Lu Chen,1,2 Guang-Qiang Chen,1,2,3
Kevin Shore,1 Orest Shklar,4 Concetta
Martins,4 Brian Devenyi,4 Paul Lindsay,4
Heather McPhail,4 Ashley Lanys,2 Ibrahim
Soliman,1 Mazin Tuma,1 Michael Kim,1
Kerri Porretta,4 Pamela Greco,4 Hilary
Every,4 Chris Hayes,1,2 Andrew Baker,1,2
Jan O. Friedrich,1,2 and Laurent Brochard
Abstract
Background
Despite their potential interest for clinical management, measurements of respiratory mechanics in patients with acute respiratory distress syndrome [ARDS] are seldom performed in routine practice. We introduced a systematic assessment of respiratory mechanics in our clinical practice. After the first year of clinical use, we retrospectively assessed whether these measurements had any influence on clinical management and physiological parameters associated with clinical outcomes by comparing their value before and after performing the test.
Methods
The respiratory mechanics assessment constituted a set of bedside measurements to determine passive lung and chest wall mechanics, response to positive end-expiratory pressure, and alveolar derecruitment. It was obtained early after ARDS diagnosis. The results were provided to the clinical team to be used at their own discretion. We compared ventilator settings and physiological variables before and after the test. The physiological endpoints were oxygenation index, dead space, and plateau and driving pressures.
Results
Sixty-one consecutive patients with ARDS were enrolled. Esophageal pressure was measured in 53 patients [86.9%]. In 41 patients [67.2%], ventilator settings were changed after the measurements, often by reducing positive end-expiratory pressure or by switching pressure-targeted mode to volume-targeted mode. Following changes, the oxygenation index, airway plateau, and driving pressures were significantly improved, whereas the dead-space fraction remained unchanged. The oxygenation index continued to improve in the next 48 h.
Conclusions
Implementing a systematic respiratory mechanics test leads to frequent individual adaptations of ventilator settings and allows improvement in oxygenation indexes and reduction of the risk of overdistention at the same time.
Trial registration
The present study involves data from our ongoing registry for respiratory mechanics [ClinicalTrials.gov identifier: NCT02623192. Registered 30 July 2015].
Electronic supplementary material
The online version of this article [doi:10.1186/s13054-017-1671-8] contains supplementary material, which is available to authorized users.
Keywords: Pulmonary function test, Respiratory physiology, Esophageal pressure, Mechanical ventilation, Quality improvement
Background
Patients with acute respiratory distress syndrome [ARDS] present various degrees of impairment in respiratory mechanics and different physiological responses to a given level of positive end-expiratory pressure [PEEP]. Applying the same ventilator regimen to every patient would be inadequate and at times can be potentially harmful. For example, the potential benefits of high PEEP in terms of oxygenation improvement and alveolar recruitment should be balanced against the risks induced by high pressures, such as hemodynamic impairment and overdistention. In other words, one needs to individualize the PEEP level by evaluating both its safety and its effectiveness for a specific patient [1]. This requires the assessment of gas exchange, respiratory mechanics, and hemodynamic variables. Additionally, partitioning lung and chest wall mechanics can also help the individualization of ventilator settings.
Despite their potential interest for clinical management, neither airway pressure [Paw]-based respiratory mechanics nor esophageal pressure [Pes]-based lung and chest wall mechanics are systematically assessed in routine practice. This discrepancy can be explained by technical issues [2] in obtaining accurate transpulmonary pressure [PL], by a lack of standardized procedures, and by the challenges of integrating the results of these measurements into ventilatory management. Even worse, the most recent large observational studies on patients with ARDS showed that simple parameters such as plateau pressure [Pplat] were not measured in the majority of the patients [3].
To improve the integration of respiratory mechanics measurement in our clinical practice, a group of physicians and respiratory therapists [RTs] at our institution introduced a respiratory mechanics test to systematically assess respiratory mechanics [i.e., performing a pulmonary function test] for patients with ARDS and be implemented as a quality improvement [QI] program. The goal was to provide clinicians with relevant physiological assessment that could be helpful for clinical practice. Because the needs for adjusting ventilator settings can be very different, this program did not include clinical recommendations or specific guidelines associated with these measurements.
Having implemented this systematic test in our clinical practice for 1 year, we retrospectively tried to assess if it had any impact. We looked for whether any changes in ventilatory settings were performed. We also assessed whether the observed changes modified physiological variables known to be associated with mortality, and we tried to understand whether the observed changes were consistent with the measurements.
Methods
Design and settings
This is a retrospective study of the impact of a 1-year program [see below] with an aim of systematically evaluating respiratory mechanics in patients with ARDS by comparing the ventilator settings and relevant physiological variables before and after performing the measurements. The program was decided by the critical care department at a teaching hospital [St. Michael’s Hospital, Toronto, ON, Canada] and implemented in both the medical-surgical and the trauma-neurosurgical intensive care units [ICUs]. Of note, the measurements are entered into a registry for future studies.
