Tidal volume

In otherwise healthy individuals, large tidal volumes can be given without generating high pressures. Therefore delivered tidal volumes of 10 to 12 ml/kg of lean body weight are usually appropriate. Obese patients do not have larger lungs, and the tidal volume delivered to a patient with a reduced number of available units must be reduced accordingly (e.g. acute repiratory distress syndrome, pneumonectomy, interstitial fibrosis). Even higher tidal volumes may be needed to satisfy the demands of a hyperpneic subject with normal ventilatory mechanics. Monotonous shallow breaths (< 6 ml/kg) encourage microatelectasis unless interrupted periodically by larger inflations (sighs) or offset by PEEP. As pressure builds during inspiration, a fraction of the inspired gas is stored in tubing and other compressible elements of the ventilator circuit (internal reservoirs, filters, humidifiers, etc.). Although a typical value for such compression losses is 3 to 4 ml/cmH 2O of peak system pressure, compression volume varies with ventilator type and the length, diameter, and composition of the tubing. Under conditions of controlled ventilation, the discrepancy between set or measured inspiratory tidal volume and exhaled tidal volume can quantitate the severity of a bronchopleural fistula.

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