Rankin Js

Linearity of the Frank-Starling relationship in the intact heart: the concept of preload recruitable stroke work.

The Frank-Starling relationship generally has been examined with filling pressure as the index of preload, resulting in a curvilinear function that plateaus at higher filling pressures. To investigate this relationship further in the intact heart, 32 dogs were chronically instrumented with left ventricular and pleural micromanometers and with regional (10 dogs) or global (22 dogs) ultrasonic dimension transducers. Seven days after implantation, left ventricular pressure and regional or global dimensions were recorded in the conscious state. After autonomic blockade, preload was varied by vena caval occlusion. Myocardial function was assessed by calculating regional or global stroke work, and preload was measured as end-diastolic segment length or chamber volume. The relationship between stroke work and either end-diastolic segment length or chamber volume (termed the preload recruitable stroke work relationship) was highly linear in every study (mean r = .97) and could be quantified by a slope (MW) and x-intercept (LW). Previous nonlinear relationships between stroke work and filling pressure seemed to reflect the exponential diastolic pressure-volume curve. Over the physiologic range of systolic arterial pressures produced by infusion of nitroprusside or phenylephrine, no significant change was observed in MW or LW in the normal dog. Calcium infusion increased both regional and global MW by 71 +/- 19% and 65 +/- 9%, respectively (p less than .02), with no significant change in LW. To normalize for ventricular geometry and heart rate, stroke work was computed from circumferential stress-strain data and converted to myocardial power output, which was then plotted against end-diastolic circumferential strain. This relationship also was highly linear, and the slope, Mmp (mW/cm3 of myocardium), is proposed as a potential measure of intrinsic myocardial performance independent of loading, geometry, and heart rate.

The three-dimensional dynamic geometry of the left ventricle in the conscious dog.

The dynamic geometry of the left ventricle was assessed with the use of chronically implanted pulse-transit ultrasonic dimension transducers. The orientation of the transducers allowed the measurement of left ventricular minor and major axis diameters and equatorial wall thickness in the conscious dog. The left ventricle was modeled as a three-dimensional, prolate ellipsoidal shell. Left ventricular and pleural pressures were measured with high fidelity micTomanometers. Aortic blood flow was obtained with electromagnetic flow probes. To test the assumptions inherent in this technique, left ventricular mass, internal volume, stroke volume, and peak aortic flow were computed from the dimension data and compared to directly measured values. Correlation coefficients of 0.95 or greater were obtained for each of these comparisons. In addition, the calculated left ventricular mass was constant to within ±6% of the mean value throughout the cardiac cycle. We found that the dynamic contraction pattern of the left ventricle was dependent on the physiological state of the dog. Furthermore, in the conscious state, shortening of the minor axis diameter, lengthening of the major axis diameter, and slight thickening or thinning of the wall were noted during isovolumic contraction (isovolumic ellipticalization pattern). In the open-chested, anesthetized state, however, marked rearrangements in geometry were observed during isovolumic contraction manifested by lengthening of the minor axis diameter, shortening of the major axis diameter, and significant thickening of the wall (isovolumic sphericalization pattern). We also observed that left ventricular volume was significantly diminished in the open-chested state. The isovolumic contraction pattern in open-chested dogs could be changed from sphericalization to ellipticalization by increasing end-diastolic volume with the infusion of saline. During a vena caval occlusion in the conscious state, the contraction pattern changed from isovolumic ellipticalization to isovolumic sphericalization as the end-diastolic volume decreased. Thus, the exact pattern of left ventricular contraction was found to be a function of left ventricular volume.

The physiology of external cardiac massage: high-impulse cardiopulmonary resuscitation.

