Experimental Right Atrial Ischemia and Its Response to Stimulation with Dobutamine

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Experimental Right Atrial Ischemia and Its ~

Response to Stimulation with Dobutamine ANGEL ROMERO-CkDENAS, M.D., JESUS VARGAS-BARRON, M.D., NILDA ESPINOLA-ZAVALETA, M.D., ARTURO MUNIZ-GARCIA, M.D., MARC0 PENA-DUQUE, M.D., CARLOS MARTINEZ-ShCHEZ, M.D., CANDACE KEIRNS, M.D., MARfA RIJLAARSDAM, M.D., MARTIN ROSAS-PERALTA, M.D., and EULO LUPI-HERRERA, M.D. Instituto Nacional de Cardiologia, “Ignacio Chavez,” Colonia Seccion XVI-Tlalpan, Mexico

I n order to analyze the repercussion of experimental isolated selected right atrial ischemia on the he- modynamics of both ventricles, we investigated the response of atrial myocardium with administra- tion of dobutamine and evaluated the utility of transesophageal echocardiography (TEE) with Doppler in the examination of alterations produced by atrial ischemia. Ten dogs were studied with normal, diminished, and increased cardiac output with ligation of the visible atrial branches of the right coronary artery. Right atrial wall movement and peak A wave velocity of tricuspid flow regis- tered by TEE decreased (P < 0.05). The amplitude of the right atrial A wave decreased in hemody- namic recordings (P < 0.05). No significant modifications occurred in right ventricular wall move- ment nor in pressures registered in right or left ventricles or cardiac output. Seventy-five minutes af- ter atrial ischemia was induced dobutamine was administered. I n dogs with incomplete ligations of atrial circulation, right atrial wall movement improved (P < 0.001), and the amplitude of the peak A wave velocity of tricuspid flow increased. I n dogs with complete coronary ligation, administration of the medication produced no improvement of these variables. The findings indicate that it is pos- sible to produce selective right atrial ischemia manifested by diminished wall movement, a dimin- ished atrial component of tricuspid flow in TEE, and decreased amplitude of the A wave in atrial pressure recordings. The localized hemodynamic changes produced by right atrial ischemia are not related to variations in venous return when right ventricular function is normal. Apparently isolated right atrial damage from ischemia does not affect ventricular function i f these remain healthy. The recuperation of atrial contractility can be demonstrated with dobutamine. Transesophageal echo- cardiography is a very useful technique for studying right atrial ischemia and infarction. (ECHO- CA-RDIOGRAPHY, Volume 15, February 1998)

atrial ischemia, response to dobutamine, atrial branches, tricuspid Doppler

The first case of atrial infarction was diag- nosed in a postmortem study by Clerc and Levy’ in 1925. They found that the histologic findings in the atrium were identical t o those in the infarcted ventricle.’ In 1932, Langen- dorP described the first case of atrial infarction

Address for correspondence and reprints: Jesus Vargas- Barrbn, M.D., Department of Echocardiography, Instituto Nacional de Cardiologia “Ignacio Chavez,” Juan Badiano No. 1, Colonia Secci6n XVI-Tlalpan 14080 Mexico, D.F., Mexico. Fax: 573-0994.

diagnosed during life on the basis of electro- cardiographic findings (negative and diphasic P waves in D,, and DIII). In cases of infarction of the right ventricle, clinical and experimental studies reveal that the compensatory increase in right atrial contraction improves right ven- tricular filling and function. In contrast, dete- rioration of atrial contractility from ischemia adversely affects the function of both ventricles and increases the hemodynamic consequences of right ventricular dy~function.~

The contractile force of the atria has been

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ROMERO-CkDENAS, ET AL.

shown to depend on the intrinsic inotropic con- ditions and neurohumoral stimulation. Williams' group4 demonstrated that isopro- terenol, calcium, and acetylstrophanthidin ex- erted positive inotropic effects on the atria sim- ilar to those obtained with the ventricles.

