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# Mean Airway Pressure and Cardiac Output **Compliance** * In lungs with normal compliance, about 50% of the airway pressure is transmitted to the thoracic cavity. * In noncompliant or stiff lungs (e.g., atelectasis, ARDS), the pressure transmitted to the thoracic cavity is much less due to the da...

# Mean Airway Pressure and Cardiac Output **Compliance** * In lungs with normal compliance, about 50% of the airway pressure is transmitted to the thoracic cavity. * In noncompliant or stiff lungs (e.g., atelectasis, ARDS), the pressure transmitted to the thoracic cavity is much less due to the dampening effect of the nonelastic lung tissues. * High levels of PIP (positive inspiratory pressure) or PEEP (positive end-expiratory pressure) may be required to ventilate and oxygenate patients with low compliance. * The decrease in cardiac output due to excessive PIP or PEEP is less severe than with normal lungs. **Mechanical Ventilation Considerations** * Mechanical ventilation creates airflow by generating a pressure gradient. * Pressures in the airways, thoracic cage, and pulmonary blood vessels are altered. * Cardiovascular functions should be evaluated and monitored to prevent adverse effects of positive pressure ventilation on the heart and blood vessels. **Positive Airway Pressure (PEEP)** * PEEP is an airway pressure strategy used in ventilation. * PEEP exerts a more negative effect on cardiac output. * The effect of PEEP can be detrimental to cardiac output because PEEP is the end-expiratory pressure during ventilation, while, CPAP includes only the airway pressure during spontaneous breathing. **Other relevant factors** * Mean airway pressure (mPaw) is the average pressure in the airways during one complete respiratory cycle. * It is related to inspiratory time, respiratory frequency, peak inspiratory pressure, and PEEP. * Low airway resistance leads to a higher PIP. * High compliance leads to a lower PIP. * Tidal volume, airway resistance, and compliance are directly related to peak inspiratory flow rate and inversely related to compliance. * These factors directly affect the function of major organ systems, including the thorax, as they depend on adequate blood flow and perfusion. * A decrease in venous return (or filling of the ventricles) leads to a reduction in stroke volume and cardiac output. * Stroke volume is the blood volume output delivered by one ventricle during contraction. * Total oxygen delivery is the product of oxygen content and cardiac output. * During spontaneous inspiration, there may be a temporary decrease in arterial blood pressure, known as pulsus paradoxus. * A significant reverse pulsus paradoxus (increase in systolic pressure >15 mm Hg) during positive pressure ventilation may be a sign of hypovolemia.