HEMODYNAMICS Assessment of pericardial constraint in dogs

To determine the better method of measuring pericardial constraint, pericardial pressure was recorded by a liquid-filled open-ended catheter and a liquid-containing flat balloon in six open-chest anesthetized dogs. Left ventricular pressure was measured by a micromanometer-tipped catheter and left ventricular anteroposterior diameter was measured by sonomicrometry. Left ventricular end-diastolic pressure was raised to 20 +1.7 (mean -+ SD) mm Hg by intravenous saline. Left ventricular diastolic pressure-diameter loops were constructed (1) with incremental amounts of saline (0 to 50 ml) in the resealed pericardium, (2) with several small holes in the pericardium, and (3) with the pericardium widely open. Measured pericardial pressures were compared with what was assumed to be the correct pericardial pressure, i.e., the calculated difference between left ventricular diastolic pressure (at a given left ventricular diameter) before and after opening the pericardium. Pressure recorded by the flat balloon was similar to the calculated pericardial pressure at all pericardial liquid volumes. Pressure recorded by the open-ended catheter, however, was significantly lower (p < .05) than the calculated pressure unless there was at least 30 ml of liquid in the pericardium. After several holes had been made in the pericardium it still exerted a constraining effect, as shown by a marked rightward or downward shift of the left ventricular diastolic pressure-diameter relationships after completely opening the pericardium. After holes were made in the pericardium pressure recorded by the flat balloon was still similar to the calculated pericardial pressure. However, pressure recorded by the open-ended catheter was significantly (p < .02) lower than the calculated pressure. In four dogs the product of left ventricular anteroposterior and septum-to-free wall diameter was used as a volume parameter; comparison made between measured and calculated pericardial pressures confirmed the results obtained with use of anteroposterior diameter to assess left ventricular size. In conclusion, unless the pericardium is sealed and contains at least 30 ml of liquid, an open-ended catheter significantly underestimates pericardial constraint. However, a flat liquid-containing balloon correctly measures pericardial constraint regardless of the amount of pericardial liquid and also when the pericardium is not sealed. Circulation 71, No. 1, 158-164, 1985. THE QUANTITATIVE EFFECT of pericardial constraint on left ventricular diastolic function remains controversial." It seems clear that the resolution of this controversy depends on the magnitude of pericardial pressure that, in turn, depends on the method of measurement. In the pulmonary literature Agostoni and othersI6 have promulgated the distinction between "liquid pressure" and "surface pressure." Liquid From the Departments of Medicine and Medical Physiology. University of Calgary, Calgary, Alberta, Canada. Supported by a grant-in-aid from the Alberta Heart Foundation. Address for correspondence: Dr. John V. Tyberg, Departments of Medicine and Medical Physiology, Faculty of Medicine, The University of Calgary, 3330 University Dr. N.W., Calgary, Alberta, Canada T2N 4N1. Received June 21, 1984; revision accepted Oct. 11, 1984. Drs. Smiseth, Frais, and Kingma held postdoctoral research fellowships from the Alberta Heritage Foundation for Medical Research during the time when the study was conducted. Dr. Tyberg was a Senior Investigator of the Alberta Heart Foundation and is a Heritage Medical Scientist of the Alberta Heritage Foundation for Medical Research. 158 pressure is familiar to cardiovascular physiologists and can be measured by connecting a manometer to any fluid-filled space as with a liquid-filled catheter. The concept of surface pressure is more difficult; it is the force per unit surface area exerted by, for example, the left ventricular surface on the overlying parietal pericardial membrane. The difference between the pericardial liquid pressure and surface pressure is the compressive contact stress7 developed between the surfaces.4 The oft-quoted work by Kenner and Wood' supports the common view that pericardial pressure is approximately equal to intrathoracic pressure and unchanged by alterations in cardiac volume. They dilated the heart substantially by aortic and pulmonary artery constriction without importantly raising pericardial (liquid) pressure. Some years ago Holt et al.2 used a liquidcontaining balloon and demonstrated that blood volume expansion increased pericardial (surface) pressure CIRCULATION by gest on A ril 4, 2017 http://ciajournals.org/ D ow nladed from LABORATORY INVESTIGATION-HEMODYNAMICS nearly as much as it increased left ventricular diastolic pressure. Recently, using a similar balloon, we have demonstrated important changes in pericardial (surface) pressure following changes in preload or afterload in the failing heart.8 Thus, the