The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts

EXHALED TIDAL VOLUMES: VENTILATOR MEASURED VERSUS DELIVERED IN A SMOKE INHALATION INJURY MODEL IN SHEEP

Melissa Morales, CRT, Andrea L. Parrot, CRT, Fidanis Ndokama, CRT, Dennis A. Hastings II, CRT, Tristan M. Washington, CRT, Joshua R. Schuetz, RRT, MEd., Ronald P. Mlcak, PhD, RRT, FAARC, Daniel L. Traber, PhD, School of Allied Health Sciences, The University of Texas Medical Branch, Galveston, Texas, and the Shriner's Burn Hospital for Children, Galveston, Texas.

BACKGROUND: Ventilator displays of exhaled tidal volume (EVT) are traditionally used to indicate the delivered VT in critically ill patients. The need to know the delivered tidal volume is essential when ventilating critically ill patients to avoid the risk of lung injury, atelectasis, hypoxia and hypercapnia. We hypothesized that the EVT measured with a conventional ventilator at the expiratory valve would differ significantly from the EVT measured with a pneumotachometer placed between the airway and ventilator circuit.

METHODS: TwelveMerino ewes(33 kg) with acute lung injury induced by cotton smoke were ventilated in a volume control mode with a Servo 900 C ventilator (Siemens Corp.) Targeted VT settings were 15 ml/kg, which is the standard VT used in this animal model of injury. Comparison was made between the ventilator measured EVT and pneumotachometer-measured EVT (Bicore CP-100 Pulmonary Monitor). An additional comparison was made between the pneumotachometer-measured EVT and the calculated effective EVT. Outcome variables included actual EVT, EVT ml/kg, and Minute Ventilation (MV). Data were analyzed by ANOVA with repeated measured and students t-test, where appropriate. Significance was accepted at p< 0.05.

RESULTS: The mean EVT measured by the pneumotachometer was significantly less that the EVT determined by the ventilator (445 ± 71 ml verses 553 ± 90 ml, p=0.014). However, the EVT measured by the pneumotachometer was not statistically different from the calculated effective EVT (445 ± 71 ml verses494± 89, p=0.096). The mean EVT expressed in ml/kg showed a significant difference between the pneumotachometer-measured EVT and the ventilator EVT (13 ml/kg verses 17 ml/kg, p=0.001). Additionally, the mean expired MV showed a significant difference between the pneumotachometer-measured volume and the ventilator-measured volume (10.7 ± 4 liters/min verses 14.6 ± 4 liters/min, p=0.012).

CONCLUSION: The data demonstrate a significant discrepancy between EVT measured at the ventilator and that measured with a pneumotachometer.Our results suggest that for determining the actual volume delivered to the lungs EVT should be determined with a pneumotachometer placed between the airway and ventilator circuit.

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