The Science Journal of the American Association for Respiratory Care

2009 OPEN FORUM Abstracts

PREDICTION EQUATIONS FOR TIDAL VOLUME BASED ON THE RADFORD NOMOGRAM

Shannon E. Cook, Robert L. Chatburn; Respiratory Institute, Cleveland Clinic, Cleveland, OH

BACKGROUND: Selection of initial tidal volume for mechanical ventilation is often based on arbitrary “rules of thumb” such as 6 mL/kg ideal body weight. A more formal approach was developed by Radford (J Appl Physiol. 1955;7(4):451). However, this tool is probably underused because alignment nomograms are virtually obsolete. With the wide availability of microprocessors, we postulated that the Radford nomogram could be converted into a computer program to facilitate application, perhaps even incorporated into mechanical ventilators. The purpose of this study was to derive and evaluate mathematical models for predicting tidal volume based on patient weight and ventilatory frequency, which Radford never published. METHODS: First, the nomogram was divided into three sections: an infant, a pediatric and adult (male and female) based upon average weight (kg) for each type of patient. Using the nomogram, we selected all combinations of weight (W) and frequency (F) indicated by tick marks to predict the associated tidal volumes (V) so that each weight received several F and V to account for patients with a range of breathing patterns. Then, a linear regression equation was calculated for each patient type: infants, pediatric patients and both male and female adults. RESULTS: After an examination of the residual plot, the linear regression equations were determined to be the best fit. Equations are for V in mL, F in breaths/min and W in lb: - Infant (6-9 lb, F = 30-50): V = 7.160 - (0.265 * F) + (2.820 * W), R2= 0.98. -Pediatric (10-79 lb, F = 8- 25): V = 154.137 + (3.470 * W) - (6.861 * F), R2= 0.91. Adults (80-250 lb, F = 8 - 18), -Male: V = 466.969 + (2.4 * W) - (26.342 * F), R2= 0.97. -Female: V = 456.408 + (1.794 * W) - (22.716 * F), R2= 0.97. CONCLUSION: The high R2 values indicate that the equations were a good fit to the data from the nomogram. These equations provide a more scientific approach than the current “rules of thumb.” As such, they can easily be incorporated into programmable systems, like ventilators. The inclusion of information regarding frequency may offer the advantage of a more individualized tidal volume. Sponsored Research - None

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