Micrografts. II: Evaluation of 25:1, 50:1, and 100:1 expansion skin grafts in the porcine model.

This study was undertaken to evaluate 25:1, 50:1, and 100:1 expansions of micronized skin grafts in a porcine model. Two full-thickness skin excisions (graft and control) were performed on each of 30 immature pigs (20 pounds). The pigs were divided into three groups of 10 animals each: group A, 25 cm2; group B, 50 cm2; and group C, 100 cm2. One square centimeter of the excised skin was thinned to produce a thick split-thickness skin graft. Four 90-degree passes were made through a skin mesher with the smooth side of the plastic mesh carrier to produce uniform pieces of skin. These pieces were applied to one area on each pig. Both the graft and control sites were covered with film. The film was removed on postoperative day 7, and excision sites were photographed on postoperative days 7, 10, 14, and 21. Healing was evaluated with a 12 x 12 inch digitizing pad to estimate the percent area healed. Healing was compared via analysis of variance, with percent area healed used as the dependent variable and treatment (control or graft) and postoperative day and expansion size used as the independent variables. No difference was found on postoperative day 7. On postoperative day 10, 25:1 grafts healed better than the 50:1 grafts, which were healed more than the 100:1 grafts. No difference was seen between 25:1 and 50:1 grafts on postoperative day 14; however, they were healed better than the 100:1 expansion grafts. No difference was seen between the graft sites on postoperative day 21.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrocardiographic effects of experimental myocardial infarction in pigs.

Myocardial infarctions were produced in young domestic farm pigs by ligating coronary arteries. Arteries ligated in four groups were (A) left anterior descending, (B) branches of left anterior descending, (C) left circumflex, and (D) right coronary artery. After 7 or 8 days, the hearts were removed and dissected and the infarcted areas measured. Vectorcardiograms were taken before ligation and just before termination. High recording speeds and sensitivities were used so that details of the QRS complexes could be seen. Difference vectors were computed as a function of time during QRS. In groups A and B, statistically significant differences in QRS components before and after ligation were found. Also, there were statistically significant correlations between vectorcardiographic deflections and the ratio of infarct weight to heart weight. The changes were related to known excitation patterns of the pig heart.

Dipole moment of in vivo and isolated perfused rabbit hearts.

We have measured magnitude and location of heart dipole moment during QRS in 46 New Zealand white rabbits. The spatial magnitude curve had one to three peaks. Mean values were M1 = 80 +/- 10 microA-cm (N = 5) pointing to right anterior and caudal, M2 = 260 +/- 15 microA-cm (N = 42) directed slightly to left of due anterior and caudal, and M3 = 236 +/- 9 microA-cm (N = 43) pointing towards left posterior and cephalad. The mean thorax resistivity was 250 ohm-cm. For 23 rabbits, M2/M3 greater than 1 and for 16 rabbits, M2/M3 less than 1. Mean times of occurrence of the three peaks were 5.8, 11.2, and 19.6 ms, respectively. Spatial magnitude curves for hearts perfused at the center of a sphere showed usually one major peak at about 19 ms. Locuscardiograms of in vivo hearts were also measured. By comparing M values for in vivo and isolated hearts, we found that M1 values agreed closely but mean M2 measured from the hearts in vivo was 2.5 times that for the isolated hearts, and M3 for in vivo hearts was about two-thirds that for isolated hearts. We relate these differences to the effects of intracardiac blood and lungs on the measured dipole moment.

Magnitude and location of a dipole in a circular ring with non-insulating boundaries.

It was shown that the resultant dipole moment of a system of sources and sinks within a region can be found from integrations of potential and potential gradient or normal current over the surfaces bounding the region. The method was applied in two dimensions to the case of a dipole in an infinite medium of one resistivity outside a circle having a different resistivity. The results agree with the Brody theory in that potentials at remote points due to a radial dipole were increased and potentials due to tangential components were decreased. Potentials on and within the circle are more complicated, however. For a radial or oblique dipole, potentials on the "endocardium" are increased in some areas but reduced at others. In addition, the effects are non-linear, depending on the distance of the dipole from the circle. Neglecting the effects of the low resistance circle leads to false values of dipole moment. When the integrations of potential and potential gradient are used, however, accurate values of dipole moment are obtained. The low-resistivity disk makes a radial dipole appear more centric but a tangential dipole appear further away.

Effects of intracardiac blood volume changes on the electrocardiogram.

