Using CPAP for patients with pulmonary edema
What are the physiological changes seen in acute cardiogenic pulmonary edema (ACPE)?
In acute cardiogenic pulmonary edema (ACPE), an increased left ventricular end-diastolic pressure (Fig. 1a) causes the left atrium to pump against an increased load (Fig. 1b). The atrium becomes overwhelmed, and an increased hydrostatic pressure gradient is created (Fig. 1c). Eventually, the pulmonary interstitium becomes overloaded, and the pulmonary veins widen, due to the accumulation of fluid (Fig. 1d).
Figure 1. The progress of physiological changes in the heart and central vessels in acute cardiogenic pulmonary edema (ACPE), a) increased left ventricular end-diastolic pressure, b) increased pressure in the left atrium, c) left atrium is overwhelmed and pressure gradient is created, d) widening of the pulmonary veins as fluid backs up in the pulmonary interstitium.
Respiratory system changes
The cardiovascular changes and resulting fluid overload in the pulmonary interstitium may cause both alveolar collapse and fluid accumulation in the alveoli.
Figure 2. Physiological changes to the alveoli seen in acute cardiogenic pulmonary edema (ACPE). The fluid overload in the pulmonary interstitium causes alveolar collapse and fluid accumulation.
What is the outcome of the physiological changes associated with ACPE?
Both of these processes limit the amount of oxygen that can get into the bloodstream such that, clinically, patients develop respiratory distress and hypoxic respiratory failure. Acute cardiogenic pulmonary edema can happen very quickly and has the possibility of high morbidity and even mortality. The in-hospital mortality rate can be as high as 12%!
How do I treat my patients with ACPE?
Noninvasive ventilation (NIV) for ACPE was first evaluated in the 1930s but didn't receive widespread use until the 1970s. In contemporary practice, the use of continuous positive airway pressure (CPAP) for acute pulmonary edema should be considered early and often. Its use has been strongly recommended in clinical practice guidelines with the following four aims:
- Reducing cardiac preload
- Reducing afterload
- Removing excess volume
- Recruiting areas of the lung with ventilation-perfusion (V/Q) mismatch
In fact, positive pressure NIV can address many of these issues at once and has been shown to improve a variety of clinical outcomes:
- Decreases respiratory rate
- Shortens the length of intensive care unit (ICU) stay
- Decreases need for endotracheal intubation
- Lowers mortality rate
What are the CPAP settings for a patient with ACPE?
- When possible, start with low pressures and titrate up to help with compliance.
- CPAP from 8–12 cmH2O worked best for patients in most clinical studies.
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- Berbenetz, N, Wang, Y, Brown, J, et al. 2019. Non-invasive positive pressure ventilation (CPAP or bilevel NPPV) for cardiogenic pulmonary oedema. Cochrane Database Syst Rev. 4: CD005351. PMID: 30950507