By Philip Neuwirth, BS, MICP, CCEMTP, FP-C
Medical care and treatment protocols continue to be developed and integrated into clinical practice at a record pace. What is the “standard-of-care” for the treatment of a disease process today becomes diminished over time by practicing medical scholars before a new “standard” is achieved. During this time, there are many different schools of thought. Research into “Mechanical Ventilation of Adult Respiratory Distress Syndrome (ARDS)”, netted few strategies, that practicing medical professionals agreed upon. That being the case, here is how to mechanically ventilate patients with ARDS in the prehospital setting which are often overlooked.
Ventilating a patient in a quiet, dim-lit ICU room is far different than ventilating a patient in a noisy, vibrating, moving helicopter or ambulance. Whatever the provider’s philosophy is regarding analgesia/sedation/paralytics, the goal is to make sure the patient is comfortable and not voluntary or involuntary resisting positive pressure ventilation. Positive pressure ventilation is needed for this patient population. However, it can be the deadliest weapon utilized in the treatment of ARDS patients.
Acute Respiratory Distress Syndrome (ARDS), is an inflammatory lung condition involving both lungs that may complicate severe pneumonia (including influenza), trauma, sepsis, aspiration of gastric contents, and many other conditions. Inflammation leads to injury of lung tissue (surfactant washout) and leakage of blood and plasma into the airspaces resulting in an inability of the lungs the exchange gas. Mechanical ventilation is required in order to maintain gas exchange at the alveoli, recruit sections of consolidated lung, deliver higher FiO2, and facilitate ventilation to remove carbon dioxide from the body. Inflammation in the lung may lead to inflammation elsewhere causing shock and injury or dysfunction in the kidneys, heart, and muscles .
In retrieval medicine, we often transfer ARDS patients from small community-based emergency departments, or Intensive Care Units (ICU’s) to a higher level of care. In many cases, these patients have not been given the most aggressive care, and their ventilatory status has suffered. In my experience, most ARDS patients I’ve transferred are from the ICU, and have been treated unsuccessfully for upwards of 1-2 weeks and are now extremely ill and need to be transferred. These patients are fragile and require low tidal volume ventilation (LTVV), also referred to as lung protective ventilation . The rationale for this approach is that smaller tidal volumes are less likely to generate alveolar overdistension, one of the principal causes of ventilator-associated lung injury . The critical care transport team’s goal is to minimize any additional damage while maintaining adequate gas exchange. Adequate gas exchange requires sufficient minute volume. It’s not uncommon to ventilate these patients at a rate of 18-20 breaths per minute or higher, depending on tidal volume and to match their required minute ventilation . Collectively, evidence suggests that the early application of and adherence to LTVV improves mortality, as well as other clinically important outcomes in patients with ARDS [3-5].
General Principles: There is no single optimal mode of mechanical ventilation [3-5]. Diseases and patient condition vary over time, so clinicians should regularly reassess the settings and modes of ventilation. Nevertheless, certain guiding principles should be applied in most cases :
- Minimize plateau pressure and tidal volumes, allowing hypercapnia if necessary (except in brain-injured patients), to reduce the risk of lung injury.
- Optimize positive-end-expiratory pressure (PEEP) to prevent alveolar collapse and improve oxygenation.
- Titrate inspired oxygen as quickly as possible.
- Maintain a back-up, head-elevated position (BUHE) whenever possible.
Permissive hypercapnia: LTVV frequently requires permissive hypercapnic ventilation (PHV), a ventilatory strategy that accepts alveolar hypoventilation to maintain a low alveolar pressure and minimize the complications of alveolar overdistension (e.g., ventilator-associated lung injury). Hypercapnia and respiratory acidosis are a consequence of this strategy . The degree of hypercapnia can be minimized by using the highest respiratory rate that does not induce auto-PEEP and shortening the ventilator tubing to decrease dead space .
Mode: Patients with ARDS can be supported using either a volume limited or a pressure limited mode of ventilation. Volume limited mode will deliver stable tidal volumes; pressure limited mode will deliver stable airway pressure, assuming that breath to breath lung mechanics and patient effort are stable. Abrupt changes in the airway pressure in a patient receiving volume limited ventilation, or in tidal volumes in a patient receiving pressure limited ventilation, should prompt an immediate search for a cause of an acute change in compliance (e.g., pneumothorax or an obstructed endotracheal tube) 
Regardless of whether volume limited or pressure limited ventilation is chosen, fully supported modes of mechanical ventilation (e.g., assist control) are generally favored over partially supported modes (e.g., synchronized intermittent mandatory ventilation [SIMV]). This is particularly true early in the course of the disease. Ultimately, the choice of mode depends primarily on clinician comfort and familiarity .
