Significance of the pericardium in human subjects: effects on left ventricular volume, pressure and ejection

Cardiac magnetic resonance imaging of pericardial diseases: a comprehensive guide

Cardiac magnetic resonance (CMR) imaging has been established as a valuable diagnostic tool in the assessment of pericardial diseases by providing information on cardiac anatomy and function, surrounding extra-cardiac structures, pericardial thickening and effusion, characterization of pericardial effusion, and the presence of active pericardial inflammation from the same scan. In addition, CMR imaging has excellent diagnostic accuracy for the non-invasive detection of constrictive physiology evading the need for invasive catheterization in most instances. Growing evidence in the field suggests that pericardial enhancement on CMR is not only diagnostic of pericarditis but also has prognostic value for pericarditis recurrence, although such evidence is derived from small patient cohorts. CMR findings could also be used to guide treatment de-escalation or up-titration in recurrent pericarditis and selecting patients most likely to benefit from novel treatments such as anakinra and rilonacept. This article is an overview of the CMR applications in pericardial syndromes as a primer for reporting physicians. We sought to provide a summary of the clinical protocols used and an interpretation of the major CMR findings in the setting of pericardial diseases. We also discuss points that are less well clear and delineate the strengths and weak points of CMR in pericardial diseases.

Mechanical Interaction of the Pericardium and Cardiac Function in the Normal and Hypertensive Rat Heart

The pericardium is a thin connective tissue membrane that surrounds the heart and is an integral regulatory component of cardiopulmonary performance. Pathological growth and remodeling of the right ventricle (RV) stemming from structural heart diseases are thought to include a significant role of the pericardium, but its exact role remains unclear. The objective of this study was to investigate potential biomechanical adaptations of the pericardium in response to pulmonary hypertension and their effects on heart behavior. Integrated computational-experimental modeling of the heart offers a robust platform to achieve this objective. We built upon our recently developed high-fidelity finite-element models of healthy and hypertensive rodent hearts via addition of the pericardial sac. In-silico experiments were performed to investigate changes in pericardium reserve elasticity and their effects on cardiac function in hypertensive hearts. Our results suggest that contractile forces would need to increase in the RV and decrease in the left ventricle (LV) in the hypertensive heart to compensate for reductions in pericardium reserve elasticity. The discrepancies between chamber responses to pericardium addition result, in part, from differences in the impact of pericardium on the RV and LV preload. We further demonstrated the capability of our platform to predict the effect of pericardiectomy on heart function. Consistent with previous results, the effect of pericardiectomy on the chamber pressure-volume loop was the largest in the hypertensive RV. These insights are expected to motivate further computational investigations of the effect of pericardiectomy on cardiac function which remains an important factor in surgical planning of constrictive pericarditis and coronary artery bypass grafting.

Pericardial Disease in the Intensive Care Setting

Pericardial disease commonly occurs in the intensive care setting, but its timely diagnosis may be missed. The normal pericardium serves as a lubricated sac within which the heart may beat with minimal friction. The effect of the pericardium on cardiac filling at normal diastolic pressures is not clear; however, it may limit cardiac dilation in states of acute volume overload such as mitral regurgitation and right ventricular infarction.

Pericardial disease may be divided into two catego ries : those cases that result from inflammation of the pericardium (pericarditis), and those cases in which a pericardial effusion or the thickened pericardium itself causes hemodynamic changes (tamponade and constric tion). Simple pericarditis should not lead to any hemo dynamic alteration other than tachycardia. In both tam ponade and constriction, the jugular venous pressure is elevated with low forward cardiac output; tamponade typically shows pulsus paradoxus, whereas constric tion more frequently shows Kussmaul's sign.

The electrocardiogram may show diffuse ST segment elevation with PR segment depression in pericarditis; a large pericardial effusion, even with early tamponade, may not by itself cause any changes in the electrocar diogram. The echocardiogram is invaluable in diagnos ing the presence of a pericardial effusion and recogniz ing tamponade physiology (diastolic collapse of the right ventricular outflow tract and invagination of the right atrium).

