Noninvasive Ventilation
Warren Isakow
General Principles
Noninvasive ventilation (NIV) or noninvasive positive pressure ventilation refers to the use of a mask or similar device to provide ventilatory support.
This definition is broad and could include external negative pressure devices (e.g., the “iron lung,” historically used for ventilation of patients suffering from poliomyelitis-induced paralysis), cuirass ventilation (external shell with applied negative pressure), and rocking beds—an effective means to ventilate a patient with bilateral diaphragmatic paralysis.
NIV by definition excludes any modality that bypasses the upper airway, such as laryngeal masks, endotracheal intubation, or tracheostomy.
For the purposes of this chapter, NIV refers to mechanical ventilatory support delivered through a face mask, nasal mask, or similar device.
Classification
Invasive mechanical ventilation and NIV have similar physiologic principles.
The modes of ventilatory support (i.e., the way in which the ventilator triggers, delivers, and ends the breath) are similar to invasive mechanical ventilation. However, there is no standardization between manufacturers regarding mode terminology.
Two of the most commonly encountered modes include continuous positive airway pressure (CPAP) and bilevel positive airway pressure (BiPAP).
Continuous Positive Airway Pressure
CPAP maintains a set positive pressure throughout the respiratory cycle (inhalation and exhalation) and is not ventilatory support in a strict physiologic sense.
CPAP “stents open” the upper airway with continuous pressure. This concept helps explain the utility of CPAP in disorders such as obstructive sleep apnea but does not explain why a treatment that does not provide ventilatory support can be of use in the patient who is suffering from hypoxemic or hypercapnic respiratory failure.
Clinical applications include:
Hypoxemic respiratory failure
Increases partial pressure of oxygen in the alveoli. In the alveolar gas equation, PAO2 = FiO2 (PB − 47) − 1.2 (PaCO2), if PB is barometric pressure (or in our case, the pressure delivered from the ventilator through the mask), an increase in the mean airway pressure throughout the respiratory cycle for a given fraction of inspired oxygen (FiO2) will increase the partial pressure of inspiratory oxygen and therefore oxygen tension in the alveoli (PAO2).
Provides extrinsic positive end-expiratory pressure (PEEP). It recruits the underventilated or collapsed lung, probably by preventing alveolar collapse during exhalation.
Hypercapnic respiratory failure can decrease the work of breathing, by overcoming intrinsic PEEP in patients with chronic obstructive airway disease.
In advanced chronic obstructive pulmonary disease (COPD) with hyperinflation, airflow obstruction and decreased elastic recoil lead to a prolonged expiratory phase. In respiratory distress, inspiration may occur before expiration is completed, leading to dynamic hyperinflation.
Ineffective ventilation and increasing work of breathing cause the buildup of carbon dioxide and worsening respiratory acidosis. The positive elastic recoil pressure left behind in this hyperinflated patient at the end of expiration is termed intrinsic PEEP.
Delivering PEEP via CPAP lessens the work of breathing by overcoming intrinsic PEEP. In intubated patients with acute respiratory failure, extrinsic PEEP (PEEPe) has been demonstrated to reduce the work of breathing by 50%. The same principle applies to the noninvasively ventilated patient.
Bilevel Positive Airway Pressure
BiPAP is CPAP with a second level of pressure support during inspiration, akin to pressure support ventilation for mechanically ventilated patients.
In practical terms, BiPAP requires the operator to set two variables, inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP), that are measured in cm H2O.
IPAP is the ventilatory pressure support the patient receives when either the machine or the patient initiates a breath.
EPAP is the pressure against which the patient exhales at the termination of inhalation.
“Initial settings” are often referred to by the IPAP followed by the EPAP (e.g., 12 cm H2O and 5 cm H2O).
The greater the difference between the IPAP and the EPAP, the greater the theoretical ventilatory support the patient receives.
However, progressively higher levels of EPAP and IPAP are not usually well tolerated by the patient, and as with any initiation of NIV, the patient should be observed closely to see if effective patient–ventilator synchrony occurs.
Specific Disease Indications for Noninvasive Ventilation
NIV may improve outcomes by avoiding intubation and the attendant risks of secondary infections in this patient population.
In general, NIV is most effective in patients with cardiogenic pulmonary edema, patients with hypercapnic respiratory failure, and in weaning patients from invasive mechanical ventilation.
Cardiogenic Pulmonary Edema
NIV helps to unload the respiratory muscles in respiratory failure caused by heart failure and pulmonary edema and improves cardiac performance by reducing right and left ventricular preload and mean transmural filling pressures.
CPAP is recommended for hypoxemic patients with cardiogenic pulmonary edema who remain hypoxemic despite maximal medical therapy.
A recent meta-analysis pooled the results of 34 trials in patients with acute cardiogenic pulmonary edema confirmed an overall mortality benefit utilizing NIV (both CPAP or bilevel modes), as well as a reduction in the risk of intubation.1
Noninvasive methods of respiratory support should not be used in hemodynamically unstable patients or in those with ongoing cardiac ischemia.
Chronic Obstructive Pulmonary Disease
NIV can be an effective initial respiratory support modality in the setting of acute COPD exacerbation. Studies have shown improvements in pH, PCO2, and respiratory rate, and lower intubation rates and lower mortality when compared to standard therapy.2,3
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