The diagnosis of atrial infarction is impor- tant because of the clinical complications asso- ciated with it: supraventricular arrhythmias, atrioventricular conduction disturbances, mural thrombosis, pulmonary embolism, and atrial wall rupture.' In the majority of cases the electrocardiographic findings are subtle and nonspecific and can be masked by the changes produced by the concomitant ventricu- lar infarction. In fewer than 8% of the cases can the classic electrocardiographic signs of atrial infarction, such as depression or eleva- tion of the PR segment be found.b

Transesophageal echocardiography (TEE) is superior to transthoracic technique in provid- ing satisfactory evaluation of atrial wall move- ment and permits diagnosis of right atrial in- farction based on the analysis of alterations of wall movement and transtricuspid diastolic flow, especially in the absence of the flow wave secondary to right atrial contraction.;

This experimental study was performed in dogs with hemodynamic control using TEE with three objectives: produce acute right atrial ischemia and analyze its hemodynamic effect on right ventricle and systemic circula- tion; investigate the presence of atrial myocar- dial response by administering dobutamine as a postive inotropic agent once right atrial isch- emic damage was established; and evaluate the utility of TEE in the examination of the al- terations produced by atrial ischemia.

Materials and Methods

Experimental Preparation

Ten male mongrel dogs weighing between 20 and 28 kg were studied. They were anes- thetized with 30 mgkg intravenous pentobar- bital maintained by continuous infusion and were ventilated through a tracheostomy tube connected to a mechanical pressure respirator. The femoral arteries and veins of both sides

were dissected as well as both jugular veins and the left carotid artery. Catheters were in- troduced into right atrium, right ventricle, pul- monary artery (Swan Ganz, Baxter Healthcare Corp., Puerto Rico), aorta, and left ventricle through the corresponding vessels. Bilateral arteriovenous fistulae were created in femoral vessels. Subsequently, a medial sternotomy was performed. The pericardium was opened over the atrioventricular groove, and all visible atrial branches of the right coronary artery dis- sected and individually referenced with silk su- ture (Fig. 1). The pericardium was closed with separate stitches, and the silk references of the atrial coronary branches drawn through small orifices between the stitches. The catheters were connected to Statham p23 ID pressure transducers (Statham, Los Angeles, CA, USA) and these to a multichannel Electronics for Medicine-Honeywell VA6 Simultrace Record (Honeywell, Denver, CO, USA) polygraph to ob- tain simultaneous recordings of right atrial, right ventricular, left ventricular, and arterial systemic pressures. The transducers were cali- brated to a middle level of the left ventricular

Figure 1. Diagram o f the technique for ligation of right coronary artery atrial branches. Each visi- ble branch is dissected and marked separately for 1 igat ion.

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cavity. The transesophageal echocardiographic study was performed with a Hewlett Packard (Hewlett-Packard, Andover, MA, USA) ma- chine and a 5 MHz biplanar transesophageal transducer, which was advanced to the level of the right atrium. Arterial blood gases and hematocrits were taken at 30-minute intervals.

Acqusition of Data

Two-dimensional and M-mode echocardio- grams in short-axis and longitudinal planes were used to evaluate the dimensions of heart cavities and wall movement. The right atrial area was calculated by a computerized system (Hewlett-Packard data analysis package). Two heart beats were analyzed for each measure- ment and under each set of venous return con- ditions. Right atrial and ventricular wall movement was evaluated according to changes in wall thickness and the excursion of the en- docardial surface toward the cavity was mea- sured. The pattern of right ventricular filling flow was analyzed in the transverse plane from four chamber images with pulsed Doppler by placing the sample volume at the edge of the tricuspid leaflets during maximal opening. The variables examined by Doppler for diastolic tricuspid flow were maximum E and A wave velocities in cdsec. Measurements were taken in the postinspiration phase. An electrocardio- graphic standard bipolar derivation (DIJ signal was recorded. All images were video taped for quantitative analysis by a computerized sys- tem (Hewlett-Packard). Recordings were taken of right atrial, right ventricular, left ventricu- lar, and systemic arterial pressures. All record- ings were printed on photographic paper for quantitative analysis. Cardiac output was measured using a thermodilution computer (PPG, model T-110) by injecting 5 mL cold saline solution in the proximal part of the Swan Ganz catheter in the pulmonary artery. Three injections were performed for each mea- surement of cardiac output.