In selected patients, simple pericarditis may be managed outside of the hospital. Anyone suspected of having a hemodynamically significant pericardial effu sion should be hospitalized, usually in an intensive care unit. Pericardiocentesis should be performed under op timal monitoring conditions, although in an emergency, blind pericardiocentesis may be attempted. Recognition of the cause of the pericardial process will guide its treatment. Management of selected pericardial syn dromes is discussed later in this review.

Three-wall segment (TriSeg) model describing mechanics and hemodynamics of ventricular interaction

A mathematical model (TriSeg model) of ventricular mechanics incorporating mechanical interaction of the left and right ventricular free walls and the interventricular septum is presented. Global left and right ventricular pump mechanics were related to representative myofiber mechanics in the three ventricular walls, satisfying the principle of conservation of energy. The walls were mechanically coupled satisfying tensile force equilibrium in the junction. Wall sizes and masses were rendered by adaptation to normalize mechanical myofiber load to physiological standard levels. The TriSeg model was implemented in the previously published lumped closed-loop CircAdapt model of heart and circulation. Simulation results of cardiac mechanics and hemodynamics during normal ventricular loading, acute pulmonary hypertension, and chronic pulmonary hypertension (including load adaptation) agreed with clinical data as obtained in healthy volunteers and pulmonary hypertension patients. In chronic pulmonary hypertension, the model predicted right ventricular free wall hypertrophy, increased systolic pulmonary flow acceleration, and increased right ventricular isovolumic contraction and relaxation times. Furthermore, septal curvature decreased linearly with its transmural pressure difference. In conclusion, the TriSeg model enables realistic simulation of ventricular mechanics including interaction between left and right ventricular pump mechanics, dynamics of septal geometry, and myofiber mechanics in the three ventricular walls.

Atrioventricular nonuniformity of pericardial constraint

Physiologists and clinicians commonly refer to "pressure" as a measure of the constraining effects of the pericardium; however, "pericardial pressure" is really a local measurement of epicardial radial stress. During diastole, from the bottom of the y descent to the beginning of the a wave, pericardial pressure over the right atrium (P(pRA)) is approximately equal to that over the right ventricle (P(pRV)). However, in systole, during the interval between the bottom of the x descent and the peak of the v wave, these two pericardial pressures appear to be completely decoupled in that P(pRV) decreases, whereas P(pRA) remains constant or increases. This decoupling indicates considerable mechanical independence between the RA and RV during systole. That is, RV systolic emptying lowers P(pRV), but P(pRA) continues to increase, suggesting that the relation of the pericardium to the RA must allow effective constraint, even though the pericardium over the RV is simultaneously slack. In conclusion, we measured the pericardial pressure responsible for the previously reported nonuniformity of pericardial strain. P(pRA) and P(pRV) are closely coupled during diastole, but during systole they become decoupled. Systolic nonuniformity of pericardial constraint may augment the atrioventricular valve-opening pressure gradient in early diastole and, so, affect ventricular filling.

Real-time three-dimensional echocardiography to construct clinically ready, load-independent indices of myocardial contractile performance

BACKGROUND

Real-time 3-dimensional echocardiography (RT3DE) reliably determines intracardiac chamber volumes without left ventricular (LV) geometric assumptions, yet clinical assessment of contractile performance is often on the basis of potentially inaccurate, load-dependent indices such as ejection fraction.

METHODS

In 6 chronically instrumented dogs, RT3DE estimated LV volumes at various loading conditions. Preload recruitable stroke work and end-systolic pressure-volume relationships were constructed. RT3DE-derived indices were compared with similar relationships determined by sonomicrometry.

RESULTS

Highly linear preload recruitable stroke work and end-systolic pressure-volume relationships were constructed by RT3DE and sonomicrometry. Mean preload recruitable stroke work slopes correlated between methods, but volume intercepts differed as a result of geometric assumptions of sonomicrometry. Conversely, RT3DE-derived end-systolic pressure-volume relationships did not correlate well with sonomicrometry.

