Pulmonary contusions due to thoracic trauma can potentially lead to acute respiratory distress syndrome.1 Injurious invasive mechanical ventilation (IMV) (tidal volume greater than 6–8 ml/kg predicted body weight (PBW)) can result in ventilator-induced lung injury (VILI).2 In patients with ‘injured lungs’, low tidal volume ventilation (<6–8 ml/kg PBW) has been shown to improve outcomes.3,4 It is therefore of paramount importance that ‘non-injurious’ ventilatory strategy is adopted in the high-risk thoracic trauma patient with pulmonary contusions. A recent French observational study (published recently in the Journal of the Intensive Care Society) showed a high rate of compliance with a low tidal volume invasive ventilation strategy in patients with severe blunt chest trauma.5 We sought to determine the rate of adherence to low tidal volume ventilation, and association between tidal volume delivered and patient-relevant outcomes in a cohort of mechanically ventilated patients with chest trauma in a UK tertiary trauma intensive care unit.
Following institutional approval, a cohort of 98 consecutive trauma patients with pulmonary contusions (between August 2016 and August 2017) requiring IMV on intensive care unit (ICU) were retrospectively reviewed. The primary end-point was rate of adherence to low tidal volume for the duration of IMV. Simple descriptive comparison between groups was undertaken either by chi-squared tests (categorical data comparison) or Wilcoxon Rank Sum (non-parametric continuous data). Univariable and multivariable logistic regression was used to construct a predictive model for non-adherence to low tidal volume ventilation.
Ninety-eight patients were included in the analysis. The median (IQR [range]) age (years) was 33.4 (24–48.4 [16.7–91.8]) with 85% of the population studied being male. The median tidal volume (ml) delivered in this cohort was 541 (504–589.5 [366–820]) and tidal volume (ml/kg PBW) was 8.0 (7.2–8.6 [5.2–13.4]). Fifty patients (51%) received tidal volume greater than 8 ml/kg PBW and only six patients (6.1%) received tidal volumes between 6 and 6.5 ml/kg PBW. Apart from two variables (height, blood transfusion during IMV) there were no statistically significant differences in baseline characteristics or clinically important outcomes (ICU and hospital length of stay (LOS), ICU mortality) between high (>8 ml/kg PBW) and low (6–8 ml/kg PBW) tidal volume IMV groups (Table 1). The between-group difference in LOS [regression co-efficient (95% CI)] and ICU mortality [adjusted odds ratio (AOR), 95% CI] remained non-statistically significant: ICU LOS −1.80 (95% CI: −8.44–4.83; p = 0.595), hospital LOS 10.30 (95% CI: −5.87–26.48; p = 0.212) and mortality 0.09 (95% CI: 0.00–3.59; p = 0.198). Following univariate analysis five factors relating to baseline characteristics (sex, height, body mass index, pre-existing lung disease and blood transfusion) as well as two respiratory parameters [having a partial pressure of arterial carbon dioxide (PaCO2) > 6 kpa, and positive end-expiratory pressure] were identified to be associated with ‘injurious’ ventilation (p < 0.2). However, following multivariate analysis only two of these factors [height (AOR: 0.94; 95% CI: 0.89–100; p = 0.038) and blood transfusion (OR: 2.81; 95 CI%: 1.07–7.40; p = 0.036)] remained significant (p < 0.05). The c-statistic for the primary end-point was 0.7847 (95% CI: 0.69–0.88) (Figure 1). The relationship (correlation) between tidal volume and height, body mass index and PaCO2 is shown in Figure 2.
Table 1.
Baseline characteristics of patients receiving < 8 and > 8 ml/kg predicted body weight.