Experimental Procedure

After basal hemodynamic and echocardio- graphic recordings were taken, venous return was decreased by partial occlusion of the infe-

rior vena cava until cardiac output was reduced by one Wmin in comparison to the basal value, and hemodynamic and echocardiographic mea- surements were repeated. Venous return was then increased by releasing the occlusion of the vena cava, opening the femoral arteriovenous fistulae, and administering liquid (100 cdh) un- til cardiac output was increased by one Wmin in comparison to basal levels. Hemodynamic and echocardiographic measurements were taken at each step. Atrial branches were lig- ated to produce right atrial ischemia, and he- modynamic and echocardiographic measure- ments were performed under the three condi- tions of venous return (normal, decreased, and increased) after 15, 45, and 75 minutes of isch- emia. Later, with atrial branches still ligated, dobutamine was administered by infusion pump over 5 minutes at a dose of 5 pgkg per minute, and echocardiographic recordings were taken with normal venous return.

At the end of the experiment the dogs were sacrificed and the hearts removed. Right and left coronary arteries were identified, mercury was injected into them, and radiographs were taken in two projections to determine whether the occIusion of the atrial branches had been total or partial.

Statistical Analysis

stan- dard error (SE). ANOVA multivariate analysis was performed comparing experimental values with basal values. Statistical signficance was considered as P c 0.05.

Data were expressed as mean values

Results

Of the echocardiographic variables analyzed, it was noteworthy that when basal values were compared with those obtained at 15,45, and 75 minutes of selective right atrial ischemia in the entire group, right atrial wall movement and maximum A peak velocity showed statisti- cally significant decreases in the three condi- tions of systemic venous return (normal, de- creased, and increased). Right atrial area and right ventricular wall movement did not un- dergo signficant changes (Table I).

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194

x

Of the hemodynamic variables examined un- der the same conditions, only right atrial A wave amplitude demonstrated a significant de- crease between basal recordings and those taken after 15, 45, and 75 minutes of atrial ischemia. Mean right atrial pressure, right ventricular systolic and diastolic pressures, left ventricular end-diastolic pressure, and sys- temic systolic pressure did not manifest statis- tically significant changes (Tables IIa and IIb).

After 75 minutes of atrial ischemia, dobuta- mine was administered to all of the dogs, and right atrial wall movement was evaluated again. Four dogs showed recuperation of atrial wall movement from 1.9 2 0.5 mm to 4.8 2 0.9 mm (P < 0.001) (Figs. ZA, ZB, and ZC). These dogs showed significant decreases without complete disappearance of the tricuspid flow A peak during ischemia, and the A peak recuper- ated with dobutamine. Postmortem examina- tion of the atrial coronary branches showed in- complete occlusion (Fig. 3). In six dogs, dimin- ished atrial wall movement showed no important modifications, from 1.9 2 0.5 mm t o 1.98 2 0.6 mm (not significant) (Figs. 4A, 4B, and 40. In these cases, the loss of the tricus- pid flow A peak was complete with ischemia and did not recuperate with the administration of dobutamine. Occlusion of atrial coronary branches proved to be complete (Fig. 5). When atrial wall movement and tricuspid " A peak velocity flow were compared separately during ischemia, movement diminished in the same way and during the same period in the two groups. Transvalvular flow velocity decreased immediately in the group with complete liga- tion and later in the group with incomplete lig- ation. Recuperation of wall movement when dobutamine was infused was statistically sig- nificant in the dogs with incomplete ligation.

