2007 OPEN FORUM Abstracts
THE EFFECTS OF PATIENT CIRCUIT DESIGN ON AUTOPEEP AND DELIVERED MINUTE VENTILATION
M. Lemin1, R. L. Chatburn1, J. K. Stoller2
AutoPEEP may contribute to hemodynamic instability, barotrauma, and increased work of breathing for patients ventilated for COPD. While there are many types of patient circuits, some are markedly different in design and may have mechanical properties that contribute to autoPEEP. The purpose of this study was to determine the effects of circuit mechanics on autoPEEP and delivered minute ventilation (MV) for a simulated COPD patient.
Methods: We assessed 3 different circuit designs (1) two limb circuit, Airlife; CardinalHealth, (2) divided circuit, Limbo;Vital Signs Inc., and (3) co-axial circuit, King F; King Systems Corp. Circuit resistance and compliance (total insp + exp) were calculated by regression from measured pressure (U tube manometer, Dwyer Instruments Inc), volume (10-60 mL; syringe) and flow (10-50 L/min; mass flow meter model 4000, TSI Inc.) Ventilation with each circuit was simulated with a Servo 300 (Siemens Inc.) and ASL 5000 lung model (IngMar Medical Inc.). Ventilator settings were volume control, tidal volume = 500 mL, insp time = 0.6 s, PEEP = 0. Lung model settings simulated an adult COPD patient; compliance = 60 mL/cm H2O, resistance = 26 cm H2O/L/s. MV was varied by changing rate from 10 to 40 breaths/min. AutoPEEP was measured using the exp hold feature on the ventilator. Set tidal volume was measured by the ventilator and delivered volume was measured by lung simulator. Set and delivered MV (rate x tidal volume) were calculated along with error (set â delivered). Experiments were repeated using two different circuits of each type.
Results: The Limbo and King circuits had similar pressure (P) vs flow (F) curves (P = 0.0033F^2 - 0.0031F + 0.26; P = 0.0023F^2 + 0.0389F - 0.24). The Airlife had much lower (linear) resistance; P = 0.02F - 0.2. The King and Airlife had similar compliances (3.4 and 2.6 cm H2O respectively) versus 1.3 cm H2O for Limbo. Mean values for autoPEEP and MV error are shown in the Figure. As expected, both autoPEEP and MV error increased as MV increased. Generally, the Limbo circuit created the most autoPEEP and the Airlife the least but the differences were small (less than 2 cm H2O up to 20 L/min). In contrast, the Limbo resulted in the least MV error (7%) compared to the other two circuits (14%).
Conclusions: Different circuit designs have measurable but probably clinically unimportant effects on autoPEEP for a simulated COPD patient. MV error may be clinically important and was related to circuit compliance.