Since during PSV the patient controls the respiratory rate and contributes actively to the ventilatory output, integrity and preservation of spontaneous neural activity are essential. The pressure support level and the patient-ventilator synchrony may greatly affect a patient's neural drive , therefore information concerning neural drive may be very useful in setting and monitoring PSV function.
To investigate the respiratory drive during PSV, many authors use the pressure drop developed during the first 100 ms following the onset of an expiratory occlusion (P0.1). P0.1 is widely accepted as an index of respiratory drive [8,26,27], though it is worth remembering that the patient's neural drive, and thus P0.1, are affected also by drugs, gas exchange, respiratory muscle function , and lung volume . Since the measurement of P0.1 could be limited by the need of special equipment, a simplified technique has been proposed . Automatic measurements implemented on many ventilators make P0.1 easily available in clinical settings .
As an alternative, the patient's respiratory drive and control of breathing may be inferred from output ventilatory variables such as Vt, RR, and mean inspiratory flow. The ratio of RR to Vt, named shallow breathing index (SBI), has been used as a predictor of weaning outcome . The combination of P0.1 with SBI has shown comparable sensitivity but better specificity as a weaning predictor than either P0.1 or SBI alone .
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