CONCLUSIONS

These data are unique in reporting load-independent measures of LV performance using RT3DE. These techniques would strengthen evaluation of LV function after myocardial ischemia or cardiac operation, in which frequent changes in ventricular geometry or loading conditions confound functional assessment by more traditional methods.

Diastolic ventricular interaction and ventricular diastolic filling

Because the ventricles share a common septum, the filling of one may influence the compliance of the other, a phenomenon known as direct diastolic ventricular interaction (DVI). This interaction is markedly enhanced when the force exerted by the surrounding pericardium is raised (pericardial constraint). In health, in the resting state, we operate near the top of the flat component of a J-shaped pericardial stress-strain relation. Therefore, pericardial constraint (and hence DVI) is only minor. When right ventricular volume/pressure acutely increases, such as during exercise, massive pulmonary embolism, or right ventricular infarction, pericardial constraint increases and significant DVI develops. In this setting, the measured left ventricular intracavitary diastolic pressure markedly overestimates the true left ventricular filling pressure, because the external forces must be subtracted. Although the pericardium can grow during chronic cardiac enlargement, we present evidence that in certain chronic disease processes, including heart failure, DVI may also be important.

Pericardial repair depresses canine cardiac catecholamines and met-enkephalin

Decreased cardiac catecholamines were observed following incision and repair of the pericardium in sham-operated vs. unoperated control dogs. Animals were assigned to five groups: unoperated, sham-operated intact pericardia, open pericardia, sutured pericardia and complete ventricular sympathectomy. Hearts were collected four weeks after surgery. Sympathectomy decreased catecholamine content when compared to all other groups. Hearts with open/sutured pericardia contained significantly less catecholamines than controls. When the pericardium was intact or left open following incision, cardiac catecholamines were unchanged compared to unoperated controls. Since opioid peptides are colocalized with catecholamines, we measured met-enkephalin and met-enkephalin-arg-phe, proenkephalin A peptide products, in parallel samples. Similar to norepinephrine, met-enkephalin was decreased following both sympathectomy and pericardial repair. However, met-enkephalin-arg-phe, which may be more associated with the myocardium than its innervation, was not changed by any treatment. The sutured pericardium more than the stress of surgery apparently alters the tissue catecholamines and enkephalin. This may have resulted from the mechanical friction at the site of repair. Epinephrine and met-enkephalin contents in sympathectomized hearts were significantly lower than unoperated controls but were not significantly different from the intermediate values observed in the sutured group. The functional consequences of these changes on neuroendocrine status are unclear and will require further evaluation. The results also emphasize the need for careful attention to proper controls for surgical studies.

Role of pericardial constraint for right ventricular function in humans

STUDY OBJECTIVE

To analyze the extent of pericardial constraint on right ventricular function in humans.

PATIENTS AND METHODS

Twenty patients, 59 +/- 2 (mean +/- SEM) years old, undergoing coronary bypass surgery. Right ventricular volumes and pressures were evaluated using a rapid response Swan-Ganz thermodilution catheter.

INTERVENTIONS

Parameters were determined before and after pericardiotomy, both before and during increased right ventricular systolic pressure by partial compression of the pulmonary artery (before pulmonary compression: 25 +/- 1 mm Hg; during: 39 +/- 1 mm Hg).

RESULTS

Pericardiotomy alone did not significantly affect right ventricular end-diastolic volume (before: 79 +/- 4 mL m-2; after: 78 +/- 3 mL m-2), right ventricular ejection fraction (before: 48 +/- 1%; after: 48 +/- 2%), and right atrial pressure (before: 4.3 +/- 0.8 mm Hg; after: 4.3 +/- 0.7 mm Hg). Before pericardiotomy, the increase in right ventricular afterload significantly increased right atrial pressure (to 5.5 +/- 0.7 mm Hg, p < 0.05) and reduced right ventricular ejection fraction (to 43 +/- 2%, p < 0.01). Right ventricular end-diastolic volume remained unchanged. After pericardiotomy, the increase in right ventricular afterload significantly increased right ventricular end-diastolic volume (to 85 +/- 3 mL m-2, p < 0.01) and also reduced right ventricular ejection fraction (to 42 +/- 2%, p < 0.01), while right atrial pressure was not significantly changed. During increased right ventricular afterload, the right ventricular diastolic pressure-volume relation was shifted rightward.

