How does the pericardium influence haemodynamics during the respiratory cycle?

The Pericardium's Role in Respiratory Cycle Hemodynamics

The pericardium plays a crucial role in cardiac hemodynamics during the respiratory cycle by mediating ventricular interdependence, limiting cardiac chamber distension, and transmitting intrathoracic pressure changes to the heart chambers, which significantly impacts venous return and cardiac output. 1

Normal Respiratory Cycle Hemodynamics

Spontaneous Breathing

• Inspiration:

  • Pleural pressure (Ppl) decreases relative to atmospheric pressure
  • Increased gradient for venous return to the right ventricle (RV)
  • Transient increase in RV filling and output
  • Left ventricular (LV) afterload increases as intracavitary cardiac pressures lower relative to atmosphere 1

• Expiration:

  • Ppl rises as inspiratory effort releases
  • Small expiratory rise in arterial pressure
  • Decreased venous return to the RV 1

Positive Pressure Ventilation

• Inspiration:

  • Increased Ppl decreases venous return to the RV
  • Decreased LV afterload (beneficial effect offset by reduced venous return)
  • Transiently decreased cardiac output 1

• Expiration:

  • Falling Ppl tends to decrease systemic arterial pressure
  • Venous return gradient partially restored 1

The Pericardium's Specific Functions

1. Ventricular Interdependence

• The pericardium enhances ventricular interdependence by creating a shared space where both ventricles must function
• During inspiration with increased RV volumes, the interventricular septum shifts leftward, reducing LV filling
• This septal shift, mediated by the pericardial constraint, contributes to pulsus paradoxus (≥10 mmHg fall in systolic blood pressure during inspiration) 1

2. Limitation of Cardiac Distension

• The pericardium limits acute cardiac chamber distension
• In conditions of RV overload (e.g., pulmonary embolism), RV dilatation within the pericardial space causes:
  • Leftward shift of the interventricular septum
  • Reduced LV preload and compliance
  • Decreased cardiac output 1

3. Transmission of Intrathoracic Pressure

• The pericardium transmits intrathoracic pressure changes to the heart
• Changes in pleural pressure affect cardiac filling pressures with slopes of approximately 0.71-0.80 for the RV and 0.39-0.78 for the LV 2
• This differential transmission explains why the LV is more sensitive to pericardial pressure changes than to general intrathoracic pressure changes 2

Pathophysiological Implications

Cardiac Tamponade

• Pericardial fluid accumulation increases intrapericardial pressure above intracavitary pressure
• Reduced transmural pressure decreases chamber filling and cardiac output
• Inspiratory increase in venous return to the RV exaggerates septal shift
• Results in pulsus paradoxus and decreased LV stroke volume during inspiration 1, 3, 4

Pulmonary Hypertension and RV Dysfunction

• In pulmonary hypertension, increased RV afterload leads to RV dilatation
• The pericardial constraint causes:
  • Decreased LV preload
  • Abnormal interventricular septal geometry
  • Overall decrease in cardiac index 1
• This is particularly problematic in acute conditions (e.g., pulmonary embolism) when the RV is not hypertrophied 1

Mechanical Ventilation in ARDS

• Positive pressure ventilation increases mean airway pressure (mPaw)
• The pericardium transmits this pressure to the heart chambers
• High PEEP and driving pressures can:
  • Decrease venous return gradient
  • Increase RV afterload
  • Compromise LV filling through ventricular interdependence 1, 5

Clinical Applications

Cardiac Cycle-Specific Ventilation

• Appropriately timed increases in intrathoracic pressure can augment ventricular ejection
• Systolic-timed pressure increases can improve LV performance in ventricular failure
• This demonstrates how understanding pericardial mechanics can be leveraged therapeutically 6, 7

Ventilation Strategies in RV Dysfunction

• Low tidal volumes (6-8 mL/kg)
• Peak pressures <30 cmH₂O
• Limited PEEP (≤10 cmH₂O)
• Avoidance of hypercapnia and acidosis 5

Summary

The pericardium serves as a critical mediator between respiratory mechanics and cardiac function. By constraining cardiac chambers, enhancing ventricular interdependence, and transmitting intrathoracic pressure changes, it significantly influences hemodynamics throughout the respiratory cycle. Understanding these relationships is essential for managing patients with cardiopulmonary disease, particularly those requiring mechanical ventilation or those with conditions affecting the pericardium itself.