Discussion

The functions and hemodynamic signifi- cance of the right atrium have been amply

Its functions include reservoir dur- ing ventricular systole; conduit for the flow of blood from the venae cavae to the right ventri- cle during ventricular diastole; and pump to push part of its contents into the right ventri-

ECHOCARDIOGRAPHY: A Jml. of CV Ultrasound & Allied Tech. Vol. 15, No. 2, 1998

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cle during end-diastole to increase the ventric- ular filling volume. Because right atrial con- tractile function can be modified by venous re- turn and venous pressure,12 systemic venous return varied in this experimental model. De- crease was produced by occluding the inferior vena cava and increase by opening femoral ar- teriovenous fistulae. Selective acute right atrial ischemia was induced by ligating the vis- ible atrial branches from the right coronary artery. One of the first observations was the colapse of right atrial walls when ischemia was produced. In spite of the fact that no statisti- cally significant changes in mean atrial pres-

Figure 2. (A) Example of basal transesophageal two-dimensional and M-mode recordings at atrial level. In M-mode the image lef? atrium, interatrial septum, right atrium, and right atrial wall can be identified. Right atrial wall movement (arrow) is normal. (B) After ligation of visible right atrial branches right atrial wall movement is diminished. (C) After administration of 5 pgJkg per minute of dobutamine right atrial wall movement shows recu- peration. In this dog interruption of right atrial cir- culation was not complete.

sure occurred with any of the conditions of ve- nous return, the function of reservoir was al- tered. This is worthy of note, since in clinical situations we have observed that in patients with left ventricular posteroinferior, right ven- tricular and right atrial infarctions the right ventricle and atrium are dilated. Atrial dilata- tion rather than colapse would be expected. Di- latation is observed when right ventricular dysfunction, tricuspid insufficiency and/or atrial damage occur. I t can be inferred that in this experimental model, inasmuch as isch- emic damage to the right ventricular was not produced, its function remained normal. This

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Figure 3. Radiograph of selective injection of coro- nary arteries with mercury showing partial occlu- sion of right atrial circulation.

assumption was surmised by normal hemody- namic and echocardiographic variables.

The function of conduit between the venae cavae and the right ventricle was not signifi- cantly modified; no alterations of rapid ventric- ular filling flow were recorded by Doppler. The function that showed important affection was that of pumping. This was documented by vari- ables with statistically significant changes: atrial wall movement and diminished atrial component to transtricuspid flow (A wave). In the analysis of the values of these noted in Ta- bles I and IIa, atrial wall movement, tricuspid flow A wave, and the amplitude of the A wave from the right atrial pressure tracing show de- creases at 15 and 45 minutes of observation in comparison to basal values. A mild increase, which did not achieve statistical significance, occurred later and could be explained by the finding that in a number of the dogs occlusion of the atrial branches was not complete. It is possible that a degree of partial atrial reperfu- sion from the unligated branches occurs and originates the increase in the atrial component of tricuspid flow between 45 and 75 minutes of observation.

Atrial contraction contributes between 18% and 60% of ventricular filling,13 so it is to be ex- pected that if atrial contraction fails, right ven- tricular filling will be affected and cardiac out- put will fall, especially if this is associated with ventricular dysfunction. In this model, no dete- rioration of right ventricular function or car- diac output was observed, perhaps because

ischemic damage was limited to atrial walls, and ventricular walls were left untouched.

In the clinical setting, it is exceptional that an isolated atrial infarction is diagnosed. How- ever, in a postmortem study atrial infarction without concomitant ventricular infarction was found in 20.8% of cases.14 In four published series, right atrial infarction has been demon- strated to be 81-98% more frequent than left atrial infar~tion.'~-'~

