Mechanical ventilators have evolved significantly in the last decade. Advances in technology, as well as an increased understanding of the physiology and effects of mechanical ventilation have had a profound impact on ventilator use.[1]
Ventilators are used in many clinical areas including operating rooms, emergency departments, critical care transport units, and air medical transports. Smaller, user-friendly portable ventilators show great potential for more widespread use in the prehospital setting, including 911 responding ambulances.
Why a ventilator?
Positive pressure ventilation with a bag-valve mask (BVM) device is a fundamental skill at the BLS level. Delivering effective BVM ventilations can be difficult, and requires considerable practice to be done correctly.[2] Bag-valve mask ventilations, while a critical and proven intervention for respiratory failure, are limited in their ability to provide consistent and accurate tidal volume and do not provide the protections of an advanced airway device.[3]
Positive pressure ventilations with an advanced airway in place can provide a more efficient and protected means of ventilation over a bag-mask device. However, variables in pressure, tidal volume, rate, and oxygen concentration all present potential complications for patients undergoing positive pressure ventilation in the pre-hospital setting.[4] The use of a mechanical ventilator allows for control of ventilation rate, volume, pressure, and oxygen concentration, as well as continuous monitoring of carbon dioxide and oxygen levels.
Ventilator basics
Mechanical ventilation is provided using two basic methods: volume controlled modes and pressure controlled modes. As the name implies, volume controlled modes are designed to achieve a programmed tidal volume with each ventilation at whatever pressure is necessary within a safe limit. Pressure controlled modes are targeted at delivering ventilations until a set pressure is achieved, with tidal volume being regulated by lung compliance, and airway resistance. All types of mechanical ventilation use some combination of these two basic modes.[5]
Ventilators also allow for adjustment of positive end-expiratory pressure (PEEP), and control over the fraction of inspired oxygen (FiO2). Delivering a more precise FiO2 can better ensure an adequate amount of oxygen is available to the patient, without causing oxygen toxicity or hyperoxia.
Standard initial settings for volume delivery usually fall between 6-8 mL/kg, with a maximum recommended volume of 10 mL/kg. Pressure settings usually max out at 20 cm H2O, but may vary by machine.[6] The standard recommended setting for positive end expiratory pressure is anywhere from 0-15, depending on patient complaint. FiO2 levels are also patient dependent.[7]
Many ventilators also provide settings for non-invasive ventilation methods, such as continuous positive airway pressure (CPAP), bi-level positive airway pressure (BiPAP), and proportional assist ventilation (PAV). Non-invasive methods of ventilation are useful in a variety of conditions where respiratory failure is imminent.[8]
While EMS personnel commonly use CPAP in the prehospital setting, BiPAP capabilities provide more customization for varied patient presentations. PAV settings allow for dynamic inspiratory assistance in patients experiencing respiratory difficulties in order to achieve pre-set tidal volume targets.
Pre-hospital Use
Currently, the use of a portable mechanical ventilator is a mainstay of critical care patient transport via both air and ground. A small number of EMS agencies have introduced automatic transport ventilators as an addition to ALS protocols. These simplified ventilators provide more consistent minute volume than traditional positive pressure ventilation with a bag-valve device.[9] There is also evidence that the use of automatic transport ventilators may allow paramedics to complete other tasks related to patient care, as they are not directly involved in manual ventilation.[10]
Prolonged manual ventilation with a bag-valve device is harmful and often increases patient mortality.[11] The introduction of automatic transport ventilators into 911 responding ambulances could have significant impact on patient outcomes, particularly in rural areas with longer transport times.
Pre-hospital use of non-invasive ventilation methods (CPAP, BiPap, and PAV) has been shown to reduce in-hospital mortality rates.[12] The addition of automatic transport ventilators with greater ability to customize settings for CPAP, BiPAP, and PAV would allow pre-hospital personnel to deliver respiratory support tailored to each patient’s specific needs.
Future technology
With traditional ventilators, ventilator supported breaths are initiated by pressure changes in the machine’s ventilator circuit. This causes a delay in the delivery of ventilations. Developments in technology now allow for neutrally adjusted ventilator assist (NAVA). With NAVA, bipolar electrodes detect signals travelling down the phrenic nerve to the diaphragm, and use this signal as the stimulus to initiate mechanical ventilations. This allows for very precise volume delivery and a decreased chance of pressure-related complications.[13] Other developments in ventilator technology include the ability to enter a patient’s weight to generate suggested initial settings.
The potential benefits of widespread automatic transport ventilator use on ALS ambulances are significant. Advances in technology should continue to create ventilators that are smaller, more user friendly, and easily adaptable to the pre-hospital environment.
References
1. Bristle, Timothy J., et al. “Anesthesia And Critical Care Ventilator Modes: Past, Present, And Future.” AANA Journal 82.5 (2014): 387-400.
2. Cummins RO, Austin D, Graves JR, Litwin PE, Pierce J.
Ventilation skills of emergency medical technicians: a teaching challenge for emergency medicine. Ann Emerg Med 1986; 15: 1187–92
3. NA, J. U., et al. “Influence Of Face Mask Design On Bag-Valve-Mask Ventilation Performance: A Randomized Simulation Study.” Acta Anaesthesiologica Scandinavica 557.9 (2013): 1186-1192.
4. Prekker, Matthew E., et al. “The Process Of Prehospital Airway Management: Challenges And Solutions During Paramedic Endotracheal Intubation.” Critical Care Medicine 42.6 (2014): 1372-1378.
5. “Anesthesia And Critical Care Ventilator Modes: Past, Present, And Future.”
6. “Anesthesia And Critical Care Ventilator Modes: Past, Present, And Future.”
7. Archambault PM, St-Onge M. Invasive and noninvasive ventilation in the emergency department. Emerg Med Clin North Am. May 2012;30(2):421-49, ix.
8. McNeill, GBS, and AJ Glossop. “Clinical Applications Of Non-Invasive Ventilation In Critical Care.” Continuing Education In Anaesthesia, Critical Care & Pain 12.1 (2012): 33-37.
9. Gervais HW, Eberle B, Konietzke D, Hennes HJ, Dick W, “Comparison of blood gases of ventilated patients during transport”. Critical Care Medicine 1987;15:761-763
10. Weiss, Steven J., et al. “Automatic Transport Ventilator Versus Bag Valve In The EMS Setting: A Prospective, Randomized Trial.” Southern Medical Journal 98.10 (2005): 970-976.
11. Maharjan, R. K., R. P Aacharya, and P. N. Prasad. “Impact Of Duration Of Prolong Manual Bag Ventilated Patients In The Emergency Service.” Journal Of Institute Of Medicine 36.2 (2014): 57-65.
12. Rowe, Brian H. “Review: Prehospital Noninvasive Ventilation For Severe Respiratory Distress Reduces Hospital Mortality.” ACP Journal Club 160.10 (2014): 1.
13. “Anesthesia And Critical Care Ventilator Modes: Past, Present, And Future.”
