Influence of inertance on respiratory mechanics measurements in mechanically ventilated puppies

The complete equation of motion for a single compartment model (SCM) includ es an inertance term to describe pressure changes in phase with acceleratio n, as well as terms for resistance and elastance. Inertance has traditional ly been excluded from the model when measuring respiratory mechanics at con ventional ventilatory frequencies in mature respiratory systems. However, t his omission has been questioned recently for measurements of respiratory m echanics in intubated infants where higher ventilation frequencies and smal ler tracheal tubes are the norm. We investigated 1) the significance of ine rtance in an immature respiratory system during mechanical ventilation, and 2) the effect of omitting it from the model on estimates of respiratory me chanics. Six anesthetised, paralysed and mechanically ventilated puppies (2 .6-3.9 kg) were studied. A SCM, including an inertance term was fitted to m easurements of flow and airway opening (P-AO) or transpulmonary (P-TP) pres sure using multiple linear regression to estimate respiratory system and lu ng resistance (R-RS, R-L), elastance (E-RS, E-L) and inertance (I-RS, I-L) respectively, at various ventilation frequencies (0.2-2 Hz). Data obtained at each ventilation frequency were also fitted with a similar model without the inertance term. Inertance contributed significantly to the model at frequencies greater tha n approximately 0.3-0.5 Hz (20-30 breaths per minute), with I-RS dominated by the lung. The importance of including the inertance term in the model in creased as ventilation frequency increased. Exclusion of inertance from the model led to underestimation of E-RS and E-L, but no errors in estimates o f R-RS or R-L. The errors increased with ventilation frequency to similar t o 10-20% for E-RS and -10-40% for E-L at 2 Hz. While inertance contributed significantly to the SCM at ventilation frequencies typically required to m aintain normal gas exchange in puppies, the errors from excluding this term were small: <3% for E-RS and <9% for E-L. Pediatr Pulmonol. 1999; 28:130-1 38. (C) 1999 Wiley-Liss. Inc.