CPAP and BiPAP are two distinct types of noninvasive ventilation. This is also known as noninvasive positive pressure ventilation, or NPPV. It is a less intrusive method of providing support for those who require assistance breathing without the need for endotracheal tube intubation.
Hypoxemic Respiratory Failure
When an endotracheal tube is required, this is called invasive ventilation. There are two types of respiratory failure. The first is Type I, known as "hypoxemic." This is a failure of oxygen exchange or a failure to oxygenate. This is described as having a PaO2 (partial pressure of oxygen) less than 60.
Hypercapnic Respiratory Failure
Type II, or hypercapnic respiratory failure, is another type of respiratory failure that patients may experience. Failure to exchange or eliminate carbon dioxide results in failure to ventilate. It is described as having a PaCO2 (partial carbon dioxide pressure) greater than forty-five.
Fraction of Inspired Oxygen
Importantly, these categories can overlap and be combined. There are various therapeutic options available for these respiratory failures. There are two different treatments for hypoxemic respiratory failure. Firstly, FiO2 (fraction of inspired oxygen) can be raised. This increases the amount of oxygen delivered to the patient. FiO2 is represented fractionally, with 100 percent represented as 1.0 and typical room air (21 percent oxygen) represented as 0.21.
Another way to help with hypoxemic respiratory failure is to increase the patient's mean airway pressure and use that to open any collapsed alveoli.
The therapy options for hypercapnic respiratory failure are to enhance the patient's minute ventilation, respiratory rate, or tidal volume.
Given that the tidal volume is determined by the patient's ideal body weight, there is only one option to alter the patient's minute breathing.
CPAP versus BiPAP
Noninvasive ventilation is classified into two forms. The first is known as continuous positive airway pressure, or CPAP. The other is known as bilevel positive airway pressure, or BiPAP.
CPAP (Continuous Positive Airway Pressure)
In the context of ventilators, this is equivalent to positive end expiratory pressure, or PEEP. Patients with hypoxemic respiratory failure benefit from CPAP. As a result, CPAP can be a useful alternative for individuals with congestive heart failure (CHF), as it raises the patient's intrathoracic pressure, reducing preload.
This can return the heart to the far end of the Frank Starling curve and enhance cardiac output. Furthermore, the pressure inside the alveoli can help to force the fluid causing the congestion out and open them up to improved gas exchange.
The capacity of CPAP to open or recruit and hold open collapsed alveoli and splint them open reduces the patient's work of breathing (WOB) and provides more alveoli for gas exchange.
CPAP can help patients with obstructive sleep apnea (OSA) by keeping their airways open. CPAP helps to increase the functional residual capacity (FRC) of the patient, which is their reserve for their own respiratory function.
Fixed Pressure CPAP
There is only one pressure setting with traditional fixed pressure CPAP. This pressure is applied to the patient during inspiration and expiration. When dealing with critically ill patients, pressure should be in the 5 to 25 cmH2O range but should not exceed twenty cmH2O.
The explanation for this is that increasing that pressure will result in an increase in intrathoracic pressure and this can cause hemodynamic instability, as well as the increased risk of insufflating a patient's stomach, which can increase the risk of emesis and aspiration.
Bilevel Positive Airway Pressure (BiPAP)
BiPAP is a suitable alternative to assist and preventing intubation. It can also be used to wean difficult patients off invasive ventilation. BiPAP therapy has been shown to reduce mortality in these patients. The term "bilevel" refers to the presence of two pressures. The two pressures are the inspiratory positive airway pressure (IPAP) and the expiratory positive airway pressure (EPAP).
So, the inspiratory positive airway pressure is the pressure that the BiPAP machine is going to provide to patients when it senses inspiration. This is support for the patient taking a breath, and it will help with that ventilation.
The typical IPAP setting range is the same as that of CPAP, ranging from 5 to 25 cmH2O. EPAP is a lower pressure that will be applied during expiration. This will help to ensure oxygenation for the patient by providing support and keeping the alveoli open.
If oxygenation is not the problem, the patient's EPAP can be set to zero. The human body has a natural physiological PEEP of 2 to 3 cmH2O. This is a significant distinction between noninvasive and invasive ventilation. This is not a possibility for an intubated patient.
The EPAP can be increased from zero to any level chosen, but it must be less than the IPAP since a difference in pressures is required. The pressure support is determined by the difference between these two pressures, IPAP and the EPAP.
Pressure support is what controls the patient's tidal volume; the greater the difference, the more tidal volume that the patients will receive. The greater the tidal volume, the greater the ventilation and consequently the clearance of CO2.
BiPAP machines can be programmed with a respiratory rate and an inspiratory time. The respiratory rate is the time between when the BiPAP equipment switches from IPAP to EPAP automatically.
Average Volume Assured Pressure Support (AVAPS)
AVAPS, which stands for Average Volume Assured Pressure Support, is a newer version of the BiPAP machine. This device automatically adjusts the pressure support breaths to attain the target tidal volume. It uses a feedback loop to either increase or decrease pressure to maintain the tidal volume at the desired level. The distinction is that the target tidal volume is predetermined and the machine in AVAPS mode will attempt to adjust the pressures to match that objective.
Noninvasive ventilation is not always sufficient for the patient. If hypoxemia or hypercapnia does not improve despite extensive therapeutic titration, intubation and mechanical ventilation may be required. Patients who are nauseous and have emesis are also at risk for aspiration. Likewise for patients who have profuse thick secretions.
The patient must be able to protect their own airway. If the patient is drowsy or unconscious, they will not be a viable candidate for noninvasive breathing. The decision to transition from noninvasive ventilation to intubation should be made proactively rather than waiting until the patient is fatigued and requires emergency intubation.