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In comparison with the outcome figures in the articles, our incidence of complications of pneumothoraces (PTX), intraventricular hemorrhage (IVH) and retinopathy of prematurity (ROP) were significantly less. As explained in our original article3 and subsequently shown by others, pressure and flow triggered systems perform suboptimally in infants less than 1500 g. While bench testing may suggest an adequate response time, clinical practice indicates that these systems are compromised by the following: (1) chest wall and lung compliance, (2) airway resistance, (3) leak around the endotracheal (ET) tube, (4) ET tube resistance, and (5) systems compliance. The trigger delay may be aggravated by each of these factors, especially in the very low birth weight infants.
We believe that the inability of the patient to terminate the insufflation of gases at the onset of exhalation leads to increased intra thoracic pressure and even intra cranial pressure. Thus, if there is trigger delay as postulated above, the ventilator continues to force gases into the infant during the expiratory phase causing active exhalation and with consequent deleterious effects.
The system used in our unit is triggered by modified impedance technology. Peak detectors within the system detect onset of inspiration and exhalation with sensitivity and rapidly. Furthermore, since the sensitivity depends on the rate of change of impedance, it is more sensitive when applied to very low birth weight infants or with increased rate of respiration. This may explain the marked difference in outcome, compared with the pressure triggered system, as shown by the application of the system in 1701 infants weighing less than 1500 g over ten years. There were 1270 infants in the same group weighing less than 1250 g. The only problem we have encountered is that of some cardiorespiratory monitors are incompatible with the triggering device. The signal processing through the monitors is crucial to the optimal performance of the respiratory analog input signal to the trigger/terminator. Prototypes of the system were used initially but since 1993, the commercially available system (Sechrist SAVI, Sechrist Industries, Anaheim, CA, USA) was used exclusively.
In large multicentre studies, derivation of consensus and consistent application of a standardised “conventional ventilation” protocol is very difficult. This may skew some of the outcome data. Perhaps the limitations of flow and pressure triggered systems need to be considered prior to abandoning triggered systems in the respiratory support of newborns. Active exhalation predisposes some of these infants to the complications cited. The incidence of ROP in our experience is less than that reported in the literature. Possibly the same mechanism described above also predisposes the infants to ROP.
Given all of the above, further studies and analysis may be prudent. Such studies of patient triggered ventilation should also incorporate the capability of patient terminated ventilation.
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