Negative Pressure Ventilation and Pulmonary Vascular Resistance

Negative pressure ventilation (NPV) mimics normal physiological breathing by creating a subatmospheric pressure around the thorax, allowing airflow to flow into the lungs passively. Unlike positive pressure ventilation (PPV), which forces air into the lungs, NPV reduces intrathoracic pressure, leading to significant cardiovascular and pulmonary benefits. This includes improved pulmonary vascular resistance (PVR), making negative pressure ventilation a crucial strategy for optimizing both respiratory and cardiac function.

How Negative Pressure Ventilation Affects Pulmonary Vascular Resistance (PVR)

Pulmonary vascular resistance is a key determinant of right ventricular after load. High PVR increases the strain on the right ventricle, potentially leading to right heart failure over time.

NPV lowers PVR through several mechanisms. By creating a more negative intrathoracic pressure, it facilitates venous return, allowing greater right ventricular filling. Additionally, it reduces external compression on pulmonary vessels, enhancing blood flow.

IPPV often increases PVR due to the elevation of intrathoracic pressures. As a result, the forced inflation of alveoli can compress pulmonary capillaries, increasing resistance and impeding right ventricular ejection. This makes NPV particularly beneficial for patients with conditions like:

• Pulmonary Hypertension
• Chronic Obstructive Pulmonary Disease (COPD)
• Acute Respiratory Distress Syndrome (ARDS)

For these patients, minimizing pulmonary vascular resistance is crucial for hemodynamics stability.

How Negative Pressure Ventilation Improves Ventilation-Perfusion Matching (V/Q Matching)

Another way NPV benefits PVR is through improved ventilation-perfusion (V/Q) matching. By allowing more physiological lung expansion, NPV promotes better alveolar recruitment in well-perfused lung regions, reducing the risk of hypoxic vasoconstriction.

Hyper pulmonary vasoconstriction (HPV) is a natural response to low oxygen levels that diverts blood flow from poorly ventilated alveoli to better-oxygenated areas. However, in disease states, widespread HPV increases PVR and right ventricular workload.

By optimizing alveolar ventilation, NPV mitigates this response, further reducing pulmonary vascular resistance.

Functional Residual Capacity (FRC) and Cardiac Function

Functional residual capacity (FRC) is the volume of air remaining in the lungs at the end of a normal exhalation. It plays a viral role in:

✔ Maintaining pulmonary compliance
✔ Preventing alveolar collapse (atelectasis)
✔ Ensuring efficient gas exchange

Why Proper FRC management is Essential

Maintaining optimal FRC is critical for cardiac function, as it influences:

• Intrathoracic pressures
• Pulmonary vascular resistance
• Venous return

FRC is closely tied to lung and chest wall mechanics. For instance:

• If FRC is too low, alveolar units collapse (atelectasis), leading to impaired oxygenation and increased PVR. Hypoxia triggers pulmonary vasoconstriction, increasing right ventricular after load and reducing overall cardiac efficiency.
• If lung inflation is excessive (as seen with high positive end-expiratory pressure [PEEP] or high tidal volumes in PPV), intrathoracic pressure rise. This reduces venous return, elevates right ventricular after load, and impairs cardiac function.

Maintaining an optimal FRC ensures:

• Alveoli remain open without overinflation
• Hypoxic vasoconstriction is minimized
• Left ventricular performance is stabilized

Since the heart and lungs share the thoracic cavity, excessive lug inflation can compress the left atrium and pulmonary veins, reducing left ventricular preload. This leads to:

• Decreased cardiac output
• System hypotension

NPV vs. PPV in Maintaining FRC

Negative pressure ventilation (NPV) plays a role in maintaining FRC in a more physiologically natural manner than positive pressure ventilation.

✔ NPV enhances diaphragmatic function and allows for normal chest wall movement
✔ It promotes even lung expansion and optimizes alveolar recruitment without overdistension
✔ This helps sustain appropriate FRC levels, balancing both pulmonary and cardiac function

Conclusion

Negative pressure ventilation offers significant advantages in reducing pulmonary vascular resistance by:

✔ Lowering intrathoracic pressure
✔ Improving venous return
✔ Enhancing alveolar recruitment

By contrast, positive pressure ventilation can inadvertently elevate PVR, increasing stress on the right heart.

Additionally, maintaining optimal functional residual capacity is crucial for cardiovascular stability, as it:

✔ Prevents atelectasis
✔ Minimizes pulmonary vasoconstriction
✔ Ensures proper ventricular filling

These physiological principles why Negative Pressure Ventilation and Pulmonary Vascular Resistance go hand in hand, offering better support for both pulmonary and cardiac health.