Goldstein et aL3 has examined the contribu- tion of the right atrium to filling of the in- farcted right ventricle. He has shown that right atrial contraction depressed by ischemia adversely affects the function of both ventricles and increases the hemodynamic repercussions of right ventricular dysfunction. In this series of experiments, the lack of normal atrial con- traction induced by selective ischemia was not associated with significant right ventricular dysfunction. Wall movement examined by echocardiography and diastolic and systolic pressures recorded by catheterization demon- strated no significant changes. In general terms, selective right atrial ischemia only pro- duced localized wall changes without signifi- cant effects on right or left ventricular systolic or diastolic function or cardiac output. Like- wise, it produced no alterations associated with changes in systemic venous return. Nev- ertheless, we feel that in clinical situations it is important to determine whether right atrial ischemic damage exists and whether it is re- versible, since this implies modifications in therapeutic management and in prognosis. Pe- riods of ischemia longer than 60 minutes gen- erally produce irreversible damage to intracel- lular structures, especially if they are not asso- ciated with reperfu~ion. '~~~~ This concept is now changing, since the therapeutic window for ad- ministration of thrombolytic agents is at least 6 hours. Consequently, in this study, after atrial ischemia was produced and maintained for at least 75 minutes, dobutamine was in- fused at a dose of 5 pglkg per minute in order to look for recuperation of wall movement. It is generally accepted that dobutamine exerts its inotropic action by direct stimulation of beta, receptors in ventricular and atrial myocar- dium.20~2' Various studies have shown that

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when dobutamine is administered at low doses and contractile capacity lost during an isch- emic event is recuperated, viable myocardium can be supposed to exist and recuperation of atrial function with therapeutic measures pre- dicted.22 When right atrial wall movement un- der the effects of dobutamine was evaluated by TEE, we found that in four dogs movement in- creased (recuperation from ischemia). This cor- responded to a group with incomplete ligation of the right atrial branches as demonstrated by postmortem radiographs of the coronary arter- ies. Six of the dogs showed no significant changes in atrial wall movement with the in- fusion of dobutamine. These had complete lig-

Figure 4. (A) Basal transesophageal two-dimen- sional and M-mode images at atrial level. Right atrial wall movement (arrow) is normal. (B) Liga- tion of visible right atrial branches produces notably diminished right atrial wall movement. (C) After administration of 5 pg / kg per minute dobutamine the right atrial wall remains akinetic, corroborating complete occlusion of the atrial circulation.

ation of right atrial branches in the radi- ographic control. It can be inferred that dobut- amine is capable of detecting recuperation of atrial wall moment after ischemia.

Another important difference we found was that dogs with complete occlusion of right atrial circulation showed absence of the tricuspid flow diastolic A wave, unlike the dogs with incom- plete ligation, in which the A wave did not com- pletely disappear. This might be related to the fact that incomplete occlusion of right atrial cir- culation can produce segmental ischemia and segmental alterations of atrial wall movement, which would allow viable atrial myocardium to participate in ventricular filling.

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Figure 5. Radiograph of selective injection of coro- nary arteries with mercury showing complete occlu- sion of right atrial circulation.

Conclusions

It is possible to produce selective right atrial ischemia that manifests loss of contractile ca- pacity with diminished wall movement, a di- minished atrial component of transtricuspid flow demonstrated by Doppler analysis, and reduction of the amplitude of the A wave in right atrial pressure recordings.

The localized hemodynamic alterations of se- lective right atrial ischemia are not associated with variations in systemic venous return when right ventricular function is normal. Ex- perimentally induced isolated right atrial myo- cardial ischemia does not originate significant hemodynamic effects on right ventricular func- tion, left ventricular function, nor cardiac out- put. Apparently, right atrial ischemic damage does not affect ventricular function if these are healthy.

In dogs with incomplete ligation of right atrial coronary circulation, the response to stim- ulation with low doses of dobutamine (5 pg /kg minute) was recuperation of atrial wall move-

ment and increase in the tricuspid “A” peak ve- locity flow. TEE is the diagnostic method of choice in right atrial ischemia or infarction.

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ECHOCARDIOGRAPHY: A Jrnl. of CV Ultrasound & AUied Tech. Vol. 15, No. 2,1998