Effects of changes of intracardiac blood volume were studied using a simple model consisting of a circle of resistivity 150 ohm-cm (representing the blood filled cavity) immersed in an infinite two-dimensional medium of resistivity 400 ohm-cm (representing the myocardium and external medium). For each case, potentials due to an artificial dipole just outside the circle were computed. Both radial and tangential dipoles were used. The area of the circle was changed to simulate blood volume changes. At remote points, the results agreed with the Brody theory in that potentials due to tangential dipoles were decreased and potentials due to radial dipoles were increased. On the circle (representing the endocardium), however, potentials were decreased for both radial and tangential dipoles in the vicinity of the dipole. All effects were enhanced by increasing the area of the circle. As area increased, radial dipoles appeared to be located more centrically, but tangential dipoles appeared to be located farther away, relative to true dipole location. Possible reasons for conflicting results on the effects of intracardiac blood volume changes on the electrocardiogram were examined and suggestions made for reduction of errors.

Experimental foundation of the Gabor-Nelson theory applied to boundaries which are non-insulating.

In order to found the application of the Gabor-Nelson theory to non-insulating boundaries, we have used a network which we have divided into two parts: a core energized by a source sink pair and an appendage, the conductivity of which may or may not differ from that of the core. By ignoring the appendage and by applying the Gabor-Nelson method to the restricted perimeter as if it were totally insulating, we stress the errors made in computing the dipole strength, orientation and position and how they are influenced by the dipole eccentricity, by its orientation with respect to the junction between the added portion and the core, and by a change in conductivity between the same compartments. Finally, we restore the dipole characteristics by using the appropriate correction derived from theory. Comparing the later results to those obtained by applying the Gabor-Nelson method to the whole insulating boundary leads to the conclusion that the correction is founded and must be taken into account.

Cardiac electrical resultant dipole moment of spontaneously hypertensive rats.

The cardiac electrical resultant dipole moment (RDM) of 17 spontaneously hypertensive rats (SHR) was compared with that of 17 Wistar-Kyoto rats (WKY), 65-100 days of age. Relative to body weight, left ventricular weight was 23% greater and right ventricular weight was 18% greater for SHR than for WKY. Left ventricular wall thickness was 11% larger and myocyte diameter was 13% larger for SHR than for WKY. RDM orientation for SHR was more dorsal, leftward, and cranial from middle to end of qRS. The second spatial magnitude peak of QRS, M2, was significantly smaller for SHR than for WKY (P less than 0.001) whereas M3 for SHR was significantly greater than for WKY (P less than 0.001). The alterations in RDM of SHR are greater than can be accounted for simply on the basis of increased cell size. The excitation sequence for SHR might be different from that of WKY. Results show the necessity of considering the details of the spatial magnitude curve during QRS.

Relative influence of intracardiac blood hematocrit and volume on the electrocardiogram.

Theoretical and experimental model studies on the effects of intracardiac blood have indicated that voltages due to a radial dipole are smaller than have been found in in-vivo experiments. The possible reasons for these differences have been examined. It is concluded that the lungs may tend to increase voltages over the anterior and posterior thoracic walls because of a current-channeling effect. This effect depends on the direction of cardiac excitation relative to the lungs. Voltages on the lateral and postero-lateral sides of the thorax are reduced because of the relatively large mass of the lungs in these regions. Changes in intracardiac blood volume for a constant hematocrit have less effect than changes in hematocrit per se.

Application of an isolated heart model to investigate blood-oxygen delivery.

To avoid the compensatory hemodynamic responses, which have limited interpretation of hemoglobin-oxygen affinity modifications in animal experimentation, an isolated blood-perfused rabbit heart model providing metabolic, functional, and vectorcardiographic measurements has been developed. Fixed-flow perfusions of unchanged or affinity-modified red blood cell suspensions were carried out to assess the benefits of high affinity during hypoxic hypoxia and of low affinity during posthypoxic recovery. Using fully saturated suspensions, the influence of affinity level during restricted flow and reperfusion was also studied. Higher myocardial oxygen consumption (MVO2) was associated with high-affinity blood during mild hypoxia and low-affinity blood during posthypoxic recovery. At low flows, heart rate and MVO2 tended to be lower in high-affinity perfusions, and to recover more completely during low-affinity reperfusions. Ventricular function, vectorcardiographic patterns, and lactate levels were affected by hypoxia and ischemia, but not by level of affinity. The relevance of these observations to the therapeutic potential of hemoglobin-oxygen affinity modification is discussed.

Effect of ventricular end-diastolic volume on vectorcardiographic potentials of the pig.

