It was observed in this study that DP×RR index was greater in the patients who failed weaning comparable to those who were successfully extubated from MV. In contrast with the most accurate parameter RSBI [3], our index showed similar sensitivity and higher specificity. Even RSBI has limitations, including mean RSBI value less than 105 in the failure group [12]. Moreover, RSBI requires dynamic monitoring of tidal volume during SBT, which is not convenient.
Imbalance between respiratory energy load and muscle function contribute to failure of ventilation weaning. Notwithstanding, several mechanisms may be involved, namely, alteration in airway resistance, respiratory system compliance, intrinsic positive end-expiratory pressure, and increased additional work (mechanism or intubation) are associated with excessive respiratory load. The alternative contribution of each factor affects the energy load. Actually, pressure, Vt and RR are considered as patient’s respiratory power which is affected by excessive respiratory load.
Tidal breath starts from the potential energy corresponding to PEEPtot and inflates the end-inspiratory value. The respiratory energy load of each tidal inflation can be kinetic (resistance and tidal elasticity) and potential (PEEPtot). This energy defines the product of pressure applied while the volume change quantifies the work accomplished. Power is product of inflation energy per cycle while the number of cycles per minute which is defined as work per unit time, usually describes the intensity of energy application [13].
In this study, patients received the same Vt. Thus, pressure×RR could be referred to respiratory power of patients amidst being affected by excessive respiratory load. Three major components comprise the inflating pressure, viz., flow resistive, tidal elastic (DP), and starting pressure above the baseline value [14]. There are at least three different ways to calculate mechanical power (energy per breath times respiratory rate) with different degrees of complexity [15]. While, the pressure due to the flow is usually considered fully dispersed within the airways, this is not taken into account for the calculation of energy during tidal breathing [16]. Furthermore, for the two groups, PEEP was fixed at 0 cmH2O. Therefore, DP relating to incremental dynamic strain plays a key role in inspiratory energy. Actually, DP is defined as the amount of cyclic parenchymal deformation imposed on ventilated and preserved lung units [17], which is most accessible at the bedside, and may serve as a marker of cyclic lung strain [18]. Mathematically, Vt/system compliance (C) is the DP, i.e. the applied pressure above PEEP to deliver the Vt.[19]
On the other hand, it was found in this study that the DP×RR and DP index were significantly greater in the failure group compared with the success group. Pertinently, DP×RR had higher sensitivity and specificity than the DP to predict weaning failure. Noticeably, most of the patients who were ready to withdraw after meeting the standard had their high airway resistance being relieved. The main factors that affected the results of the withdrawal are cardiopulmonary and diaphragm functions. It was observed that when the patient changed from mechanical ventilation to SBT, the compliance of thorax or lung was poor, while the elastic load and the driving pressure of the patient increased but the evacuation failed. In addition, when the patient's primary disease did not improve, the respiratory load was high, or with poor heart and diaphragm functions. Besides, during SBT, the patient's respiratory frequency accelerated, indicating that the respiratory endurance was not enough to balance the patient's respiratory load. As a result, the patient could not maintain a stable breath, while the final result was also a failure to withdraw the machine.Weaning is a complex process involving transition from ventilatory support to spontaneous breathing. Any single parameters cannot guide the weaning well. A more global perspective which combines DP and RR does not only reflects respiratory system compliance, but also the capacity to perform work (energy) and the intensity of energy application (power). When the required pressure to produce the fixed volume is high, work rate is increased, or when RR is increased, work per unit time is high coupled with high power, indicating that the patient is more likely to fail to withdraw from mechanical ventilation.
In this study, some few limitations were identified. Different etiologies were studied together, which thereby increased the heterogeneity of the study. Despite this, the DP×RR was significantly greater in the failure group compared with the success group. There are also limitations of the study generalizability as this study did not include obese patients (a growing part of the population), patients with "organ failure" (many ICU patients), and was still a single center study. In addition, the SBT failure was included in the definition of machine withdrawal failure, thus the result was greatly affected by RR index in SBT. The observation of weaning outcomes does not mean that an intervention targeted at achieving that profile will accurately determine the timing of weaning.