1998 OPEN FORUM Abstracts
EFFECTS OF PATIENT RESPIRATORY MECHANICS ON THE UNLOADING OF INSPIRATORY MUSCLES IN PSV- A SIMULATION STUDY
Hong-Lin Du, MD, Newport Medical Instruments, Inc., Costa Mesa, CA92627
Purpose: To investigate how the patient respiratory mechanics (patient demand, resistance, and compliance) affect the efficiency of PSV in unloading the inspiratory muscle.
Method: A drive-dependent test lung model (Michigan TTL) was used to simulate spontaneous breathing. The driving compartment of the model was driven by a Bear-5 ventilator with sine-flow pattern. The compliance of the driving compartment was set at 200 ml/cmH2O (i.e., chest wall compliance) and the pressure in this compartment was therefore taken as the pressure of the inspiratory muscles (Pmus). Integration of the Pmus with volume provided the work of the inspiratory muscles (Wmus). The flow rate of the Bear-5 was adjusted to obtain a peak inspiratory flow rate (PF) at the airway opening of the dependent lung of 30 or 60 LPM when no ventilator was attached to the dependent lung, at the dependent lung compliance of 80, 40 or 20 ml/cmH2O with a resistor of Rp5 or Rp20. A Newport Wave E200 ventilator or Servo 300 ventilator was then attached to the dependent lung and PSV 10 cmH2O was applied with trigger sensitivity of -0.5 cmH2O. The Wmus when no ventilator (Wmus-0) was attached was compared that when PSV10 was applied (Wmus-p), and the percent unloading of the Wmus was calculated by (Wmus-0 - Wmus-p)/Wmus-0. The percent unloading of the Pmus was calculated in the same way as Wmus. Mean values from 3 breaths for each setting were calculated and presented because the variation among the 3 breaths was negligible.
Results: At the same patient demand level, unloading efficiency of PSV decreased as the resistance increased and/or the compliance decreased. At normal resistance and compliance with PF of 30 LPM, PSV10 unloaded 90% of patient inspiratory muscle work. However, when PF increased to 60 LPM, PSV10 only unloaded 40% of the patient inspiratory work. The inspiratory muscles was rarely unloaded at high resistance with high demand by PSV 10. There was no recognizable difference between the Wave E200 and Servo 300.
(See original for figure)
Conclusion: Although both ventilators provided nearly-square pressure waveform during PSV, the targeted pressure in these ventilators is the pressure at the Y connector (e.g., the E200) or at intra-ventilator (e.g., the Servo 300), instead of the pleural pressure. As the result, as the patient airway resistance increases (or the inspiratory demand increases / compliance decreases), the efficiency of PSV in unloading the inspiratory muscle decreases. From the work-unloading standpoint, PS level should be set individually based on the patient respiratory mechanics and inspiratory effort.
The 44th International Respiratory Congress Abstracts-On-Disk®, November 7 - 10, 1998, Atlanta, Georgia.