| Baseline characteristics | Under 8 ml/kg PBW group (n = 48) | Over 8 ml/kg PBW group (n = 50) | p value |
|---|---|---|---|
| Age (years) | 32.9 (23.1–44.7 [17.8–91.8]) | 35.8 (25.7–51 [16.7–84]) | 0.4908 |
| Men | 43 (89.6%) | 40 (80%) | 0.188 |
| Height (cm) | 180 (170–180 [156–190]) | 170 (165–179 [143–188]) | 0.001 |
| Weight | 80 (70–87.5 [51–120]) | 80 (70–86 [45–130]) | 0.611 |
| Predicted body weight (kg) | 75.3 (70.7–75.3 [52.3–84.5]) | 66.2 (57–73 [36.3–79.8]) | <0.0001 |
| BMI (kg/m2) | 24.7 (22.7–27.8 [19.0–39.8]) | 26.0 (24.2–29.4 [15.6–63.6]) | 0.0852 |
| ISS score | 39.5 (29–45 [9–66]) | 36 (29–50 [11–66]) | 0.4044 |
| Pre-existing disease | 14 (29.2%) | 22 (44.9%) | 0.109 |
| Anticoagulation | 1 (2.1%) | 0 (0%) | 0.305 |
| Chronic lung disease | 2 (4.2%) | 1 (2.0%) | 0.545 |
| Extent of contusions (number of quadrants) | |||
| 0 | 1 (2.1%) | 0 (0%) | |
| 1 | 23 (47.9%) | 28 (56.0%) | |
| 2 | 24 (50.0%) | 22 (44.0%) | 0.464 |
| Other chest injury | 45 (93.8%) | 45 (90.0%) | 0.498 |
| Shock on admission to ITU | 21 (43.8%) | 27 (54%) | 0.31 |
| Vasopressors in first hour | 40 (83.3%) | 39 (78.0%) | 0.504 |
| Blood transfusion during IMV | 14 (29.2%) | 28 (56.0%) | 0.007 |
| RRT during IMV | 1 (2.4%) | 2 (4.7%) | 0.585 |
| Ventilation characteristics | |||
| Tidal volume (ml) | 524.5 (490.8–557 [376–602]) | 574.5 (530–630 [366–820]) | 0.0002 |
| Tidal volume/PBW | 7.3 (6.8–7.7 [5.2–8.0]) | 8.6 (8.3–9.5 [8.0–13.4]) | <0.0001 |
| PIP (cmH2O) | 20 (18–23 [14–32]) | 20 (18–22 [14–31]) | 0.9373 |
| Proportion of PIP >25 cmH2O | 4 (8.3%) | 4 (8.0%) | 0.952 |
| PEEP (cmH2O) | 5 (5–7 [5–8]) | 5 (5–5 [5–10) | 0.1467 |
| Proportion of PEEP >10 cmH2O | 0 (0%) | 0 (0%) | n/a |
| PaCO2 (kpa) | 5.64 (5.22–6.11 [3.62–7.11]) | 5.44 (5.11–5.78 [4.58–7.15]) | 0.1638 |
| PaO2:FiO2 ratio | 40.4 (32.1–57.1 [17.7–65.7]) | 37.6 (32.3–45.9 [17.8–57.6]) | 0.2207 |
| Duration of invasive ventilation | 4 (1–7 [1–24]) | 3 (1–6 [1–17]) | 0.7116 |
| Outcomes of interest | |||
| Length of ICU stay (days) | 10 (5.5–18.5 [1–57]) | 9 (4–17 [1–33]) | 0.7061 |
| Hospital LOS (days) | 20 (13–41.5 [1–154]) | 31.5 (17–55 [1–115]) | 0.1058 |
| Died in ICU | 7 (14.6%) | 4 (8.0%) | 0.302 |
Values are number (proportion) or median (IQR [range]).
BMI: body mass index; ICU: intensive care unit; IMV: invasive mechanical ventilation; ISS: injury severity score; LOS: length of stay; PaO2:FiO2 ratio: ratio of partial pressure of arterial oxygen and fraction of inspired oxygen; PaCO2: partial pressure of arterial carbon dioxide; PBW: predicted body weight; PEEP: positive end-expiratory pressure; PIP: peak inspiratory pressure; RRT: renal replacement therapy.
Figure 1.
Receiver operating characteristic curve displaying the c-statistic for non-adherence to low tidal volume ventilation.
Figure 2.
Correlation between tidal volume and height (a), body mass index (b), and PaCO2 (c). PaCO2: arterial partial pressure of carbon dioxide; PBW: predicted body weight.
In chest trauma patients, low tidal volume ventilatory strategy is underutilised within our institution. Shorter patients (with lower PBW) where more likely to receive ‘injurious’ tidal volume (>8 ml/kg PBW) with a consequent tendency for higher body mass index. Low adherence rate in obese patients could potentially be explained by prescription of tidal volumes based on actual body weight and concerns relating to lung collapse at low tidal volumes. The association between blood transfusion requirements and use of ‘injurious’ tidal volume may reflect transfusion-related circulatory overload or even lung injury leading to respiratory insufficiency and attempts to correct impaired gas exchange. Non-adherence was predicted with moderate discriminative ability. There is a need for continuous education and further investigation focusing on identifying barriers to adherence to best practice and strategies to overcome them. The performance of our prediction model needs to be externally validated in a larger cohort.
Acknowledgement
This paper has been presented in part as a poster at the Intensive Care Society State of the Art meeting, London, United Kingdom, December 2018.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Dr Zochios is supported by an Academic Clinical Fellowship from the National Institute for Health Research (ACF-2016-09-011).
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