Protective mechanical ventilation strategies in the prehospital setting
Lowering tidal volumes in an effort to reduce lung injury following initiation of mechanical ventilation is far from a new idea, the original ARDSNet data are nearly 20 years old. Recent ED-based studies have shown decreased mortality when lower tidal volumes are used early in ventilated patient management.1
Montgomery County Hospital District (MCHD), which provides EMS services to Montgomery County, just outside the city of Houston, recently implemented protocol changes to allow paramedics to begin lung protective ventilatory steps in an effort to decrease ventilator-induced lung injury (VILI).
Mechanical ventilation, although often lifesaving, comes with side effects like any other medical procedure or treatment. When paramedics intubate and place patients on the ventilator, the potential for lung injury must be considered.
There are four ways that invasive positive pressure can harm the lungs:
1. Barotrauma: High pressures can damage the alveolar basement membrane. This can occur at much lower pressures than the classically taught barotrauma-related pneumothorax.
2. Volutrauma: Large volumes of gas also over-distend the alveoli and cause structural damage leading to secondary pro-inflammatory cytokine release.
3. Atelectrauma: Repeated atelectasis (i.e., the opening and closing of alveolar openings) is damaging to alveolar membranes and pro-inflammatory.
4. Oxygen toxicity: Evidence from patients with ST-elevation myocardial infarction, stroke/cerebrovascular accident and chronic obstructive pulmonary disease (COPD) now demonstrates that hyper-oxygenation not only isn’t beneficial, but is often harmful.
What are the potential complications? As mentioned above, pneumothorax/pneumomediastium can result from extreme elevations in pressure and volume, but VILI can occur quickly at much lower pressures in the form of acute respiratory distress, ventilator-associated infections, pulmonary edema and atelectasis.2
Acute Respiratory Distress Syndrome (ARDS)
ARDS is defined as acute respiratory distress (i.e., < 1 week) with severe hypoxemia (i.e., PaO2/FiO2 < 200 mmHg) and no sign of volume overload as a cause.3 The chest X-ray in these patients will demonstrate diffuse infiltrates, which can mimic both congestive heart failure and infection.
ARDS can occur from a direct lung insult, such as pneumonia, aspiration or non-fatal drowning. It can also occur as a result of indirect lung injury from sepsis, severe pancreatitis and multisystem trauma.
Much of the initial data supporting the benefits of lowering tidal volume comes from the ARDSNet trial. These were ICU patients with ARDS who received either weight-based tidal volumes or predicted tidal volumes based on height. Those with lower tidal volumes had lower mortality and more ventilator-free days. This was a significant finding, as ARDS has a reported mortality rate from 25–50%. In the ARDSNet trial, a decrease in mortality from 40% to 31% was seen in the lower tidal volume group.4
From the ICU to the Street
How does this ARDSNet data extend into the ED and prehospital settings?
VILI can occur quickly and is shown to be related to ED ventilator management. The LOV-ED trial was an ED-based before-and-after study that examined the effects of introducing lung protective ventilatory strategies in the ED.
The investigators used the ARDSNet-predicted tidal volume height-based table, advocated for rapid weaning of oxygen therapy, elevated the head of the patient beds to 30 degrees, ventilated appropriately with a rate of 20–30 breaths per minute, minimized plateau pressures, and used standard positive end expiratory pressure (PEEP) of 5–10 mmHg depending on body mass index.1
The study results demonstrated decreased VILI, decreased ventilator time, decreased ICU and hospital days, and most importantly, decreased mortality (from 34% to 19% after implementation).1
The focus of lung protective ventilation is often tidal volume reduction to decrease the volutrauma/barotrauma, which was discussed earlier. Other strategies include PEEP usage, which helps to decrease atelectrauma, and head-of bed elevation to 30 degrees/suction to prevent aspiration. Additionally, the oxygen concentration is rapidly weaned to 30–40% to reduce oxygen toxicity.
The authors of a randomized controlled trial of COPD patients found mortality is more than halved with prehospital oxygen therapy titrated to SpO2 88–92%, compared with high concentration oxygen therapy.5
Lung protection with mechanical ventilation is a multipronged effort that involves multiple, simple-to-enact protocol changes each targeting the different sources of pulmonary injury.
