◆ Continuous positive airway pressure (CPAP) is a mode of respiratory assistance in which the patient breaths spontaneously at higher than atmospheric pressure.
◆ In heart and/or respiratory failure patients, CPAP application can provide favourable haemodynamic and respiratory effects.
◆ CPAP is indicated in patients with parenchymal respiratory failure, and normal respiratory drive and muscular pump.
◆ The CPAP apparatus not only keeps the airway pressure positive throughout the respiratory cycle, but also regulates FiO2, humidity and temperature of inspired gases.
◆ CPAP may be delivered by different flow generators and interfaces—the final choice depends on patient’s disease and compliance, environment, material availability, and clinical practice.
Continuous positive airway pressure (CPAP) is the simplest mode of respiratory assistance. A CPAP respiratory device is less sophisticated than a ventilator and is designed to keep the patient’s airway pressure at a higher level than atmospheric pressure during the whole respiratory cycle. CPAP also conditions the inspired gas in terms of oxygen concentration (FiO2), humidity, and temperature. Essentially, ventilation is performed by the patient’s own respiratory drive and muscular pump.
Mechanical ventilation (MV) was introduced in clinical practice for patients with respiratory pump failure and healthy lungs (during the epidemic of poliomyelitis in the 1950s) as opposed to CPAP, which is indicated in patients with respiratory failure and a normal drive and muscular pump. CPAP was first employed in the early 1970s to treat acute respiratory distress syndrome (ARDS). However, in 1938, Barach used it for acute cardiogenic pulmonary oedema (ACPE), while others found CPAP useful in military medicine and aeronautics. Nowadays, CPAP is largely used in critically-ill patients with parenchymal respiratory failure in various clinical settings.
Physiological effects of CPAP
The use of CPAP leads to an increase of transpulmonary pressure (PTP). PTP is the pressure that distends the lung parenchyma, and will end up in an increase of lung volume at end expiration, i.e. the functional residual capacity (FRC). When an increase in FRC is due to recruitment of previously collapsed alveoli, lung compliance improves, whereas shunt and low ventilation/perfusion conditions decrease within the lungs . Therefore, CPAP can significantly improve gas exchange and reduce the work of breathing (WOB). On the other hand, when an increase in FRC is due to overdistention of already open alveolar spaces, lung compliance can decrease, whereas dead space (both anatomical and functional) and pulmonary vascular resistance increase .
Although CPAP does not directly assist patients’ inspiratory efforts, its use often provides favourable effects on spontaneous ventilation. CPAP can decrease every single component of patients’ WOB (resistive, elastic, and threshold loads) by improving respiratory mechanics (both airway flow resistance and compliance)  and by opposing flow limitation. Progressive collapse of small airways during expiration promotes intrinsic positive end-expiratory pressure (PEEPi), i.e. a pressure gradient between alveoli and proximal airways at the end of expiration. PEEPi represents a threshold load for the inspiratory muscles, which can be substantially decreased by PEEP and CPAP in chronic obstructive pulmonary disease (COPD) flow-limited patients. In asthmatic patients, PEEP and CPAP use can be detrimental due to pulmonary hyperinflation. Finally, CPAP increases the ability of the expiratory muscles to share the workload with the inspiratory muscles.
CPAP use has some haemodynamic effects, mainly mediated by an increase of intrathoracic pressure secondary to greater elastic recoil of the chest wall and/or lower inspiratory efforts. The corresponding fall in right atrial transmural pressure decreases venous return and preload, especially in hypovolaemic patients, whereas the fall in left ventricle transmural pressure translates into lower afterload, particularly in heart failure patients. Moreover, the CPAP-induced increase in oxygen delivery may result in a better myocardial oxygen supply-demand balance, improving diastolic function, ventricular relaxation, and compliance.
Acute cardiogenic pulmonary oedema
By the early twentieth century, evidence showed the application of a positive pulmonary pressure to a patient experiencing ACPE allowed haemodynamic and respiratory mechanics to improve together with less clinical symptoms of respiratory distress. CPAP has been shown to reduce hospital mortality, endotracheal intubation rate, and ICU length of stay in patients with acute pulmonary oedema . In addition, CPAP can also effectively treat ACPE patients with established respiratory muscle fatigue . Accordingly, hypercapnic ACPE patients had similar recovery times and similar gas exchange responses when treated by CPAP or NIV . Compared with NIV, CPAP is less dependent on the experience of the care team and is a simpler technique. This also makes CPAP useful in the out-of-hospital setting.