CONCLUSIONS

At normal levels of right ventricular diastolic filling, the pericardium does not exert constraining effects on right ventricular function. However, with increasing levels of right ventricular preload, pericardial constraint significantly influences right ventricular function in humans.

Increased left ventricular mass after thoracotomy and pericardiotomy. A role for relief of pericardial constraint?

BACKGROUND

Myocardial stretch and increased ventricular filling can lead to increased rates of myocardial protein synthesis. In animal studies, left ventricular mass increases after pericardiectomy, suggesting relief of a biologically meaningful restraining role and a resultant stimulus for growth. The present study was designed to test the hypothesis that combined thoracotomy and pericardiotomy leads to left ventricular hypertrophy in patients with normal left ventricular ejection fraction undergoing elective bypass surgery.

METHODS AND RESULTS

Twenty-five patients with normal left ventricular ejection fraction without active myocardial ischemia underwent Doppler and quantitative two-dimensional echocardiography 1 day before and 6 weeks and 7 months after elective coronary artery bypass surgery. The pericardium was left widely incised in all patients. Left ventricular end-systolic volume, end-diastolic volume, stroke volume, ejection fraction, end-systolic circumferential wall stress, and mass were measured. Left ventricular end-diastolic volume index increased from 51 +/- 11 mL/m2 to 62 +/- 14 mL/m2 (p < 0.05) at 6 weeks and to 66 +/- 14 mL/m2 (p < 0.05 versus baseline, p = NS versus 6 weeks) at 7 months. Left ventricular mass index increased from 109 +/- 23 g/m2 to 127 +/- 24 g/m2 (p < 0.05) at 6 weeks and to 131 +/- 23 g/m2 (p < 0.05 versus baseline, p = NS versus 6 weeks) at 7 months. There were no changes in systolic or diastolic blood pressures, end-systolic circumferential wall stress, or end-systolic volume.

CONCLUSIONS

Patients with normal left ventricular ejection fraction develop increases in left ventricular end-diastolic volume and mass after coronary artery bypass surgery. These findings support the hypothesis that the increase in left ventricular end-diastolic volume associated with thoracotomy and pericardiotomy leads to myocardial growth and an increase in left ventricular mass.

Pericardial adaptation in severe chronic pulmonary hypertension. An intraoperative transesophageal echocardiographic study

BACKGROUND

The pericardium both limits cardiac distension and accentuates ventricular interdependence. Although this effect appears minimal under normal circumstances, the pericardium markedly restricts acute cardiac enlargement. Animal studies have demonstrated gradual pericardial adaptation and expansion in chronic volume overload and cardiomegaly, but the pericardial response in humans with cardiac hypertrophy and enlargement has not been examined fully. To investigate this further, 14 patients with right ventricular hypertrophy and cardiomegaly secondary to chronic pulmonary thromboembolic disease and severe pulmonary hypertension were studied during pulmonary thromboendarterectomy.

METHODS AND RESULTS

Simultaneous intraoperative transesophageal Doppler echocardiography and direct biventricular hemodynamic measurements were performed at steady state immediately before and after pericardiotomy. All hemodynamic variables showed no significant change before and after pericardiotomy, including heart rate (76 +/- 16 versus 75 +/- 15 beats per minute), mean pulmonary arterial pressure (46.3 +/- 11.1 versus 45.5 +/- 11.7 mm Hg), cardiac index (1.8 +/- 0.5 versus 2.0 +/- 0.6 l/min/m2), left ventricular end-diastolic pressure (5.9 +/- 5.7 versus 7.1 +/- 5.0 mm Hg), and right ventricular end-diastolic pressure (7.9 +/- 6.6 versus 8.0 +/- 6.7 mm Hg). Similarly, there were no significant changes in all Doppler echocardiographic parameters, including right ventricular end-diastolic area (23.2 +/- 5.7 versus 22.6 +/- 5.4 cm2), left ventricular end-diastolic area (15.3 +/- 5.9 versus 15.5 +/- 4.4 cm2), the position of the interventricular septum, and the Doppler-derived mitral inflow measures of diastolic function.

