A.2. Describe the pathophysiology of cardiac tamponade.
Answer:
The pericardium is a two-layered blind sac surrounding the atria, ventricles, proximal great vessels, venae cava, and pulmonary veins. Normally, 10 to 50 mL of fluid exists within the pericardial space. This plasma-like fluid is produced by the serous pericardium and is drained by the lymphatic system. The serous pericardium consists of a visceral portion, which is adherent to the epicardium, and a parietal layer, which is adherent to the outer, fibrous pericardium. The relatively noncompliant fibrous pericardium constrains the heart within the thoracic cavity. The normal physiologic functions of this pericardial constraint include limiting acute ventricular dilatation with increases in preload, preserving the "optimal" ventricular shape for mechanical efficiency, and maintaining the normal coupling of right ventricular (RV) and left ventricular (LV) function.
Cardiac tamponade is defined as a pathologic increase in intrapericardial pressure causing a decrease in CO. In normal conditions, intrapericardial pressure is very close to intrathoracic pressure, being slightly negative relative to atmosphere at rest. In cardiac tamponade, intrapericardial pressure increases and approaches atrial and ventricular diastolic pressures. This forms a Starling resistor system that progressively impairs diastolic cardiac filling and SV, eventually causing hypotension and shock depending on the rate of pericardial pressure increase relative to intracardiac filling pressures and the compliance of the pericardial sac (Table 12.2).
Table 12.2: Three Phases of Cardiac Tamponade
| Phase | Pathophysiologic Events | Pressure Relationship |
|---|---|---|
| I | Pericardial fluid increases ventricular stiffness, increasing filling pressures. | LVEDP > RVEDP > IPP |
| II | Pericardial pressure approaches right-sided filling pressures, exerts Starling resistor forces on RV filling. Respirophasic variation in stroke volume appears. | LVEDP > IPP = RVEDP |
| III | Pericardial pressure approaches left-sided filling pressures. Hemodynamic instability is prominent. | IPP = LVEDP = RVEDP |
In defining cardiac tamponade as a state where intrapericardial pressure approaches cardiac diastolic pressures, it is important to note that cardiac tamponade can result from a decrease in cardiac filling pressures in the presence of a modest elevation in intrapericardial pressure. This has been described as "low-pressure cardiac tamponade."
References
- Ariyarajah Y, Spodick DH. Cardiac tamponade revisited: a postmortem look at a cautionary case. Tex Heart Inst J. 2007;34:347-351.
- Imazio M, De Ferrari GM. Cardiac tamponade: an educational review. Eur Heart J Acute Cardiovasc Care. 2020;10:102-109.
- Saito Y, Donohue A, Attai S, et al. The syndrome of cardiac tamponade with "small" pericardial effusion. Echocardiography. 2008;25:321-327.