Recruitment manoeuvres for patients with acute respiratory distress syndrome
thesisposted on 26.05.2017, 07:58 authored by Hodgson, Carol Lynette
Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are inflammatory conditions resulting from direct or indirect lung injury that affects over 11,000 people annually in Australia1-2. Mortality associated with ARDS is high (30-41%)1,3. Ninety percent of people affected by ARDS require mechanical ventilation to maintain gas exchange during the critical phase of the condition. A protective mechanical ventilation strategy characterized by low tidal volume and limitation of plateau airway pressure (Pplat) is now widely accepted4-8. However, this strategy may fail to expand the most dependent lung regions and inadequately reduce cyclic opening and closing of atelectatic alveoli. Both of these effects may contribute to the progression of lung injury. Recruitment manoeuvres (RMs) may have an important role in the management of ventilated patients with ARDS and ALI by opening collapsed alveoli during low tidal volume ventilation9-11. Recruitment manoeuvres used with PEEP may recruit dependent lung regions with prolonged collapse and reduce alveolar derecruitment and shear forces across the alveoli, resulting in reduced barotrauma, atelectrauma and biotrauma12. However, little information is available regarding the most effective type of RM to use for patients with ARDS. The aim of the research detailed in this thesis was to investigate the safety and short-term effectiveness of a new staircase RM in patients mechanically ventilated with ARDS and to establish the longer term effects of a staircase RM on patients with ARDS compared to current best practice. A Cochrane review of the effects of RMs, that identified seven relevant randomised trials, concluded that RMs transiently improved oxygenation in patients with ALI without adverse effects of barotrauma or hypotension. There was substantial heterogeneity in methods used to deliver RMs, including peak pressure, time at maximum pressure, concurrent ventilatory strategies and end PEEP levels. The most common RM used in protective ventilatory strategies was a static RM of 40 cm H2O pressure for 40 seconds. There was no long term benefit to patients with ALI of a static RM in a protective ventilation strategy, perhaps because the static RM was not performed for an adequate time or with adequate pressure to open collapsed alveoli in patients with ALI. The staircase recruitment manoeuver (SRM) was examined in an observational study of 20 patients with ARDS to evaluate the safety of the SRM13. Eighty percent of participants responded to the SRM with improved shunt fraction (36.3 ± 10% at baseline to 26.4 ± 14% after the SRM, P<0.01). In addition, desaturation during the SRM, a marker previously thought to imply non-response, did not indicate a failure to respond at the end of the SRM or one hour later. Two investigations were undertaken to facilitate selection of outcome measures for assessing response to the new SRM. The accuracy of oxyhaemoglobin saturation using a finger probe was compared to a forehead probe in patients with ARDS who may have compromised peripheral circulation that could affect accurate reading of oxyhaemoglobin saturation (SpO2) from a finger probe14. Oxygen saturation measured using a forehead probe was less reliable that finger probe during periods of low oxygen saturation and high positive end expiratory pressure (PEEP). Finger probes were employed in subsequent experiments in favour of forehead probes. In a second study, the reliability and validity of digital chest X-rays as an objective measure of lung area and radiolucency in intensive care was assessed. If digital chest X-rays were informative, they would provide a safe and cost-effective method for monitoring lung recruitment compared to the current best practice of using a CT scan to measure lung volume. The lung area score was found to be adequately reliable and valid while the radiolucency score was determined to be useful as an adjunctive but not for the primary assessment of patients with ARDS. Finally a randomised controlled trial was conducted to assess the effects of the SRM in a protective ventilation strategy. Twenty patients with ARDS were randomised to conventional “best practice” ARDS ventilation according to the ARDSnet15 recommendations or to PHARLAP ventilation (permissive hypercapnia, alveolar recruitment and low airway pressure)16. Lung compliance and oxygenation were significantly better and inflammatory cytokines reduced in the PHARLAP group compared to the comparison group across a 7 day period. Those receiving the PHARLAP strategy had, on average, reduced time on mechanical ventilation, in ICU and in hospital, however this did not reach statistical significance. The SRM appears to be safe and can be effectively incorporated into a protective ventilation strategy in patients ventilated in intensive care with ARDS. The effects of the PHARLAP strategy on important outcomes such as length of mechanical ventilation and hospital stay warrant further investigation in larger trials.