aim of the present study was to determine whether an open-ended catheter or a flat, liquid-containing balloon could accurately measure pericardial constraint. To arrive at a "gold standard" of pericardial pressure we postulated a simple, static-equilibrium concept: at a given left ventricular end-diastolic size (diameter) the correct pericardial pressure is that pressure which must be added to left ventricular transmural pressure to equal the intracavitary left ventricular pressure. We measured left ventricular transmural pressure directly as a function of left ventricular diameter at the end of the experiment after removal of the pericardium with the chest widely open and the lungs retracted. Thus, we defined the calculated pericardial pressure as the difference between the left ventricular end-diastolic pressure measured when the pericardium was closed and the left ventricular end-diastolic pressure measured at the same diameter when the pericardium was removed. To determine the dependence of the measured pericardial pressure on the volume of pericardial liquid we infused saline into the resealed pericardial cavity. To test the hypothesis that even a netlike pericardium would induce a measurable, physiologically significant constraint on left ventricular diastolic filling, measurements were repeated after several small holes had been cut in the pericardium. Methods Animal preparation. Experiments were done in six mongrel dogs (22 to 33 kg). Anesthesia was induced by 25 mg/kg iv sodium thiopental (Pentothal, Abbott Laboratories, Montreal, P.Q.) and was maintained by 1.5% halothane and nitrous oxide/ oxygen with use of a constant-volume respirator (model 607, Harvard Apparatus Co. Inc., Millis, MA) and a closed rebreathing system. A midline stemotomy was performed with each dog in the supine position. Left ventricular pressure was measured by a No. 8F micromanometer-tipped catheter with a reference lumen (model PC-480, Millar Instruments, Houston, TX). The ventral surface of the pericardium was incised transversely along the base of the heart. A flat balloon and open-ended catheter were positioned on the anterolateral surface of the left ventricle at the mid left ventricular level and were stitched loosely to the epicardium. The open-ended catheter was composed of a 4 cm terminal silicone rubber segment with an endhole and three side-holes fixed to a stiff 60 cm No. 8F cardiac catheter. Another multiple side-hole catheter was inserted into the pericardium to be used for drainage and saline infusion. The pericardium was sutured and sealed water tight by applying small amounts of glue (The Gripper Super Glue, Via Chem. Inc., Quebec, P.Q.) along the sutures. A snare was placed around the posterior vena cava to transiently restrict left ventricular filling and a catheter for intravenous infusion was placed in a femoral vein. Aortic pressure was monitored through a cathVol. 71, No. 1, January 1985 eter introduced into a femoral artery. Left ventricular anteroposterior diameter (DA-P) and in four dogs septum-to-left ventricular free wall diameter (DS-FW) were measured with ultrasonic crystals. The free wall crystals were sutured to the epicardium at the mid left ventricular level. The septal crystal was pushed halfway through the septum at the mid left ventricular level. A limb-lead electrocardiogram was monitored and body temperature was maintained by a warming lamp. Pressures, diameters, and the electrocardiogram were recorded (Electronics for Medicine/Honeywell, model VRl6, White Plains, NY) at a paper speed of 75 mm/sec. Data were also recorded on analog tape (Hewlett Packard, model 3968A, Palo Alto, CA) for later analysis. Pericardial balloons. The balloons were made from a 0.025 cm thick folded sheet of silicone rubber (Armet Industries Corp., Concord, Ont.) sealed at the edges. (Internally, the balloon measured 3 x 3 cm.) A short silicone rubber tube protruding from the balloon was connected to a 60 cm No. 8F stiff cardiac catheter. The balloon could hold up to 1.8 ml of fluid without developing measurable pressure. Before insertion into the pericardium, the balloon was calibrated by a procedure similar to that used by McMahon et al. ,6 the results of which are illustrated in figure 1. The balloon was also found to accurately measure negative pressures (0 to -20 mm Hg) that were created in a water-containing chamber. The frequency response of the balloon was tested in a water chamber (WGA-200, Millar Instruments, Inc., Houston, TX). The pressure amplitude ratio (balloon-micromanometer) was 1.0 below 14 Hz and increased to 1.1 at 25 Hz (both amplifiers filtered at 2500 Hz). Experimental protocol. To increase pericardial pressure, the dogs received intravenous infusions of saline adjusted to maintain left ventricular end-diastolic pressure at approximately 20 mmHg (mean + SD = 20.2 + 1.7 mmHg) throughoutthe period of pericardial saline infusion. Pericardial suction was applied initially and was discontinued immediately before the fi