Conversely, the administration of positive pressure of 5 cm H2O will generate an increase in lung volume of 500 mL.
This means that in a normal lung the administration of 500 ml of air via positive pressure ventilation will increase the alveolar pressure by 5 cm H2O. Normal lung compliance is around 100 ml/cmH20. Proper management of mechanical ventilation also requires an understanding of lung pressures and lung compliance. At the same time, the decreased return may improve overdistension in the left ventricle, placing it at a more advantageous point in the Frank-Starling curve and possibly improving cardiac output. In these patients with volume overload, decreasing venous return will directly decrease the amount of pulmonary edema being generated, by decreasing right cardiac output. The effects of mechanical ventilation with positive pressure on the venous return may be beneficial when used in patients with cardiogenic pulmonary edema. Reducing the work from respiratory muscles also reduces the generation of CO2 and lactate from these muscles, helping improve acidosis. This, in turn, decreases blood flow to respiratory muscles and redistributes it to more critical organs. On the other hand, mechanical ventilation with positive pressure can significantly decrease the work of breathing. This is a very important consideration in patients who may not be able to increase their SVR, like in patients with distributive shock (septic, neurogenic, or anaphylactic shock). Less 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, which will result in a drop in mean arterial pressure (MAP) if there is not a compensatory response by increasing systemic vascular resistance (SVR). This has a double effect in decreasing cardiac output: Less blood in the right ventricle means less blood reaching the left ventricle and less blood that can be pumped out, decreasing cardiac output. The increased RA pressure and decreased venous return generate a decrease in preload.
The positive pressure generated by the ventilator transmits to the upper airways and finally to the alveoli, this, in turn, is transmitted to the alveolar space and thoracic cavity, creating positive pressure (or at least less negative pressure) in the pleural space. The application of positive pressure ventilation changes this physiology. This same negative intrathoracic pressure decreases the right atrial (RA) pressure and generates a sucking effect on the inferior vena cava (IVC), increasing venous return. When the diaphragm pushes down during inspiration, negative pressure in the pleural cavity is generated, this, in turn, creates negative pressure in the airways that suck air into the lungs. Normal respiratory physiology works as a negative pressure system. Mechanical ventilation has several effects on lung mechanics. Since having a patient on mechanical ventilation allows a practitioner to modify the patient’s ventilation and oxygenation, it has an important role in acute hypoxic and hypercapnic respiratory failure as well as in severe metabolic acidosis or alkalosis. In respiratory physiology, total compliance is a mix of lung and chest wall compliance as these two factors cannot be separated in a patient. Compliance: Change in volume divided by change in pressure.
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