Annals of translational medicine
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A protective ventilation strategy is based on separation of lung and chest wall mechanics and determination of transpulmonary pressure. So far, this has required esophageal pressure measurement, which is cumbersome, rarely used clinically and associated with lack of consensus on the interpretation of measurements. We have developed an alternative method based on a positive end expiratory pressure (PEEP) step procedure where the PEEP-induced change in end-expiratory lung volume is determined by the ventilator pneumotachograph. ⋯ Thus, the most crucial factors of ventilator induced lung injury can be determined by a simple PEEP step procedure. The measurement procedure can be repeated with short intervals, which makes it possible to follow the course of the lung disease closely. By the PEEP step procedure we may also obtain information (decision support) on the mechanical consequences of changes in PEEP and tidal volume performed to improve oxygenation and/or carbon dioxide removal.
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Ventilator management of patients with acute respiratory distress syndrome (ARDS) has been characterized by implementation of basic physiology principles by minimizing harmful distending pressures and preventing lung derecruitment. Such strategies have led to significant improvements in outcomes. Positive end expiratory pressure (PEEP) is an important part of a lung protective strategy but there is no standardized method to set PEEP level. ⋯ Although there is some spatial and positional artifact, esophageal pressures in numerous animal and human studies in healthy, obese and critically ill patients appear to be a good estimate for the "effective" pleural pressure. Multiple studies have illustrated the benefit of using esophageal pressures to titrate PEEP in patients with obesity and with ARDS. Esophageal pressure monitoring provides a window into the unique physiology of a patient and helps improve clinical decision making at the bedside.
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Despite being a promising idea that combines several variables related to ventilator-induced lung injury (VILI), the concept of mechanical power (MP) carries a number of limitations, leaves several open questions, lacks proper modelling of positive end-expiratory pressure (PEEP) effects and, more importantly, does not respect the amount of lung tissue subjected to MP. First, the assessment of MP as a measure for development of VILI would have the highest relevance when volume displacement and related pressure changes are measured directly within the lung. Thus, ideally the relationship between MP delivered to the total respiratory system, and that delivered to lung tissue is discerned. ⋯ Fourth, in its current form, MP is modelled with a positive linear relationship with PEEP, which is based on incorrect mathematical modelling to integrate the role of PEEP into MP. Fifth, the present equation used to calculate MP is qualitatively in disagreement with clinical data on VILI. The reduction of MP to its elastic part, might not only result in a higher association with VILI, but also amplifies an indirect U-shaped relationship with PEEP.
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Editorial Comment
Ultrasound to assess diaphragmatic function in the critically ill-a critical perspective.
Ultrasound of the diaphragm in critically ill patients has become a diagnostic technique of emerging interest among clinicians and scientists. The advantages include that it is widely available, non-invasive and examination can be performed after relatively short training and at low costs. It is used to estimate muscle mass by measurement of muscle thickness and diagnose weakness by the assessment of diaphragm movement during unassisted breathing. ⋯ The enthusiasm that surrounds this topic is shared by many clinicians and we agree that ultrasound is a valuable tool to screen for diaphragm dysfunction in intensive care unit (ICU) patients. However, in our opinion much more studies are required to validate ultrasound as a tool to quantify breathing effort. More sophisticated ultrasound techniques, such as speckle tracking imaging are promising techniques to evaluate respiratory muscle function in patients, including the critically ill.