TABLE 1.

Basic Pulmonary Physiology and Cardiopulmonary Interactions

Ppleural Affects RV and LV Preload and Afterload
1. Ppleural is determined by the balance of the tendency of alveolar units toward collapse (elastic recoil) versus of the thoracic wall to spring outwards and action of respiratory muscles
2. Changes in Ppleural generally influence RV inflow and LV outflow, while changes in transpulmonary pressure (Palv-Ppleural) influence RV outflow and LV inflow.
3. Negative Ppleural a) increases venous return and preload; b) decreases RV afterload; and c) increases LV afterload.
4. Positive pressure ventilation increases Ppleural and a) decreases preload; b) increases RV afterload; and c) decreases LV afterload
5. Large shifts in Ppleural (e.g., respiratory distress) can significantly increase LV afterload.
PEEP Affects RV and LV Hemodynamics
1. Total PEEP is the sum of extrinsic PEEP (generated by the ventilator) and intrinsic or auto-PEEP (due to incomplete exhalation).
2. Extrinsic PEEP is commonly used in the CICU for its beneficial effects on oxygenation, alveolar recruitment, airway patency, and preload.
3. PEEP: a) increases pulmonary vascular resistance; b) decreases RV and LV preload; c) decreases LV afterload; and d) reduces LV compliance through interventricular dependence.
4. The effect of PEEP on cardiac output varies with preload dependence and LV contractility and compliance.
Airway Pressure Influences Hemodynamics Via its Impact on Alveolar Pressure and Pleural Pressure
1. Airway pressure is determined by the flow, airway resistance, tidal volume, compliance of the chest wall and lung parenchyma, and the total end-expiratory pressure.
2. Positive pressure ventilation exerts its effects on cardiovascular hemodynamics principally through its impact on Palv and Ppleural.
3. In poorly compliant lungs, changes in intrathoracic pressure will have more pronounced effects on hemodynamics.

LV = left ventricular; P_alv = alveolar pressure; P_aw = airway pressure; PEEP = positive end-expiratory pressure; P_pleural = pleural pressure; RV = right ventricular.