The Brody hypothesis proposes that alterations in end-diastolic blood volume will produce changes in the magnitude of cardiac electrical potentials recorded at the body surface. To test this hypothesis, several procedures that affect end-diastolic volume were applied in 28 anesthetized domestic piglets. The pig dipole moment magnitude curve is typified by two peaks, designated M2 and M3, which responded to these procedures. Doubling heart rate by electrical pacing or hemorrhage of 20 ml/kg produced reduction in M2 by 30% and 29%, respectively. M3 rose by 27% and 29%, respectively, under these two conditions. Reduction of end-diastolic volume by partial occlusion of the vena cava produced similar effects. Maneuvers increasing end-diastolic volume produced directionally opposite effects. Temporary aortic coarctation of phenylephrine injection caused an increase in M2 with no change in M3, accompanied an acute elevation of arterial pressure. Thus, the pig electrocardiogram shows changes, attributable to adjustments in end-diastolic volume, consistent with the Brody hypothesis, suggesting potentials for this technique in monitoring changes in end-diastolic volume.

The difference vector: assessment of effects of changes or interventions.

Vector dipole moments were measured on young pigs before and one week after ligation of the left anterior descending coronary artery. Vectors were obtained for the three peaks of vector spatial magnitude, M. Preoperative, postoperative, and difference vectors were measured for each peak. If excitation is normal except through the infarcted tissue, the difference vector should be more closely related to the infarct because the normal excitation cancels out. It was found that the postoperative vector was changed by the infarct but that the difference vector was a better indication of infarction. This paper was designed to introduce the method using experimental data for a small number of pigs.

Effect of hematocrit on electrocardiographic potentials and dipole moment of the pig.

The Brody hypothesis posits that blood in the heart chambers enhances body surface potentials due to radially oriented excitation and diminishes those due to tangentially oriented excitation. Evidence supporting the hypothesis has been found in models and in dogs. The current study was performed in pigs (Sus scrofa), anesthetized with sodium pentobarbital, to determine whether the phenomenon could also be demonstrated in a species having a ventricular activation pattern distinctly different from that of dogs. Hematocrit was varied from normal 35.3 +/- 1.1% to as low as 19.8% by hemodilution and to as high as 68.0% by hemoconcentration. Surface potentials early in QRS increased and those late in QRS decreased in hemodilution experiments, while the reverse was true in hemoconcentration experiments. Total QRS duration was 38.0 +/- 0.8 ms. The first peak in resultant dipole moment magnitude, at 5.4 +/- 0.2 ms, was inversely related to blood resistivity with a linear regression correlation coefficient r = -0.76; the second peak, at 10.9 +/- 0.1 ms, was directly related, r = 0.52; and the third peak, at 17.5 +/- 0.2 ms, was directly related, r = 0.89. When interpreted in accordance with the Brody hypothesis, changes in body surface potentials and in resultant dipole moment were consistent with radial excitation of the apical septum, ill-defined orientation of free ventricular wall excitation, and tangential excitation of basal left ventricle and septum.

Changes in heart dipole moment after surgical correction of atrial septal defect.

Spatial dipole moment of the heart (M) was measured in patients before and 1 to 2 weeks after repair of atrial septal defect. In normals three peaks of M were found, corresponding to excitation of the septum (M1), ventricular walls (M2), and basal portion of ventricles (M3). Most of the patients also had an M2a peak at 52 to 56 per cent of QRS duration. The patients with secundum defects were divided into two groups, A and B, depending on whether right ventricular pressure was less than or greater than 30 mm. Hg. M1 was smaller for the Group B patients than for the normals for Group A and decreased slightly after surgery. M2 was about half normal for Group A and one third normal for Group B. After surgery M2 increased for both Groups but was still below normal. There were small mean clockwise rotations of the M2 vector in the horizontal plane PO. The M2a vector pointed to the right anterior or posterior and generally downward. In three Group A patients the M2a peak disappeared PO but in two others it increased in magnitude with a counterclockwise rotation in the horizontal plane. In the Group B patients, M2a increased in all cases. The M3 vectors generally increased slightly in magnitude postoperatively. M3 pointed posteriorly in the normals but to the right in the patients, with little change in direction after surgery. Both P and T curves dropped in magnitude after surgery. The changes observed may be due in part to restoration of right ventricular blood volume towards normal after surgery. The remaining VCG abnormalities should be due to right ventricular hypertrophy.

Magnitude, direction and location of the resultant dipole moment of the pig heart.

Vectorcardiograms were obtained from 50 young domestic pigs using the Nelson lead system. Compensation for body size and shape is achieved and the resultant dipole moment magnitude reflects heart size. A strong relationship was found between heart size and maximum magnitude. Dipole moment magnitude increased as four pigs increased from five to ten weeks of age. The dipole moment during QRS is considered in light of known pig heart excitation pattern. Dipole locations during QRS, calculated by computer solution of the Gabor-Nelson equations, were in agreement with heart location and excitation data.