Mechanical properties of the isolated canine pericardium

Influence of inflammation and cardiac hypertrophy on mechanical properties of human pericardium

Different devices for mechanical circulatory support (MCS) have been developed for the treatment of refractory cardiogenic shock. However, all of them are associated with direct blood contact, the need for anticoagulation and bleeding complications. To overcome these limitations the pericardial sac got into the focus as a promising implantation site for MCS. For this purpose, further knowledge about the mechanical properties of human pericardium is required. In this prospective, monocentric, experimental pilot study 56 samples of human pericardium were extracted postmortem from 13 critically ill patients. After preparation of test specimens uniaxial tensile tests were performed. The primary end points were load at fracture per sample width and strain at fracture. Acute inflammation was assessed by blood levels of C-reactive protein, white blood count and procalcitonin measured at several times during hospital stay. Inflammatory load was estimated by area under the inflammatory curves. Correlation and regression analysis were used to assess the relationship of primary end points to inflammation, comorbidities and postmortem time to preparation. Human pericardium showed a load at fracture per sample width of 1.95 [1.38–2.94] N/mm (median [inter quartile range]) and a strain at fracture of 89.29 [73.84–135.23] %. Markers of acute inflammation and cardiac hypertrophy did not correlate to load or strain at fracture. However, strain at fracture increased with higher body mass index and an increasing number of postmortem days. In contrast, higher patient age was associated with a lower strain at fracture. Inflammation and cardiac hypertrophy did not influence mechanical properties of human pericardium.

Age-related changes in material behavior of porcine mitral and aortic valves and correlation to matrix composition

Recent studies showing significant changes in valvular matrix composition with age offer design criteria for age-specific tissue-engineered heart valves. However, knowledge regarding aging-related changes in valvular material properties is limited. Therefore, 6-week, 6-month, and 6-year-old porcine aortic valves (AV) and mitral valves (MV) were subjected to uniaxial tensile testing. In addition to standard material parameters, the radius of transition curvature (RTC) was measured to assess the acuteness of the transition region of the tension-strain curve. Radially, the MV had greater stiffness and a smaller RTC compared with the AV. Circumferentially, the center of the MV anterior leaflet (MVAC) had the highest stiffness (MVAC > AV > MV free edge [MVF]), greater stress relaxation (MVAC > MVF/AV), lowest extensibility (MVAC < AV < MVF), and smaller RTC compared with MVF (AV < MVAC < MVF). AV and MV radial strips had a larger RTC compared with circumferential strips. Aging elevated stiffness for MV and AV radial and circumferential strips, elevated stress relaxation in AV and MVF circumferential strips, and increased RTC for MV radial and MVF circumferential strips. In conclusion, there are significant age-related differences in the material properties of heart valves, which parallel differences in tissue composition and structure, likely impact valve function, and highlight the need for age-specific design goals for tissue-engineered heart valves.

Sensitivity and specificity of echocardiographic evidence of tamponade: implications for ventricular interdependence and pulsus paradoxus

The reported sensitivity of the echocardiographic finding of right atrial collapse for the diagnosis of tamponade ranges from 50% to100%; specificities have ranged from 33% to 100%. Its sensitivity in identifying right ventricular collapse ranges from 48% to 100% whereas the specificity ranges from 72% to 100%. Collapse of either the right atrium or right ventricle is not reliable except in cases where the risk of tamponade is high, consistent with Bayes' theorem. If the patient has hypotension, tachycardia, dyspnea, increased venous pressure, and a pericardial effusion, the diagnosis of tamponade will likely be sustained. To explain pulsus paradoxus, most echocardiographic reports have invoked Dornhorst's theory that inspiratory filling of the right ventricle actively collapses the left ventricle by successfully competing for a fixed total pericardial space ("ventricular interdependence"). However, the pericardial space is not fixed in tamponade but increases with inspiration, and the right heart is much more likely to collapse than the left, given their relative thickness. Pulsus paradoxus depends on the inspiratory surge to the right heart, exaggerated by the small stroke volume of both ventricles induced by tamponade, and vascular coupling between the pulmonary and systemic beds, with a transit time of one to two heart beats.