Post-operative respiratory failure and post-extubation failure treatment/prevention
CPAP is a widely-used strategy to prevent or to treat post-operative pulmonary complications. Several studies have demonstrated the efficacy of CPAP to reduce atelectasis and improve oxygenation in post-operative settings . The efficacy of CPAP in hypoxaemic patients after abdominal surgery is strictly related to the length of use. Therefore, interfaces that improve patient tolerance and length of treatment, such as the helmet, are preferred . The use of CPAP in obese patients following bariatric surgery is questioned because of concerns that pressurized air may inflate the stomach and proximal intestine, resulting in anastomotic disruption. However, it was recently proved that CPAP can be a safe and effective method for respiratory support in this context, provided it is used as early as possible after extubation . CPAP has been shown to reduce reintubation rate in post-extubation hypoxaemic failure after cardiac surgery .
CPAP should be used in patients with chest wall trauma who remain hypoxic despite adequate regional anaesthesia and high-flow oxygen. Several studies have shown that CPAP resulted in fewer treatment days when compared with immediate intubation followed by intermittent positive pressure ventilation. This improvement is obtained mainly through a decrease of infection rate in non-intubated patients. In view of the risk of pneumothorax, patients with chest trauma who are treated with CPAP or non-invasive ventilation (NIV), especially in presence of multiple rib fractures, should be monitored in the intensive care unit (ICU).
In patients with community-acquired pneumonia (CAP), time is needed for conventional therapy to show its effect; during this period, the maintenance of a satisfactory oxygenation represents the main goal in the management of acute respiratory failure (ARF). CPAP can rapidly improve oxygenation in CAP patients, but the beneficial effects disappear early after its discontinuation. The choice of helmet improves tolerance by allowing expectoration and ameliorating patient–environment interaction, and could be the right choice in patients with pneumonia who may need longer periods of CPAP treatment .
Respiratory failure in immunocompromised patients
Any less invasive method able to avoid the use of endotracheal ventilation appears to be particularly useful in immunocompromised patients. CPAP used at an early stage of hypoxaemic ARF is effective in reducing intubation and ICU admission rate, often related to an increased risk of infectious complications and death in these patients .
Mild/moderate acute respiratory distress syndrome
In patients with acute lung injury (ALI), applying positive pressure to the airways has been shown to lessen the reduction in functional residual capacity, and to improve respiratory mechanics and gas exchange. However, the efficacy of CPAP to prevent subsequent clinical deterioration and to reduce the need for endotracheal intubation has never been certainly shown. Despite early physiological benefits, CPAP delivered via face mask did not reduce the need for endotracheal intubation among ALI patients, and did not impact the length of hospital stay or hospital mortality . Whether the use of the helmet interface, allowing prolonged administration of high PEEP levels, could improve outcome in patient affected by mild ARDS, has to be definitively demonstrated .
The increase in WOB during COPD exacerbations is due to both an increased resistive load and dynamic hyperinflation; PEEPi imposes an additional inspiratory threshold load, while decreasing the effectiveness of the inspiratory muscle. CPAP can counterbalance PEEPi without causing further hyperinflation, decreasing inspiratory effort and dyspnoea, while improving the pattern of breathing. Arterial blood gases tend to remain the same or improve slightly, probably due to improvement in breathing pattern at constant minute ventilation, and eventually to decreased WOB. For these reasons, despite the evidence demonstrating NIV effectiveness in COPD exacerbation, also the use of CPAP can be considered, especially in particular settings where NIV is not available.
Techniques for CPAP administration
The main goal of CPAP delivering is to maintain pressure in the proximal patient’s airways at the same desired level during the whole respiratory circle, together with full control of inhaled gas mixture. This means low difference between set and delivered oxygen fraction, low level of CO2 rebreathing, temperature, and humidity according to invasive or non-invasive setting. Additional goals are patient’s compliance, safety and monitoring, and technique’s easiness and cheapness.
CPAP can be delivered by different flow generators and interfaces. The final choice depends on patient’s disease and compliance, environment, material availability, and clinical practice.
Continuous flow systems consist of a mixer-flow meter (or two separate flow meters for air and oxygen, or a Venturi system), a heater-humidifier, a pressure stabilizer, a PEEP valve, and a manometer. Oxygen fraction delivered by Venturi systems depends on flow and pressure in the CPAP circuit, and should be monitored at every setting change. Ideally, to maintain constant positive pressure in the patient’s airways, a continuous flow at least equal to patient’s peak inspiratory flow should be set. Anyway, besides a great gas wasting, very high (50–100 L/min) fresh gas flows have some drawbacks. Gas conditioning can be suboptimal, noise substantially increases, and the effect of even small expiratory circuit resistance is magnified. The increase of pressure during expiration can put a significant brake on the patient’s expiratory flow. A large volume (10 L at atmospheric pressure) and highly compliant (500 mL/cmH2O) balloon provides good pressure stabilization in the CPAP circuit without the need of very high fresh gas flow. Its optimal position is in the inspiratory limb nearest the patient’s airways opening. Gas flow lower than 30 L/min favours CO2 rebreathing, both in an invasive and non-invasive setting.
