This is a very important point to have in mind, especially with patients who may not be able to increase their SVR, such as those with distributive shock (e.g., septic, neurogenic, or anaphylactic). This has a double effect in decreasing cardiac output: less blood in RV means less blood reaching LV and less blood that can be pumped out decreasing cardiac output, at the same time, the decreased preload means that the heart works at a less efficient point in the frank-startling curve, generating less effective work and further decreasing cardiac output and resulting in a drop in mean arterial pressure (MAP) if there is not a compensatory response by increasing systemic vascular resistance (SVR). This increases RA pressure and decreases venous return, generating a decrease in preload. The positive pressure generated by the ventilator or BPAP transmits to the upper airways and finally to the alveoli which are transmitted to the alveolar space and thoracic cavity, creating positive pressure (or at least less negative pressure). The application of extrinsic PEEP changes this physiology. This same negative intrathoracic pressure decreases the right atrial (RA) pressure and generates a sucking effect on the IVC increasing venous return. When the diaphragm pushes down during inspiration, negative pressure in the pleural cavity is generated, creating negative pressure in the airways that suck air into the lungs. Normal respiratory physiology works as a negative pressure system.
The use of Extrinsic PEEP also can cause some complications. By decreasing work of breathing, CO2 and lactate production decreases, decreasing the need for high minute ventilation (to correct the hypercapnia and acidosis) and thereby decreasing respiratory drive and further decreasing the work of breathing needed by the patient in a positive-effect loop. This increases CO2 and lactate production, both of which may be problems of their own. In intubated patients with low compliance, work of breathing can represent an important part of their total energy expenditure (up to 30%). This is especially important for stiff lungs with low compliance. If a patient needs to clear CO2 by improving ventilation, he should receive some level of pressure support for his ventilation, either via BPAP or invasive ventilation.Įxtrinsic PEEP also significantly decreases the work of breathing. Nevertheless, extrinsic PEEP should never be used for the sole purpose of increasing ventilation. By opening up airways, the alveolar surface increases, creating more areas for gas exchange and somewhat improving ventilation. The application of extrinsic PEEP will, therefore, have a direct impact on oxygenation and an indirect impact on ventilation. The application of positive pressure inside the airways can open or “splint” airways that may otherwise be collapsed, decreasing atelectasis, improving alveolar ventilation, and, in turn, decreasing VQ mismatch. This, in turn, increases the solubility of oxygen and its ability to cross the alveolocapillary membrane and increase the oxygen content in the blood.Įxtrinsic PEEP also can be used to improve ventilation-perfusion (VQ) mismatches. This applies to mechanical or noninvasive ventilation in that increasing PEEP will increase the pressure in the system. By Henry’s law, the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the surface of the solution. Extrinsic PEEP can be used to increase oxygenation.
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