In intact chronically instrumented dogs, left ventricular dynamics were studied during cardiopulmonary resuscitation (CPR). Electromagnetic flow probes measured cardiac output and coronary blood flow, ultrasonic transducers measured cardiac dimensions, and micromanometers measured left ventricular, right ventricular, aortic, and intrathoracic pressures. The dogs were anesthetized with morphine, intubated, and fibrillated by rapid ventricular pacing. Data were obtained during manual external massage with dogs in the lateral and supine positions. Force of compression was varied from a peak intrathoracic pressure of 10 to 30 mm Hg, and compression rate was varied from 60 to 150/min. Increasing force of compression increased stroke volume up to a peak intrathoracic pressure of approximately 20 mm Hg, beyond which stroke volume remained constant or declined. Stroke volume appeared to result primarily from direct transmission of manual compression force to the heart rather than from positive intrathoracic pressure because peak cardiac or vascular pressures or the change in these pressures were consistently two to four times greater than the corresponding intrathoracic pressures during manual compression. With increasing compression rate, stroke volume remained relatively constant, and total cardiac output increased significantly: 425 +/- 92 ml/min at 60/min, 643 +/- 130 ml/min at 100/min, and 975 +/- 219 ml/min at 150/min (p less than .05). Left ventricular dimensions decreased minimally at higher manual compression rates. In four patients undergoing CPR, systolic and diastolic arterial blood pressure increased with faster compression rates, correlating well with data obtained in the dog. Dynamic coronary blood flow in canine experiments decreased to zero or negative values during compression. Antegrade coronary flow occurred primarily during noncompression periods and seemed to be related to diastolic aortic perfusion pressure; coronary flow at a compression rate of 150/min averaged 75% of control. Therefore stroke volume and coronary blood flow in this canine preparation were maximized with manual chest compression performed with moderate force and brief duration. Increasing rate of compression increased total cardiac output while coronary blood flow was well maintained. Direct cardiac compression appeared to be the major determinant of stroke volume during manual external cardiac massage.

Management and long-term outcome of aortic dissection.

All 163 patients admitted to one institution between 1975 and 1988 with aortic dissection were reviewed. Type I and type II patients received grafting of the ascending aorta, with an intraoperative mortality rate of 11%. For type III dissection, management was medical in 53 patients, while 19 required surgery for aortic rupture or expansion, with an intraoperative mortality rate of 11%. The 9- or 10-year survival rates were 29%, 46%, and 29% for types I, II, and III respectively. Of 135 patients with primary aortic dissection, 17 (13%) required subsequent aortic surgery. Cause of late death was other cardiovascular disease in 38%, rupture of another aortic segment in 18%, sudden death in 24%, and other medical conditions in 21%. Although operative therapy for types I and II dissections and reserving operation for selected type III dissections provides acceptable long-term survival, careful follow-up is necessary due to concurrent cardiovascular disease and residual aortic disease.

Viscoelastic properties of the diastolic left ventricle in the conscious dog.

The mechanical properties of the normal left ventricular wall during diastole were studied in 15 chronically instrumented, conscious dogs. Left ventricular minor and major axis diameters and equatorial wall thickness were measured with implanted pulse-transit ultrasonic dimension transducers. Left ventricular and pleural pressures were measured with high fidelity micromanometers. Circumferential mural stress was calculated by using an ellipsoidal shell theory; circumferential strain was calculated by using a natural strain definition. The static elastic properties of the myocardium were estimated by fitting the stress-strain values at the points of diastasis during a vena caval occlusion to an exponential function. A modified creep test was used to evaluate the series viscous properties of the myocardium. Acute increases in systolic and diastolic loading were produced by inflating implanted aortic occluders for 15 minutes in five dogs. In these dogs, the static stress-strain curves were not altered significantly after this period of pressure loading, indicating tbat short-term series viscous properties are negligible. Parallel viscous properties were evaluated in 10 dogs by means of the variable rate stretch test of dynamic diastolic filling. A viscoelastic model incorporating a parallel viscous element fit the dynamic stress-strain data better and predicted the static elastic properties more accurately than a simple exponential model. Thus, the mechanical characteristics of the diastolic left ventricle can be represented most precisely by a viscoelastic model that includes a parallel viscous element.

Influence of compression rate on initial success of resuscitation and 24 hour survival after prolonged manual cardiopulmonary resuscitation in dogs.