Implementing the respiratory mechanics test in clinical practice
The procedure of the test was determined through discussion among ICU physicians, fellows, and RTs. A team of 22 users [3 physicians, 2 fellows, and 17 RTs] was recruited voluntarily to facilitate implementing the test in four aspects:
To increase awareness and understanding of monitoring respiratory mechanics, education sessions consisting of lectures, bench and bedside hands-on sessions, and feedback rounds were provided to ICU clinicians. The education sessions were focused on explaining the importance of measuring respiratory mechanics, the technical approaches for measurements, the physiological and clinical meanings of the measured variables based on scientific evidence, and the limitations of those variables. We did not propose to use one single parameter on which to base changes of the ventilator settings; we proposed to incorporate multiple variables [e.g., airway pressure, Pplat, driving pressure [Pdriv], chest wall component, recruitability, oxygenation, and hemodynamic response to PEEP] into the global history of the patient and let the clinical team decide what was best for the patient.
To standardize the procedures, we developed written protocols to guide esophageal catheter placement and the associated systematic measurements.
To simplify the calculations, we developed a custom-programmed Portable Document Format form [PDF; Adobe Systems, San Jose, CA, USA] to automatically calculate physiological parameters and generate a clinical report [see Additional file 1: Appendixes S1 and S2].
This clinical report was delivered to the caregivers in charge of the patients.
Patient enrollment process for the test
All patients admitted to the ICUs meeting the Berlin definition of ARDS [4] and receiving invasive mechanical ventilation were eligible. A daily screening was done, mostly on the weekdays. It was left at the discretion of the clinical team to decide to perform measurements, place esophageal catheters, and accept possible transient changes in sedation or paralysis [Additional file 1: Figure S1]. Esophageal catheter insertion was recommended when the ratio of partial pressure of arterial oxygen and fraction of inspired oxygen [PaO2/FiO2] was ≤200 mmHg. In the group of patients with mild ARDS [i.e., PaO2/FiO2 > 200 mmHg], catheters were placed at the discretion of the clinical team. In the following cases, the clinical team discussed the benefits of doing the measurements on a case-by-case basis: [a] severe hemodynamic instability [i.e., >30% increase in the dose of vasopressors in the last 6 h or need for >0.5 μg/kg/minute of norepinephrine]; or [b] a known esophageal problem, active upper gastrointestinal bleeding, or any other contraindication to the insertion of a gastric tube.
Measurements
Each patient enrolled underwent measurements of respiratory mechanics performed by one or two trained RTs and/or fellows [depending upon availability of clinicians] following a standardized protocol. More than 20 clinicians were considered as trained users. The patients were measured at the early stage of ARDS, and all of them were already deeply sedated and often paralyzed. Additional sedation with or without paralysis could be transiently necessary to suppress or minimize spontaneous breathing. This approach was accepted as part of our clinical practice to get reliable measurements of passive respiratory mechanics. Nevertheless, the decisions of deepening sedation and/or using paralysis for an individual patient were made at the discretion of the clinical team. The absence of spontaneous effort was confirmed by the absence of a negative Paw swing during a 3-second end-expiratory occlusion and by the presence of positive Pes swings during tidal breathing. Volume-controlled ventilation [VCV] was used during the measurements with a standardized tidal volume [VT] of 6 ml/kg of predicted body weight [PBW], a constant inspiratory flow of 50–60 L/minute, and 0.3-second pause at the end of inspiration. Respiratory rate [RR] was set to maintain a minute ventilation [VE] similar to premeasurement level. PEEP and the fraction of inspired oxygen [FiO2] were maintained at the clinically chosen level. Paw, airway flow, and airway volume were directly taken from the ventilator monitoring system. Pes was measured using a catheter with an air-filled balloon [CooperSurgical, Trumbull, CT, USA] with a pressure transducer connected to a bedside monitor [similarly to measuring central venous pressure]. Measuring Pes was recommended as a component of the test for patients with a PaO2/FiO2 ratio ≤200 mmHg, but the decision of placing a catheter was left at the discretion of the clinicians. The validity of the Pes was confirmed using an occlusion test during spontaneous breathing [premeasurement] or a positive pressure occlusion test by manually compressing the thorax during passive breathing [2, 5]. Elastance, resistance, and other derived variables were automatically calculated using the programmed PDF form. The process of conducting the measurements was as follows:
Paw-based respiratory mechanics were measured by using end-expiratory and end-inspiratory occlusions for 1–2 seconds. The absence of leakage during an end-inspiratory occlusion was confirmed by the equivalence of expiratory VT between the breaths with occlusion to the one without occlusion. Total positive end-expiratory pressure [PEEPtot], airway peak pressure [Ppeak], and airway Pplat were recorded. Intrinsic PEEP, Pdriv [Pplat − PEEPtot], respiratory system compliance, and resistance were then calculated automatically.
Pes-based lung and chest wall mechanics [2] were measured simultaneously using end-expiratory and end-inspiratory occlusions. Transpulmonary pressure at end expiration [PL,end-exp] and transpulmonary pressure at end inspiration [PL,end-insp], lung compliance, chest wall compliance, and the ratio of lung elastance to respiratory system elastance were calculated automatically. PL, unless specifically indicated such as elastance-derived transpulmonary plateau pressure, was calculated using direct measurement of Pes.
Oxygenation and hemodynamic responses to PEEP were assessed by increasing PEEP by 3–5 cmH2O [preferably 5 cmH2O] from the clinical PEEP level if the Pplat was