CONCLUSIONS

The pericardium appears to have little influence on the marked cardiac and septal deformations seen in patients with chronic, severe right ventricular pressure overload and cardiomegaly. This study confirms that the human pericardium is capable of adapting over time to changes in cardiac size and geometry.

Alterations in transesophageal pulsed Doppler indexes of filling of the left ventricle after pericardiotomy

The impact of pericardial constraint on patterns of left ventricular filling was measured by transesophageal pulsed Doppler echocardiography in 30 patients undergoing elective nonvalvular cardiac surgery. Peak early left ventricular filling velocity increased from 0.52 +/- 0.11 to 0.56 +/- 0.15 m/s (p less than 0.05) and early left ventricular filling fraction increased from 60 +/- 9% to 65 +/- 9% (p less than 0.005) after pericardiotomy. The study group was retrospectively subdivided into two groups based on the prepericardiotomy mean right atrial pressure, an index of intrapericardial pressure and hence pericardial constraint. In 13 patients with a mean right atrial pressure less than 6 mm Hg, no significant changes in early left ventricular filling were evident after pericardiotomy. In 17 patients with a mean right atrial pressure greater than or equal to 6 mm Hg indicative of a greater degree of pericardial constraint before pericardiotomy, significant increases in peak early filling velocity (0.52 +/- 0.13 to 0.57 +/- 0.19 m/s, p less than 0.05), peak early filling rate (4.29 +/- 0.67 to 4.66 +/- 0.86 stroke volumes/s, p less than 0.05) and early left ventricular filling fraction (57 +/- 7% to 63 +/- 8%, p less than 0.001) were measured after pericardiotomy. Thus, the pericardium does constrain early left ventricular filling and its effects are more pronounced in patients with an elevated right atrial pressure.

The pericardium exerts constraint on the right ventricle during cardiac surgery

The right ventricle may be particularly susceptible to the effects of pericardial constraint. This study examined the effects of pericardiotomy on right ventricular function. Twenty-four anesthetized patients with coronary artery disease, but without evidence of pericardial pathology, were studied. Anesthesia consisted of fentanyl (100 micrograms.kg-1), diazepam, pancuronium, and 100% oxygen. The American Edwards REF-1 Cardiac Output Computer, rapid-response thermistor pulmonary arterial catheter, and a radial arterial catheter were used to measure hemodynamic variables. Baseline measurements were obtained with the sternum fully retracted. The measurements were then repeated following pericardiotomy by a midline incision. There were significant (P less than 0.05) changes in systolic arterial pressure (+4.5%), mean arterial pressure (+3.7%), systolic pulmonary arterial pressure (+11.8%), cardiac output (+9.1%), stroke volume (+6.9%), right ventricular end-diastolic volume (+7.6%), and right atrial pressure (-8.6%). In the current study, pericardiotomy augmented right ventricular diastolic filling and stroke volume, while the right atrial pressure decreased. These results support the concept of pericardial constraint.

The normal pericardium does not affect left ventricular function

Whether the normal pericardium exerts a constraining effect on left ventricular (LV) diastolic compliance and/or systolic function is controversial. Left ventricular filling and performance were studied in 15 patients by two-dimensional transesophageal echocardiography (2D-TEE) measuring end-diastolic area (EDa), end-systolic area (ESa), ejection fraction area (EFa), and hemodynamics immediately pre- and post-pericardiotomy. To diminish the influences of other variables such as surgical stimulation, chest wall constraint, and autoregulation, measurements were performed in deeply anesthetized patients with the chest fully opened immediately before and after pericardiotomy (PC). No significant echocardiographic or hemodynamic changes were observed after PC. Although alterations in compliance cannot be excluded, no significant changes in LV diastolic filling (EDa, pulmonary capillary wedge pressure [PCWP]) or systolic performance (EFa, cardiac output [CO]) were found. Therefore, it is concluded that the normal pericardium does not exert a measurable constraining effect on LV performance.

