure controlled ventilation (PCV) will maintain a more stable minute ventilation than flow controlled ventilation (FCV). No patient data have been published to support this hypothesis. CASE SUMMARY: A 68 year old white male motor vehicle accident victim with closed head and chest trauma was transported to our Level 1 trauma center. The patient was immobilized on a back-board with C-Collar, and required oxygen at 15 L/m by non-rebreathing mask. The patient was alert and oriented, without evidence of respiratory distress. Physical exam of the chest revealed abrasions along with subcutaneous emphysema extending to the neck and face. Breath sounds were diminished bilaterally and bilateral chest tubes were placed. Shortly thereafter he demonstrated difficulty breathing and profound hypotension. EKG showed an accelerated junctional rhythm with ST elevation consistent with a left inferior wall MI. Several attempts at elective oral intubation were unsuccessful and a cricothyroidotomy was performed, using a size 6.0 cuffed Shiley tracheostomy tube. The patient remained hemodynamically unstable with a metabolic acidosis. Mechanical ventilation was initiated and a high VE, (17 L/m) targeted to prevent worsening acidosis. The Nellcor Puritan Bennett 7200 ae ventilator provided flow controlled SIMV (descending ramp flow waveform) with rate of 18/min, Vt=0.9 L, PEEP = 10 cm H2O, FIO2 = 0.85, PS = 8 cm H20. A variable airway leak contributed to unstable exhaled VE ranging from 14.0 - 17.0 L/m and peak inspiratory pressures (PIP) of 31 - 42 cm H2O. Within 12 hours, a continuous airway leak was noted resulting in a reduction in exhaled VE to 14 L/m. The leak was estimated to be 18%. Reintubation was not an option as there was significant airway trauma and edema visualized by direct laryngoscopy. C-spine immobilization and subcutaneous emphysema made initiatives for leak compensation by head and/or body repositioning difficult. The ventilator was switched to PCV with pressure limit of 33 cm H2O above the set PEEP of 10 cm H2O. This resulted in a rapid rise of VE to 28.9 L/min. Applied pressure was rapidly weaned to 28 cm H2O, achieving the target VE of 17 L/m. Reduction of pressure eliminated an audible leak. The patient eventually succumbed to cardiac compromise due to cardiac contusion. DISCUSSION: The literature defines massive airway leaks as > 10% of the tidal volume (Bernstein, Crit Care Med 1995;23(10)1739-1744). It is commonly believed that FCV with a descending ramp flow waveform simulates PCV, limiting airway pressure and improving gas exchange (Intens Care Med 1985;11;68-75). In this patient, PCV delivered more of the target VE than did FCV with a comparable PIP. This may be due to the fact that acutal (rather than simulated) pressure control can generate unlimited flow to "feed" a leak and maintain inflating pressure longer to yield higher tidal volumes. This clinical observation supports and broadens the scope of the theoretical data cited above. PCV may be preferable to FCV in patients with unstable ventilation associated with airway leaks.