The influence of chest compression rate on initial resuscitation success and 24 hr survival after prolonged manual cardiopulmonary resuscitation (CPR) was investigated in 26 morphine-anesthetized dogs (17 to 30 kg). After placement of aortic and right atrial micromanometers and induction of ventricular fibrillation, manual CPR was commenced immediately and continued for 30 min. One group of 13 dogs underwent manual CPR at a compression rate of 60/min, and the other group at a rate of 120/min. The compression durations in the two groups were not significantly different (51.7 +/- 1.8% at 60/min vs 51.6 +/- 1.9% at 120/min). No drugs other than sodium bicarbonate were administered during CPR. A maximum of three attempts was permitted to defibrillate the heart. Successfully defibrillated animals were followed for 24 hr, during which time no treatment, other than naloxone, was given to reverse the effects of morphine. Arterial blood pH, PCO2, and PO2 were not significantly different in the two groups throughout the CPR period. When compared with the compression rate of 60/min, the compression rate of 120/min produced more successfully defibrillated animals (12/13 at 120/min vs 2/13 at 60/min, p less than .002) and more 24 hr survivors (8/13 at 120/min vs 2/13 at 60/min, p less than .03). All 24 hr survivors were conscious and able to sit, stand, and drink normally. One 24 hr survivor in each group had difficulty walking. Improved survival with the high-rate compression technique was consistent with the significantly higher mean aortic (systolic and diastolic) and coronary perfusion pressures attained with high-rate compressions (all p less than .002). Although the clinical applicability of these findings has yet to be demonstrated, they provide empirical support for the recent decision to increase the chest compression rate for manual CPR recommended by the American Heart Association, and indicate that the hemodynamic and survival benefits of faster compression rates in this experimental preparation were not dependent on covariant alterations in compression duration.

Dynamic ventricular interaction in the conscious dog.

In nine conscious, chronically instrumented dogs, ultrasonic dimension transducers measured left ventricular anterior-posterior and septal-free wall minor axis and major axis diameters. Matched micromanometers measured right and left ventricular transmural and transeptal pressures. Ventricular pressures and volumes were varied by inflation of implanted vena caval and pulmonary artery occluders, and the functional significance of the two types of ventricular interaction, i.e., direct and series, was determined. The left ventricle was represented by a modified ellipsoidal geometry. Left ventricular stroke volume calculated from the dimension data correlated well with that measured directly from ascending aortic electromagnetic flow probes during all interventions (r ⩾ 0.96). Partial pulmonary artery occlusion significantly increased right ventricular diastolic and systolic pressures as compared to values obtained during control and venal caval occlusion. During pulmonary artery occlusion, latitudinal septal eccentricity was increased throughout the cardiac cycle compared to control and vena caval occlusion (P < 0.05), indicating leftward interventricular septal shifting and significant alteration of left ventricular shape. The normalized diastolic pressure-volume curve was shifted to the left with pulmonary artery occlusion compared to control and indicated a decrease in left ventricular chamber compliance. However, when selected cardiac cycles with similar end- diastolic volumes from vena caval and pulmonary artery occlusions were compared, parameters of left ventricular systolic function (stroke volume, percent systolic shortening, peak aortic blood flow, peak left ventricular pressure, and its first derivative) remained relatively constant. These data suggest that mean myocardial fiber length is the major preload determinant of left ventricular systolic function independent of chamber pressure and shape, and that direct ventricular interaction mediated by interventricular septal shifting has minimal effects on systolic myocardial performance in this model. Thus, series ventricular interaction during acute imbalances in biventricular loading, where the output of the right ventricle determines the input of the left, seems to be far more important than direct interaction to the regulation of overall cardiac function.

Intraoperative transesophageal Doppler color flow imaging used to guide patient selection and operative treatment of ischemic mitral regurgitation.

