Effects of Increasing Intrathoracic Pressure on Cardiac Parameters in a Normal Heart
Set A is the correct answer - increasing intrathoracic pressure causes an increase in right atrial pressure, a decrease in transmural LV systolic pressure, and a decrease in intrathoracic blood volume. 1
Physiological Effects on Individual Parameters
Right Atrial Pressure
- Increasing intrathoracic pressure (as occurs during positive pressure ventilation or Valsalva maneuver) directly increases right atrial pressure by compressing the heart chambers within the thoracic cavity 1
- This occurs because the right atrium is a thin-walled, compliant structure that readily transmits surrounding pressure changes to its intracavitary pressure 1
- The increased right atrial pressure acts as a "back pressure" that reduces the gradient for venous return to the heart 1
Transmural LV Systolic Pressure
- Transmural pressure is defined as the difference between intracavitary pressure and surrounding pressure (intrathoracic pressure) 1
- When intrathoracic pressure increases, the pressure surrounding the left ventricle also increases 1
- Even if intracavitary LV systolic pressure remains constant, the transmural LV systolic pressure (the effective pressure for ejection) decreases 1, 2
- This effectively reduces LV afterload, making it easier for the left ventricle to eject blood 1
Intrathoracic Blood Volume
- Increased intrathoracic pressure decreases the pressure gradient between the systemic venous circulation and the right atrium 1
- This reduced gradient impairs venous return to the heart 1
- The impaired venous return leads to blood pooling in the extrathoracic venous system and decreased intrathoracic blood volume 1
- During positive pressure ventilation, the gradient for venous return is decreased throughout the ventilatory cycle 1
Heart-Lung Interactions During Increased Intrathoracic Pressure
Effects on Right Heart Function
- Increased intrathoracic pressure has dual effects on the right heart: 1
- Decreased venous return due to increased right atrial back pressure
- Increased right ventricular afterload due to increased transpulmonary pressure
- These combined effects can significantly reduce right heart output 1
- The normal right ventricle is highly compliant but has limited myocardial thickness and contractile power, making it relatively more sensitive to afterload changes 1
Effects on Left Heart Function
- Increased intrathoracic pressure has opposing effects on the left heart: 1
- Decreased LV afterload due to reduced transmural pressure
- Initially increased LV preload due to pulmonary blood volume displacement
- The net effect typically favors improved left heart ejection in normal hearts 1
- However, reduced right heart output will eventually lead to reduced left heart filling 1
Clinical Applications and Implications
Positive Pressure Ventilation
- During positive pressure ventilation, these physiological changes occur cyclically 1
- The application of constant positive pressure (PEEP) results in a sustained decrease in venous return throughout the cardiac cycle 1
- Approximately 50% of alveolar pressure changes are transmitted to pleural pressure in normal lungs 1
- These effects form the basis for diagnosing cardiac conditions like tamponade and predicting volume responsiveness 1
Valsalva Maneuver
- During the Valsalva maneuver (forced expiration against a closed glottis), intrathoracic pressure increases significantly 1
- This produces a characteristic blood pressure response with initial increase (Phase 1), followed by decrease (Phase 2), then brief reduction after strain release (Phase 3), and finally an overshoot (Phase 4) 1
- In heart failure patients, this normal response pattern is altered, providing diagnostic information 1
In conclusion, the physiological effects of increased intrathoracic pressure on cardiac parameters follow Set A: increased right atrial pressure, decreased transmural LV systolic pressure, and decreased intrathoracic blood volume 1.