Acute Cholecystitis Dr Swapnil Pawar
Neuromuscular Blockade in Intensive Care – To Be or Not To Be Dr Swapnil Pawar
Hyperkalemia Dr Swapnil Pawar
Transfusion Thresholds in Intensive Care Dr Swapnil Pawar
Driving Pressure Vs Tidal Volume in ARDS Dr Swapnil Pawar
Recently there has been a growing interest in driving pressure as a significant parameter for titration in mechanical ventilation for ARDS patients. It has challenged our dogma of 6-8 mls/kg tidal volume ventilation. In this podcast, we discuss the current evidence and future of mechanical ventilation in ARDS.
Our understanding of mechanical ventilation in ARDS has evolved significantly since the year 2000. The key concepts that have been accepted in practice include low tidal volumes, low plateau pressure, lower driving pressures and reduced frequency of injurious strain cycles. The main focus is to avoid ventilator-associated lung injury. With the recent interest in driving pressure, the question has surfaced whether we should target low tidal volumes and monitor plateau pressure or whether we should titrate other parameters based on driving pressures.
What is Driving Pressure? What is the physiological significance of Driving Pressure?
Prevention of ventilator-induced lung injury has typically revolved around the use of tidal volumes of 5–8 ml/kg of predicted body weight and limitation of plateau pressures to 30 cm H2O largely based on the ARDS-net trial of nearly 20 y ago. However, the lung available for ventilation is significantly reduced and highly variable in patients with ARDS. Hence, the use of tidal volumes based on the predicted body weight leads to variable lung stress, proportionate to the extent of lung involvement.
The airway driving pressure represents the stress applied to the lungs. It is measured at the bedside as the difference between the plateau pressure (Pplat) and the positive end expiratory pressure (PEEP). It effectively denotes the tidal volume based on lung compliance.
Driving pressure = Pplat – PEEP
Compliance = tidal volume/Pplat – PEEP
Thus, Pplat – PEEP (driving pressure) = tidal volume divided by lung compliance
Why we titrating to Driving Pressure is better than low tidal volume?
Targeting the driving pressure effectively titrates tidal volume based on lung compliance in contrast to titration based on the predicted body weight. The use of a tidal volume based on predicted body weight does not take into account the considerable heterogeneity in lung involvement in patients with ARDS. Considering the relatively small size of the functional lung in ARDS, tidal volumes titrated to the lung available for ventilation, represented by compliance, maybe physiologically more appropriate. The driving pressure is expected to be high in a poorly recruited lung with low levels of PEEP; it will also rise if the applied PEEP is too high, with overdistension of the lungs. In other words, there is an optimal level of PEEP at which the driving pressure is lowest for a given tidal volume, wherein the lung compliance is optimal. The application of an ideal level of PEEP may be expected to result in optimal lung recruitment, and thus, reduce driving pressures.
The driving pressure needs to be considered in the context of PEEP, as PEEP sets up the platform pressure upon which the driving pressure acts. To increment pressure by a given amount, greater force is necessary when starting from a higher level of pressure. So with PEEP more inflation energy is required, as higher pressure is required to achieve a given tidal volume which may exceed the threshold pressure of inciting damage.
Pressures cannot gauge lung injury risk. The frequency with which high-risk cycles are applied determines the intensity of potentially damaging energy applications.
What’s the evidence in favour of using Driving pressure as the key titration parameter in ARDS patients?
Studies on animal models have shown that tissue damage may be more dependent on the amplitude of cyclical stretch in contrast to the level of maximal stretch (2). Lung tissue may withstand sustained stretching without injury. Driving pressures of less than 20 cm H2O was employed as part of a lung-protective strategy that resulted in a significantly lower 28-d mortality in an early randomized controlled trial (3).
Amato et al. retrospectively analyzed data from 3562 patients from nine randomized controlled trials using a special statistical methodology called multilevel mediation analysis. In this study, the driving pressure was the strongest predictor of survival. The important findings of this study were: 1. For similar PEEP levels, mortality was higher with increasing driving pressures. 2. When the driving pressure remained constant, an increase in the PEEP level did not result in higher mortality, despite higher plateau pressures. 3. Importantly, even when the plateau pressure levels remained similar, the mortality decreased with lower driving pressures. This was probably because increasing PEEP levels improved lung recruitment and compliance, thereby allowing lower driving pressures for similar tidal volumes (4).
A meta-analysis of four studies including 3,252 patients, revealed significantly higher mortality with higher driving pressures. The median upper limit of driving pressure in this meta-analysis was 15 with an inter-quartile range of 14–16 cm H2O. A sensitivity analysis of three studies that used similar driving pressure limits (13–15 cm H2O) also revealed a similar effect on mortality (5). A hospital-based registry study analyzed patients who underwent non-cardiothoracic surgery under general anesthesia with endotracheal intubation. On multivariable regression analysis, lung-protective ventilation was associated with a reduced risk of respiratory complications in the postoperative period. Driving pressure had a dose-dependent association with postoperative pulmonary complications in this study (6). Thus, there is a strong signal from largely observational data that suggests that it may be the driving pressure that really counts, in contrast to the conventionally accepted targets of tidal volume and plateau pressures.
Driving Pressure = Pplat – PEEP
Compliance = Vt /Driving Pressure
Driving Pressure = Vt /2C
Total Inflation Pressure = P res + P el
P total = Flow * Resistance + Driving Pressure + PEEP tot
Energy = P total * Volume = V * [Flow*R + Vt /2c + PEEP total]
VILI is directly proportional to power transferred from ventilator to lungs.
Driving Pressure underestimates the ventilation risks as it does not measure viscoelastance i.e Pinsp – P Plateau and does not account for undetected inspiratory muscle tone.