BackgroundIntraoperative transesophageal Doppler color flow imaging (TDCF) affords the opportunity to assess mitral valve competency immediately before and after cardiopulmonary bypass (CPB). The purpose of this study was to assess the utility of TDCF to assist in the selection and operative treatment of ischemic mitral regurgitation (MR). Methods and ResultsTwo hundred forty-six patients undergoing surgery for ischemic heart disease were prospectively studied. All had preoperative cardiac catheterization. Catheterization and pre-CPB TDCF were discordant in their estimation of MR in 112 patients (46%). Compared with patients in whom both techniques agreed in estimation of MR, patients with discordance in MR were more likely to have had unstable clinical syndromes at the time of catheterization (79% versus 40%, p < 0.05) or to have received thrombolytics (16% versus 8%, p < 0.05). Pre-CPB TDCF resulted in a change in the operative plan with respect to the mitral valve in 27 patients (11%). Because less MR was found by TDCF than catheterization, 22 patients had only coronary bypass grafting when combined coronary bypass and mitral valve surgery had been planned. Because more MR was found by TDCF than catheterization, five patients had combined coronary bypass and mitral valve surgery when coronary bypass alone had been planned. Unsatisfactory results noted by TDCF following mitral valve surgery in five patients resulted in immediate corrective surgery. Cox regression analysis identified residual MR at the completion of surgery to be an important predictor of survival (X2=21.4) after surgery-more important than patient age (X2=8.3) or left ventricular ejection fraction (x2=5.3). ConclusionsThese results indicate that TDCF is useful in guiding patient selection and operative treatment of ischemic MR and that in such patients, intraoperative TDCF should be performed routinely.

The effects of airway pressure on cardiac function in intact dogs and man.

Ventilation with positive end-expiratory pressure (PEEP) is associated with reduced cardiac output, but the mechanisms involved are controversial. Possible explanations include increased intrathoracic pressure, reflex changes in myocardial inotropism, pulmonary vascular obstruction and abnormal ventricular interaction. Three types of conscious canine preparations were developed to examine simultaneously each of these factors during ventilation with PEEP. In addition, similar measurements were obtained in patients after cardiac surgical procedures and compared with the results of animal experiments. The primary cause of reduced cardiac output during PEEP appeared to be a diminished end-diastolic volume of the left ventricle, and this appeared to be the result of elevated intrathoracic pressure and increased impedance to blood flow through the lungs. Abnormal interventricular septal shifting and reflex autonomic alterations did not appear to be significant in the normal cardiovascular system. These data provide insight into the cardiac effects of PEEP and emphasize the importance of simultaneous quantification of biventricular performance when assessing cardiopulmonary function.

The end-systolic pressure-volume relationship in conscious dogs.

The end-systolic pressure-volume relationship (ESPVR) has been shown to be an afterload-insensitive descriptor of ventricular inotropic state in the isolated heart. The purpose of this study was to examine the effects of changes in afterload, heart rate, intravascular volume, autonomic tone, and inotropic state on the ESPVR in conscious dogs. In 30 dogs, left ventricular and pleural pressures were measured with micromanometers, and left ventricular volume was assessed with global ultrasonic crystals. The ESPVR was obtained during vena caval occlusions in each dog during pharmacologic afterload interventions at control and after autonomic blockade. Analysis of variance techniques were used to compare the slopes (Emax) and intercepts (Vd) of ESPVR regression lines in a given study. All estimates of the ESPVR in conscious dogs involved large extrapolations to obtain estimates of Vd. Repeat determinations of Emax at control in the unblocked state were significantly different in six of eight dogs (p less than .05). After autonomic blockade, these differences were significant in only one of eight dogs. Changes in heart rate and volume loading had minimal effects on the ESPVR. In the absence of autonomic blockade, increases in inotropic state with either calcium or dobutamine tended to cause parallel shifts in the ESPVR. After autonomic blockade, Emax increased with augmentation of inotropic state, while Vd was unchanged. ESPVRs obtained at different afterloads showed statistically significant differences in Emax and in Vd in 12 of 14 dogs. However, no statistically significant relationship of Emax to afterload was observed. Thus, the ESPVR is probably valid in conscious dogs, but measurement with an intact cardiovascular system is hampered by statistically significant variability in Emax and Vd with changes in afterload. Baseline variability is magnified by the autonomic nervous system, probably mediated through sympathetic reflexes.