Assessment of pericardium in cardiac bioprostheses. A review

Cardiac valve bioprostheses are assessed in terms of their present and future clinical utility. The problems concerning durability basically involve early failure due to tears in the valve leaflets and late failure mainly associated with calcification of the biological tissue. New strategies for selection and chemical treatment of the biomaterials employed are analyzed, and the available knowledge regarding their mechanical behavior is reviewed. It is concluded that the durability of these devices, and thus their successful use in the future, depends on the knowledge of the interactions among the different biomaterials of which they are composed, the development of new materials, and the engineering design applied in their construction.

Altered mechanical properties in aortic elastic tissue using glutaraldehyde/solvent solutions of various dielectric constant

The extent to which elastic tissue can be crosslinked in aldehydes and the mechanism of such action is unresolved in the literature. We have used glutaraldehyde/solvent solutions of decreasing dielectric constant (phosphate buffer, methanol, 95% ethanol, n-propanol, n-butanol) to alter the mechanical properties of aortic elastic tissue obtained from autoclaved and CNBr-purified bovine aortae. Treated and untreated hoop samples were examined for stress-strain and stress relaxation behavior and for residual stress using opening angle experiments as per Fung. The extent of exogenous crosslinking was analyzed through amino acid analysis. Mechanical properties of autoclaved elastic tissue varied with dielectric constant in glutaraldehyde/solvent treatments; however, solvent treatment alone produced no effect. Extensibility decreased with decreasing dielectric constant while tensile modulus changed over a range from -2.4% (-0.86 kPa) for glutaraldehyde/buffer to +35.3% (+14.3 kPa) for glutaraldehyde/n-propanol (untreated-treated). Residual stress experiments similarly showed a systematic decrease in opening angle with decreasing dielectric constant. Differences ranged from 10.5 degrees for glutaraldehyde/buffer to 22.2 degrees for glutaraldehyde/n-butanol. Interestingly, purification of aortae with CNBr reduced the effects of glutaraldehyde/n-butanol treatment. We hypothesize that CNBr differentially degraded the elastin-associated microfibrillar proteins in aortic elastic tissue, thus producing the observed differences in mechanical behavior. The observed phenomena in this study may be attributed to the composite structure of elastic tissue: elastin and microfibrillar protein. During treatment, conformational changes in elastin facilitated by polar/nonpolar interactions occurred which then were "locked" in by glutaraldehyde crosslinking of the microfibrillar proteins. By this mechanism the increases in both stiffness and time-dependent behavior observed after treatment may be explained.

Nonuniform regional deformation of the pericardium during the cardiac cycle in dogs

We hypothesized that local contact forces between the pericardium and the heart cause regional variation in pericardial deformation during the cardiac cycle, reflecting volume changes of the underlying cardiac chambers. To test this, we measured regional pericardial area over the right atrium (RA) and right ventricle (RV) with orthogonal pairs of sonomicrometers in six open-chest dogs. At a left ventricular end-diastolic pressure of 5 mm Hg, RV pericardial area paralleled RV volume, that is, shrinkage during ejection by 10 +/- 8% and expansion during filling. RA pericardial area was reciprocally related to RV pericardial area, with average expansion during ventricular ejection of 2 +/- 2%, thus paralleling RA volume during RV ejection. With volume loading, RV pericardial shrinkage during ejection increased to 14 +/- 6%, but the RA pericardial area change was no longer reciprocal (0 +/- 3% change during RV ejection). Elimination of contact forces by cardiac tamponade resulted in both marked attenuation of RV pericardial area changes and synchronization of the RV and RA pericardial area pattern; that is, both shrank during RV ejection. In two additional dogs, measurement of pericardial area over left ventricle and atrium showed similar results. We conclude that dynamic pericardial contact forces cause regional variation in pericardial deformation, which reflects volume changes of the underlying chambers. These findings imply that the influence of the pericardium on filling and ejection may be more complex than previously recognized, varying both by chamber and dynamically over the course of the cardiac cycle.