- When moving your patient over to your portable ventilator, take note of your patient’s exhaled volume, compared to delivered volume on the hospital ventilator. This will give you a breath by breath indication of your patient’s lung compliance and any air-trapping that may be occurring.
- Clamp your tube so you do not lose the recruitment you have already achieved before you disconnect the ETT and reconnect.
- Adjust your settings for lung protective ventilation.
- Increase your ventilatory sensitivity to prevent auto-triggering.
- Generally, a fully supported mode of mechanical ventilation, rather than a partially supported mode is recommended. Volume limited and pressure limited modes are both acceptable; however, volume limited approaches have been more extensively studied, and there is no evidence that pressure limited approaches have additional benefit .
- Tidal volume and respiratory rate: For all patients, tidal volume and respiratory rate should be managed using the strategy of low tidal volume ventilation. After setting the initial tidal volume at 6 mL/kg based on predicted body weight, we suggest adjusting the tidal volume to achieve an inspiratory plateau airway pressure ≤30 cm H2O. Lower tidal volume requires higher rates to maintain adequate minute volume. It’s not uncommon to have respiratory rates between 18-20 breaths per minute, depending on your tidal volume to maintain adequate minute volume.
- Positive end-expiratory pressure (PEEP): For patients with ARDS who have a PaO2/FiO2 ≤200 mmHg (26.6 kPa), we suggest using a strategy of high PEEP. A preferred approach to determining the optimal level of high PEEP has not been established. For patients who have a PaO2/FiO2 of 201 to 300 mmHg (26.7 to 40 kPa), there is no evidence that a strategy of high PEEP is beneficial. Thus, the applied PEEP can be determined using the approach employed in low tidal volume ventilation (see table below) .
|Calculate Predicted Body Weight (PBW)|
|Male =||50 + 2.3 [height (inches) – 60] OR|
|50 + 0.91 [height (cm) – 152.4]|
|Female =||45.5 + 2.3 [height (inches) – 60] OR|
|45.5 + 0.91 [height (cm) – 152.4]|
|Plateau Pressure Goal: Pplat ≤30 cm H2O|
|Check inspiratory plateau pressure with 0.5-second inspiratory pause at least every 15 min in flight and after each change in PEEP or tidal volume.|
|If Pplat >30 cm H2O, decrease tidal volume in 1 mL/kg PBW steps to 5 or if necessary to 4 mL/kg PBW.|
|If Pplat <25 cm H2O and tidal volume <6 mL/kg, increase tidal volume by 1 mL/kg PBW until Pplat >25 cm H2O or tidal volume = 6 mL/kg.|
|If breath stacking (autoPEEP) or severe dyspnea occurs, tidal volume may be increased to 7 or 8 mL/kg PBW if Pplat remains ≤30 cm H2O.|
|Arterial oxygenation and PEEP|
|Oxygenation goal: PaO2 55 to 80 mmHg or SpO2 88 to 95 percent|
|Use these FiO2/PEEP combinations to achieve oxygenation goal:|
|PEEP||5||5 to 8||8 to 10||10||10 to 14||14||14 to 18||18 to 24|
|PEEP should be applied starting with the minimum value for a given FiO2.|
- NHLBI ARDS Network, Retrieved from http://www.ardsnet.org/
- Hou, P, Alejandro Baez, A, Walls, R, Grayzel, J (June 7, 2016). Mechanical Ventilation of Adults in the Emergency Department. Up To Date, Retrieved on July 16, 2016, from http://www.uptodate.com/contents/mechanical-ventilation-of-adults-in-the-emergency-department
- Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000; 342:1301.
- Putensen C, Theuerkauf N, Zinserling J, et al. Meta-analysis: ventilation strategies and outcomes of the acute respiratory distress syndrome and acute lung injury. Ann Intern Med 2009; 151:566.
- Richecoeur J, Lu Q, Vieira SR, et al. Expiratory washout versus optimization of mechanical ventilation during permissive hypercapnia in patients with severe acute respiratory distress syndrome. Am J Respir Crit Care Med 1999; 160:77.
- Bauer, E. (2015) Ventilator Management, A pre-Hospital Perspective, Scottsville, KY, FlightBridgeED, LLC