Commercially-available PEEP valves, such as water valves, and adjustable or precalibrated spring-loaded valves, are threshold resistors. Ideally, exhaled gas freely flows away through the valve until airway pressure decreases to a preset level, at which time the valve abruptly closes. In practice, these valves are not pure threshold resistors, but offer a small (and variable among different devices) resistance to flow, eventually to be added to the resistive behaviour of the expiratory limb of the circuit, and of the expiratory port of mask and helmet interfaces . This small resistance to flow has significant effects on patient’s airways pressure when high flow rates are used. Therefore, when administering CPAP with continuous flow systems, pressure must be monitored with a manometer placed as near as possible patient’s airways, to control the effective pressure applied, and detect respiratory oscillations. Excessive pressure oscillations (>3–4 cmH2O) correspond to significant additional respiratory load for the patient. In this case, the use of a high-volume high-compliance balloon becomes mandatory, an increase of gas flow, and the use of the helmet interface should be considered. Recently, the Boussignac valve has been proposed for non-invasive CPAP delivery . This device consists of a plastic tube open to atmosphere and provided with microchannels in the tube wall. The gas injected into the system accelerates through the microchannels, leading to turbulence, and finally positive pressure, corresponding to a ‘virtual’ valve at the patient’s side of the tube. Generated pressure depends on the flow administered; therefore, CPAP level and oxygen concentration cannot be adjusted separately. Moreover, the Boussignac device is not able to maintain a stable positive pressure value in the case of forced breathing and/or high respiratory rate . Anyway, special features of this device (an ‘open’ system with small dimensions and easy to use) make it a good choice in particular settings, like pre-hospital treatments  and bronchoscopy procedures.
Mechanical ventilators can be used as demand-flow systems to administer CPAP, where the alternate opening of the inspiratory and the expiratory valves should provide the inhalational gas volume required by the patient and allow its exhalation, respectively. When the control systems of the mechanical ventilator assure both fast detection of patient’s respiratory activity and a rapid reaction of mechanical valves, pressure oscillation in the airways will be not clinically significant. This was not the case with old generation ventilators—the inspiratory decrease and expiratory increase of pressure were significant, leading to additional work to inhale and braked exhalation. Technological improvements allow new generation ventilators to perform, as well as continuous flow systems, with some differences among models. Additional benefit of ventilator-delivered CPAP is respiratory monitoring and alarming that is necessarily incomplete with continuous flow systems. Higher costs and low availability of both instruments and practice, limit the use of mechanical ventilators to specific settings.
CPAP can be administered by different interfaces: nasal and face masks, nasal pillows and helmets. Nasal devices need cooperation of the patient that has to breathe with a closed mouth to effectively receive CPAP. Among full-face devices, the helmet is more efficient than the mask because of its high internal volume and compliance, making it an additional pressure stabilizer in the CPAP circuit . Moreover, the helmet provides a good tightness by non-traumatic adhesion of the soft collar to the neck of the patient with less discomfort and skin lesions compared with face masks. Because high pressures inside masks—but not inside helmets—increase leaks , higher PEEP levels can be administered with helmets. Therefore, acute hypoxaemic patients needing high-level CPAP without interruptions may particularly take advantage from the helmet device [11,12]. CO2 rebreathing during mask CPAP is limited mainly by leaks, both unintentional and intentional through expiratory ports. Conversely, during helmet CPAP significant CO2 rebreathing may occurs when the amount of fresh gas flowing through the device is not well matched with patient’s CO2 production. A minimum fresh flow delivery of 30–35 L/min is always mandatory, to be increased to 50 L/min or more in case of higher metabolic requirements. This also means that the use of mechanical ventilators, that provide fresh gas flow equal to patient’s minute ventilation, must be avoided for helmet CPAP. Mean inhaled CO2 can be easily monitored by gas sampling at the outlet port of the helmet ; this value should be maintained below 0.5% by adjusting fresh gas flow in order not to increase patient’s ventilatory workload.
Comfort data suggest that humidity of inspired gases during CPAP should be at or above 15 mgH2O/L with temperatures ranging from 25 to 30°C. Heated humidifier can provide this gas conditioning .
We are indebted to Doctor Ilaria Currò (Section of Anesthesia, Intensive Care and Pain Therapy; Department of Clinical, Surgical, Diagnostic, and Pediatric Sciences; University of Pavia, Italy) and Doctor Marco Pozzi (Anesthesia and Intensive Care Division I; Emergency Department; Fondazione IRCCS Policlinico San Matteo, Pavia, Italy), for assisting with the literature revision, and with Doctor Houman Amirfarzan (Harvard Medical Instructor; V.A. Hospital, Boston, USA), for the language revision.
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