Evidence that deformations which occur during mechanical conditioning of bovine pericardium are not permanent

Natural and glutaraldehyde fixed bovine pericardium samples were mechanically conditioned by a cyclic uniaxial load procedure. The samples tested were controlled for both position and direction in the pericardial sac. The natural tissue demonstrated a significant increase in length and a significant decrease in width after mechanical conditioning. The deformations were not permanent. The test specimens had returned to their original length by 10.5 h after the mechanical test. The control samples of natural tissue showed no significant changes in length during this time. The chemically modified tissue showed a significant increase in length but no significant changes in width or thickness after mechanical conditioning. As in the natural tissue, the length changes were not permanent. Twenty four hours after returning the fixed tissue to its normal buffered glutaraldehyde storage medium the test samples had returned to their original length. A subsequent mechanical retest 8 d after the initial test procedure suggested that the history of the original test had been removed. These observations may be important in the interpretation of 'in vitro' hydrodynamic tests for heart valve substitutes.

The standardisation of gauge length: its influence on the relative extensibility of natural and chemically modified pericardium

Cyclic uniaxial load tests were performed on natural and chemically modified bovine pericardium which is used in the construction of heart valve substitutes. A template was employed to select specimens from the same sites in different pericardial sacs. When the pericardium was chemically modified by glutaraldehyde fixation as an entire sac the tissue showed increased extensibility after modification compared with the natural tissue. The undeformed stress-free length (gauge length) in both the natural and modified tissue was determined by a highly reproducible experimental method giving a coefficient of variation of less than 0.5%. Specimens excised from a natural pericardial sac demonstrated a significant increase (p less than 0.03) in length, 4.97 +/- 3.49%, after a single load cycle, compared with controls placed in isotonic saline but not mechanically tested. The test pieces had returned to their original length by 8.5 h after the mechanical test. After fixation the same specimens decreased significantly in length (p less than 0.001) by 11.18 +/- 4.28%. This shrinkage was not significantly different to that of the control specimens (11.09 +/- 2.47%) which had not been tested. Uniaxial loading of these chemically modified strips demonstrated a similar increase in tissue extensibility compared to the natural tissue if the undeformed length of the test specimen after shrinkage was used as the gauge length. After mechanical conditioning the chemically modified tissue also demonstrated a significant increase (p less than 0.001) in stress-free length (5.35 +/- 0.59% after 36 cycles, 8.92 +/- 1.50% after 2085 cycles). These deformations were not permanent. The tissue had returned to its original length after 38 h in its normal buffered glutaraldehyde storage medium. On the basis of these observations, recommendations for the clarification and standardisation of gauge length definitions were made in natural, chemically modified and mechanically conditioned tissue.

Pericardial adaptations during chronic cardiac dilation in dogs

The manner in which the pericardium adapts to chronic cardiac dilation is not known. Recent work from our laboratory indirectly suggested that the size of the pericardium and/or its pressure-volume relation was altered by chronic cardiac enlargement. To examine this question further, we compared pericardial pressure-volume and stress-strain relations, surface area, mass, and average thickness in seven normal dogs and seven with chronic volume overload hypertrophy due to a systemic arteriovenous fistula. Dogs with significant cardiac hypertrophy had an increased pericardial volume at any pressure and a proportionality constant for the slope of the entire curve as determined by nonlinear regression analysis. This was associated with parallel increases in pericardial surface area and mass such that average thickness was unchanged. Stress-strain analysis of the data revealed that the mechanical properties of the pericardium were not significantly different in dogs with chronic cardiac dilation. These results indicate that during chronic cardiac dilation the pericardium enlarges in size and mass. The pericardial chamber is more compliant, although the intrinsic stiffness of the pericardium appears to be unchanged. Further, since pericardial mass is increased, the response to chronic dilation appears to also involve the addition of new pericardial tissue.

Pericardial influences on ventricular filling in the conscious dog. An analysis based on pericardial pressure

