Continuous positive airway pressure (CPAP) is a form of non-invasive positive pressure ventilation (NIPPV) that helps improve work of breathing and oxygenation for individuals with different cardiopulmonary complaints. The concept of positive pressure ventilation started in the 1930s and gained momentum in the 1950s during the polio epidemic. It was not until the 1980s that noninvasive forms of CPAP were adopted to help patients with obstructive sleep apnea and chronic obstructive pulmonary disease. The use of CPAP in the prehospital setting gained traction in the late 90s as the primary form of non-invasive positive pressure ventilation as an alternative to endotracheal intubation or use of supraglottic devices and over the past several years has become the standard of care of patients with acute respiratory distress.
Initial models of CPAP used a control unit (flow generator) attached to the oxygen source to produce the necessary positive pressure. Newer models of CPAP devices deliver a specific amount of pressure based on either adjustment of a control valve (PEEP valve) or by adjusting the amount of flow that is delivered to produce the necessary positive end-expiratory pressure (PEEP). These newer models have all the necessary parts built into the device and subsequently, have a significantly reduced cost versus the original devices.
With each breath a person takes, the negative, intrathoracic pressure causes the alveoli and distal pulmonary structures to collapse, as demonstrated with deflating a balloon or in the case of individuals with stiff lungs [chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis patients]. Following this, there is an abnormal residual pressure due to air trapping referred to as auto-PEEP (positive end-expiratory pressure). Similarly, the hardest part of blowing up a balloon is that initial breath which has to overcome the surface tension of the walls touching each other. Usage of continuous positive airway pressure allows for a constant pressure throughout the ventilatory cycle (inspiration and expiration phases) and does not allow for the alveoli to completely collapse during the exhalation phase. This positive pressure will reduce the surface tension on the walls of the alveoli by increasing the intrathoracic pressure which helps to reduce the work of breathing. Similarly, there is also an increase in the surface area of the alveoli which allows for an increase in gas exchange. The positive pressure from CPAP allows for individuals to overcome the auto-PEEP and will help reduce the work-of-breathing. With the increase in intrathoracic pressure, there is also a reduction in preload coming back to the heart which in turn allows for a fluid shift out of the lungs and back into the pulmonary vasculature.
Individuals that can breathe spontaneously throughout the complete respiratory cycle can use CPAP. The primary function of non-invasive positive-pressure ventilation is to improve pulmonary compliance, improve alveoli aeration by recruiting and stabilizing collapsed alveoli, and ultimately improve ventilation-perfusion mismatches. Continuous positive airway pressure increases intrathoracic pressure which in turn reduces venous return (preload), transmural pressure, and afterload. This ultimately allows for enhancement in cardiac function and reduces pulmonary edema.
Common indications for CPAP include
Absolute contraindications for non-invasive, positive-pressure ventilation include cardiac arrest, respiratory arrest, coma, or any condition requiring immediate intubation.
Relative contraindications for non-invasive, positive-pressure ventilation include:
There are multiple commercial grade devices available for the prehospital providers that combine the flow generator with the face mask as a disposable product. The major brand of devices includes the Pulmodyne and Flow-Safe II. The Pulmodyne CPAP device has an attached PEEP valve that can be adjusted for a maximum of 10 cm of water. The Flow-Safe II was created using a venturi effect where an increase in airflow turbulence causes an increase in pressure. Both of these devices have closed circuit nebulization features. Philips Respironics created a nasal CPAP device (ComfortFusion Nasal) that is supposed to be more comfortable for patients.
Common settings for prehospital NIPPV include an inspiratory pressure of 5 to 10 cm water with an expiratory pressure of 5 cm water. Patients with asthma, bronchitis, COPD usually start with a pressure of 5 cm water versus those with congestive heart failure (CHF), severe pneumonia and pulmonary edema due to near-drownings at 10 cm water. With these levels, the work of breathing can be reduced by as much as 60% and have an improvement of inspiratory muscle endurance up to 95%.
The scope of practice for emergency management service (EMS) providers over the years have changed, and over time emergency medical technicians in most states have lost the ability to intubate a patient. This has been partly due to the advances in CPAP equipment and the reduction in the need to perform invasive airway techniques. In most states, the emergency management technician (EMT) is allowed to use a CPAP device with appropriate training and approval from their medical director.
Most patients who have used a noninvasive positive-pressure ventilation device before need very little education about the device. It is the patient who has never used the device before or the ones that are extremely anxious who will need help to ensure that they do not "fight" the device. The full face mask devices can cause some claustrophobe feelings for patients. Education and coaching from the medical provider are extremely important for the patient to become comfortable and use the device. When dealing with a patient that is in an acute respiratory distress situation the prehospital provider will need to assemble the NIPPV device quickly. Ensure that the device is attached to a properly filled oxygen source because the majority of the devices will use quite a bit of oxygen (refer to specific manufacturers devices). Initially, just apply the mask to the face of the patient and coach them to take in deep breathes. Once the patient can tolerate the mask, then you can apply the head strap. Reassess the patient frequently for improvement in their respiratory status, lung sounds and how well they are tolerating the device.
Complications of NIPPV include patient discomfort, anxiety, and agitation, pulmonary barotrauma including pneumothorax, hypotension secondary to increased intrathoracic pressure and reduced preload, gastric distention (usually with elevated inspiratory positive airway pressure greater than 20 cm water) leading to abdominal compartment syndrome. Most of these problems can be alleviated by either the administration of a sedative medication, placement of a nasogastric tube and for the barotrauma either needle decompression or chest tube placement.Hypotension can be treated with either administration of intravenous (IV) fluids, starting the patient on a vasopressor (norepinephrine, dopamine, or epinephrine) or discontinuing the CPAP.
Non-invasive positive pressure ventilation and continuous positive pressure ventilation has a clinical significance with patients in acute respiratory distress from a number of etiologies, but the mainstay treatment for prehospital providers has been for those individuals with congestive heart failure. In the 1998 article from Pang et al., they performed a literature review that demonstrated a reduction in the need for invasive ventilation with the use of CPAP devices. Ucgun et al. demonstrated that the most important predictor of mortality in the COPD patient pertains to invasive ventilation and complications related to mechanical ventilation.