Immediate hemodynamic effects of pericardial closure after open-heart surgery

Acute hemodynamic effects of a routine pericardial closure after cardiopulmonary bypass was studied in 29 patients undergoing cardiac surgery. Clinically, the pericardial closure was well tolerated. Pericardial closure resulted in an 8% decrease of cardiac output (p less than 0.01) while cardiac index remained normal (2.9 l/min/m2 +/- 0.6 SD). The effect of the pericardium on pulmonary arterial and wedge pressures, and on systemic arterial pressure was not significant. Central venous pressure increased from 8 +/- 2 mmHg to 9 +/- 3 mmHg (p less than 0.05) after pericardial closure and decreased to 7 +/- 3 mmHg (p less than 0.05) when the pericardium was reopened. Left ventricular end-diastolic cavity diameter by echocardiography decreased in 19 of the patients studied from 46 +/- 6 mm to 41 +/- 5 mm (p less than 0.01) when the pericardium was closed, and increased to 45 +/- 6 mm (p less than 0.01) after re-opening of the pericardiotomy incision. The hemodynamic effects of pericardial closure seem to result from limited ventricular filling.

Intraoperative measurement of pericardial constraint: role in ventricular diastolic mechanics

The pressure of pericardial constraint was measured in 20 patients undergoing elective cardiac surgery (10 in Group I with normal cardiac size; 10 in Group II with cardiomegaly) using a catheter with a collapsible latex end balloon. Right atrial pressure and other hemodynamic variables including right ventricular stroke work index were also measured before and after the pericardium was widely opened. The pericardium was grossly normal in all patients and only small physiologic effusions were present. In Group I mean pericardial pressure was 8 +/- 2 mm Hg as was mean right atrial pressure. In Group II mean pericardial pressure was 6 +/- 2 mm Hg versus mean right atrial pressure of 10 +/- 5 mm Hg (p less than 0.05). Excluding 2 of the 20 patients with outlying data, pericardial pressure showed linear correlation with right atrial pressure (r = 0.689). In Group I right ventricular stroke work index rose from 5.0 +/- 2.0 to 6.4 +/- 2.1 g-m/m2 (p less than 0.01) after pericardiotomy with no significant increase in mean right atrial pressure; similar findings in Group II were consistent with removal of external constraint. Thus, even in the absence of an abnormal effusion the normal pericardium exerts a significant pressure on the heart, which is often similar in magnitude to right atrial pressure. In certain notable exceptions, however, right atrial pressure far exceeds pericardial pressure. Such pericardial constraint has important implications for ventricular diastolic mechanics.

The relationship between pericardial pressure and right atrial pressure: an intraoperative study

The objective of this study was to determine the constraining effect of the normal human pericardium. Accordingly, immediately after thoracotomy in nine patients undergoing elective cardiac surgery, we measured mean pericardial surface pressure over the lateral free wall of the left ventricle with a flat balloon as well as mean right atrial pressure while incrementally infusing up to 2.1 liters of Ringer's solution to increase right atrial pressure. In each case, the slope of the relationship between right atrial (range -4 to 20 mm Hg, overall) and pericardial pressures was near unity (1.16 +/- 0.20 mean +/- SD) and the intercept was approximately zero (0.71 +/- 2.48 mm Hg). Correlation coefficients ranged from .86 to .97. These observations suggest that right atrial pressure can be used as an estimate of pericardial surface pressure. If this is the case, true left ventricular preload (i.e., effective distending pressure or transmural diastolic pressure) might be estimated from the difference between left ventricular filling pressure and right atrial pressure, both conveniently measurable clinically by means of a triple-lumen, flow-directed catheter.