Twenty-five dogs were chronically instrumented to investigate the effects of the normal pericardium on cardiac function. Pulse-transit ultrasonic transducers were implanted to measure multiple ventricular dimensions. The pericardium was incised transversely at the base of the heart and precisely reapproximated, so as to disturb its characteristics minimally. One week later, the dogs were studied in the conscious state, and left ventricular, right ventricular, pericardial, and pleural pressures were measured with matched micromanometers. Data were recorded before and after blood volume expansion. Absolute end-diastolic pericardial pressure varied directly with pleural pressure during the respiratory cycle. Transpericardial pressure (pericardial-pleural pressure) varied little with respiration and was related directly to ventricular diameter during the cardiac cycle with peak transpericardial pressure uniformly occurring at end-diastole. With volume infusion, normalized end-diastolic minor axis diameter and left ventricular transmural pressure (left ventricular-pleural pressure) increased significantly from 0.14 +/- 0.01 and 9.5 mm Hg +/- 1.0 mm Hg, respectively, in the control state to 0.20 +/- 0.01 and 19.3 mm Hg +/- 1.2 mm Hg after volume loading. End-diastolic transpericardial pressure also increased significantly from 2.3 +/- 0.5 mm Hg to 4.1 +/- 0.3 mm Hg, and represented approximately 21% of transmural left ventricular pressure. When measurements were obtained sequentially after implantation, transpericardial pressure was initially high but decreased with time, presumably due to pericardial creep. After volume loading, right ventricular end-diastolic transmural pressure averaged 9.6 mm Hg, and pericardial pressure constituted 42% of right ventricular pressure. Thus, pericardial restraining effects may predominantly influence right ventricular filling and affect the left ventricle through series interaction. In the normal conscious dog, transpericardial pressure remains low over the entire physiological range, and the direct influence of the normal pericardium on diastolic filling of the left ventricle appears to be minimal.

Viscoelastic behavior of human connective tissues: relative contribution of viscous and elastic components

Stress-relaxation tests were performed at successive strain levels on strips of human aorta, skin, psoas tendon, dura mater, and pericardium. The elastic fraction, the equilibrium force divided by the initial force, was calculated at each strain increment. In the aorta, the elastic fraction decreased with strain and was modeled as the transfer of stress from elastic to collagen fibers, while in skin it increased with strain, probably due to the rearrangement of individual collagen fiber orientations, resulting in an aligned collagen network at high strains. The strain-independent elastic fractions for tendon, dura mater, and pericardium were similar, and approximately equal to the values found for aorta and skin at high strains. It was hypothesized that the elastic fraction is related to the type of fiber loaded, and the tissue geometry. This analysis may be useful in studying disease-induced changes in the mechanical properties of connective tissues.

Mechanical and structural correlates of canine pericardium

We have assessed viscoelastic properties of pericardium within the physiological range of stresses and related mechanical behavior to fiber direction as defined by scanning electron microscopy. Stiffness, stress relaxation, and creep were measured in samples taken from the anterior surface of 14 canine pericardia. Stress-strain relations generally were not exponential; stiffness at a stress of 1 g/mm2 ranged from 12.9 to 239 g/mm2 during stretch and varied both from pericardium to pericardium and with the orientation of the strip within the sample (anisotropy). The strips exhibited hysteretic behavior which was not promotional to rate of strain. Following a rapid increase in stress, creep averaged less than 1% and stress relaxation, 34% in a 30-minute test period. The orientation of the strip with the greatest stiffness was consistent from pericardium to pericardium, and correlated with a layer of collagen fibers oriented along the major axis of te strip.

Comparison of haemodynamic responses to positive-end-expiratory ventilation and pericardial effusion in dogs

1. The haemodynamic responses to progressive increments in positive end-expiratory pressure ventilation (PEEP) were assessed in dogs in the presence and absence of pericardial effusion (2.2 ml saline/kg). 2. Increasing levels of PEEP (3, 4, 8, 16 cm H2O) were associated with increases in right atrial, left atrial, pericardial and pulmonary artery diastolic pressure; increases in pulmonary vascular resistance; decreases in aortic blood pressure; and decreases in cardiac index or stroke index. 3. Pericardial effusion that was associated with an increase in pericardial pressure of 3 mmHg was not associated with significant changes in aortic blood pressure or stroke index. In contrast 8 cm PEEP was associated with significant decreases in both these haemodynamic variables yet pericardial pressure increased only 2.2 mmHg. 4. For these levels of PEEP and pericardial effusion, the same haemodynamic response to PEEP was observed regardless of the presence or the absence of pericardial effusion. 5. Similar changes in aortic blood pressure and stroke index despite different pericardial pressures is reflected by significantly different intercepts in the linear model relating these haemodynamic variables to pericardial pressure in the presence and absence of pericardial effusion. 6. The results suggest that these haemodynamic consequences of PEEP are primarily a function of pulmonary hyperinflation and are less dependent on increases in extracardiac pressure per se.