MCHD Protocol & Results
In April 2017, MCHD introduced a prehospital lung protective ventilation bundle, which focused on taking the lessons learned from trials like ARDSNet and LOV-ED to the EMS setting.
Head-of-bed positioning, both during intubation and post-intubation is essential and encouraged in our delayed sequence intubation (DSI)/rapid sequence intubation (RSI) protocols as well as post-intubation, with 15 degrees advised during intubation and an increase to 30 degrees during transport. This positioning increases first-pass success rate during RSI/DSI and decreases aspiration-related complications during the post-intubation period.6
Early titration of oxygen down from 100%, if oxygen saturation allows, is encouraged. We recommend PEEP of 5 mmHg, and calculate tidal volume based on height as opposed to weight, as we had done in the past.
To assess the feasibility of paramedic-initiated lung protective ventilatory settings in the prehospital setting, MCHD conducted a retrospective before-and-after chart review. The study compared consecutive ventilated patients from August 2016 through March 2017 who received weight-based ventilator settings to ventilated patients after the introduction of lung protective ventilatory strategies from April 2017 through December 2017.
Preceding protocol initiation, all paramedics completed didactic classroom lectures and hands-on ventilator training in a mandatory continuing education program. Ventilator tidal volume was changed from a weight-based calculation (7 mL/kg) to a rapid, height-based predicted tidal volume formula created at MCHD (and which is meant to approximate the ARDSNet tidal volume table).
Paramedics estimate the patient’s height in feet, round to the closest half of a foot, and use the following formula: (Height in feet - 1) x 100 = tidal volume.
This allows us to eliminate the need for cumbersome charts during this calculation. Our demographics and results are summarized in Tables 1 and 2. Our patient demographics before and after protocol initiation were markedly similar. After the initiation of lung protective strategies, MCHD reduced tidal volume by more than 20%, from an average of 625 mL to 489 mL.
Lung protective ventilatory strategies have progressively been shown—from the ICU and now into the ED setting—to decrease ventilator days, ICU/hospital stay and overall mortality.1,3 Data also suggests that lower tidal volumes during air transport leads to lower initial ED tidal volumes.7
The concept of therapeutic momentum is likely responsible for the improved outcomes seen following the implementation of early lung protective measures. When the correct initial choice is made earlier, it’s more likely to be carried throughout the course of care. This occurs and has been described in multiple treatment modalities, from tidal volume calculation to antibiotic choice. More importantly, the converse is also true: the wrong initial treatments are delayed in correction throughout the hospital stay.
MCHD’s experience has shown that early lung protective ventilatory strategies, including rapidly predicted tidal volume calculation using estimated height, is feasible for ground-based paramedics. Further studies are needed for confirmation, but based on the mounting evidence, early lung protective ventilation efforts should result in an overall decreased mortality for EMS transported ventilated patients.
1. Fuller BM, Ferguson I, Mohr NM, et al. Lung-protective ventilation initiated in the emergency department (LOV-ED): A study protocol for a quasi-experimental, before-after trial aimed at reducing pulmonary complications. BMJ Open. 2016;6(4):e010991.
2. Boyer AF, Schoenberg N, Babcock H, et al. A prospective evaluation of ventilator-associated conditions and infection-related ventilator-associated conditions. Chest. 2015;147(1):68–81.
3. Ranieri VM, Rubenfeld GD, Thompson BT, et al. Acute respiratory distress syndrome: The Berlin definition. JAMA. 2012;307(23):2526–2533.
4. Brower RG, Matthay MA, Morris A, et al. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301–1308.
5. Austin MA, Wills KE, Blizzard L, et al. Effect of high flow oxygen on mortality in chronic obstructive pulmonary disease patients in prehospital setting: Randomised controlled trial. BMJ. 2010;341:c5462.
6. Turner JS, Ellender TJ, Okonkwo ER et al. Feasibility of upright patient positioning and intubation success rates at two academic emergency departments. Am J Emerg Med. 2017;35(7):986–992.
7. Stoltze AJ, Wong TS, Harland KK, et al. Prehospital tidal volume influences hospital tidal volume: A cohort study. J Crit Care. 2015;30(3):495–501.