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Ann Thorac Surg 2003;78:679-684
© 2003 The Society of Thoracic Surgeons
a Heart Lung Center Utrecht, University Medical Center, Utrecht, and Antonius Hospital, Nieuwegein, The Netherlands
Accepted for publication July 18, 2003.
* Address reprint requests to Dr Gründeman, Experimental Cardiothoracic Surgery, Experimental Cardiology Laboratory, Heart Lung Institute, Utrecht University Medical Center, Rm G02.523, Heidelberglaan 100, 3584 CX Utrecht, The Netherlands
e-mail: exp.cardio{at}hli.azu.nl
Abstract
PURPOSE: In off-pump coronary surgery through sternotomy, exposure of posterior circumflex branches causes circulatory deterioration in both patients and pigs. We assessed cardiac pump function when displacing the pig heart anteriorly with a suction cardiac positioner.
DESCRIPTION: Six pigs (±80 kg) underwent sternotomy for hemodynamic instrumentation using catheter-tipped manometers and paced at 80 beats/min. Ultrasound flow probes were placed around the aorta and proximal coronary arteries. The heart was retracted anteriorly to 90 degrees with the Starfish cardiac positioner attached to the apex by means of suction (–400 mm Hg). Retraction was guided by cardiac output monitoring.
EVALUATION: Anterior displacement to 90 degrees facilitated full exposure of posterior arteries. Stroke volume and mean arterial pressure decreased to 94% ± 13% (mean ± SD, p = 0.135) and 95% ± 13% (p = 0.09) of control values, respectively. Right and left ventricular end-diastolic pressure increased to 129% ± 37% (p = 0.009) and to 128% ± 57% (p = 0.235), respectively. Coronary flow remained unchanged. Additional 15-degree head-down positioning increased stroke volume to 113% ± 17% (p = 0.015) and mean arterial pressure to 113% ± 25% (p = 0.087) at the expense of further increased right and left ventricular end-diastolic pressure (186% ± 63%, p < 0.001 and 157% ± 49%, p < 0.001, respectively).
CONCLUSIONS: When lifting the porcine heart ninety degrees anteriorly, the Starfish cardiac positioner facilitated exposure of posterior branches and, when guided by cardiac output, preserved stroke volume and arterial pressure.
In off-pump coronary surgery, lateral and posterior wall vessels are more difficult to approach than anterior wall vessels and the right coronary artery because of limited exposure and hemodynamic deterioration seen with cardiac displacement. Lifting the beating heart to expose circumflex branches decreases stroke volume (SV) by up to 40% in both patient [1, 2] and pig [3–5], which is attributed to right heart deformation [4] and impaired diastolic filling [4]. Several interventions will mitigate the stroke volume drop: the Trendelenburg maneuver [1–9] or by increasing preload otherwise [1, 2, 6–8], inotropic support [1, 2, 6], the increase in heart rate by pacing, preservation of right heart filling by pericar-dial incision [7, 8], efforts to drop the heart posteriorly in the right chest [7] or moderate right lateral decubitus body positioning [9].
The objectives of this study were (1) to assess exposure of the posterior wall by lifting the pig heart with the Starfish apical suction cardiac positioner; (2) to monitor the accompanying hemodynamic changes; and (3) to study the modifying effect of the Trendelenburg maneuver.
Material and methods
Six Dutch land race pigs (range, 75 to 85 kg) were used. All animals received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (National Institutes of Health publication 85-23, revised 1985). The study protocol was approved by the Animal Experimentation Committee of the Utrecht University.
Anesthesia and instrumentation
The pigs were premedicated and anesthetized, and underwent midsternotomy to expose the heart for insertion of catheter-tipped manometers (Millar Instruments, Houston, TX) as described previously [3–5, 9]. After the administration of propranolol (range, 15 to 25 mg), pacing at a fixed rate of 80 beats/min was started. An ultrasound transit time flow probe (20 or 24 mm, Transonic Inc, Ithaca, NY) was placed around the ascending aorta for on-line measurement of the cardiac output (CO). Stroke volume was calculated by dividing CO by the heart rate. Flow probes were placed around the most proximal part of the main coronary vessels [5]. The heart was retracted anteriorly to 90 degrese with the Starfish cardiac positioner (Medtronic Inc, Minneapolis, MN). A personal computer–based data acquisition system stored hemodynamic variables as described previously [3–5, 9].
Experimental protocol
The protocol was comparable with the one used in previous studies [3–5, 9]. Baseline cardiovascular values were recorded after stabilization after at least 15 minutes of pacing (phase 1, anatomic position). Subsequently, values were taken 3 (phases 2 to 5) and 15 minutes (phase 6) after each intervention. Phase 6 served as baseline for the second sequence of displacement. In phase 2, the suction cardiac positioner was fixed to the apex while the heart remained in its anatomic position. In phase 3 (displacement), the beating heart was hoisted with the Starfish cardiac positioner until a 90-degree extraanatomic position relative to the spine was achieved (apex pointing ventrally). To minimize hemodynamic deterioriation, displacement was guided by the instantaneous CO reading. In phase 4, after stabilization of hemodynamics, the operating table was tilted 15 degrees in the head-down position (Trendelenburg maneuver) without changing the position of the heart relative to the body. After return from Trendelenburg, the heart was released from the Starfish cardiac positioner and fell back into the pericardial cradle (phase 5 and 6, freely beating). Afterwards, the entire protocol was repeated once.
Statistical analysis
The duplo data from each animal were averaged for analysis. In Table 1, data are presented as the mean value ± SD (absolute values). In the results section and in Figure 1, hemodynamic variables are presented as mean ± SEM of percentage of control values (the heart in anatomic supine position, phase 1). Statistical analysis was performed using multiple analysis of variance to assess the influence of changing the apex position from anatomic position into a 90-degree extraanatomic position relative to the spine. A paired Student's t test was used to assess the modifying effect of Trendelenburg compared with control values (supine position, phase 1).
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All animals survived the entire procedure without the need to defibrillate or administer inotropic drugs. The results are summarized in Table 1 and Figure 1. The paced heart remained in the regular rate of 80 beats/min throughout the experiment.
Phase 2: attachment of the cardiac suction positioner to the apex
Care was taken to avoid inclusion of the distal left descending coronary artery (LAD) in one of the three suction cusps of the Starfish cardiac positioner. After a couple of seconds of suction (–400 mm Hg suction force), the heart became firmly fixed to the device, which occasionally elicited premature ventricular beats. No inadvertent detachment from the apical suction device occurred during the entire period of the protocol. Initially, SV and MAP decreased on average to 99% ± 2% and 95% ± 2% (p =0.629 and 0.020, respectively (see Table 1). Right atrial pressure (RAP) and right ventricular end-diastolic pressure increased to 105% ± 4% (p = 0.275) and 104% ± 5% (p = 0.260), respectively. Left atrial pressure increased to 106% ± 4% (p = 0.175) and left ventricular end-diastolic pressure increased to 102% ± 4% (p = 0.956). Coronary flows remained unchanged.
Phase 3: cardiac retraction with the Starfish cardiac positioner
Displacement of the heart by the suction device was performed by first pulling the apex in the axial direction of the left ventricle and next moving it ventrally (Fig 2). Using the device, the maneuver was easy to perform. It resulted in a transient minor drop in MAP. The posterior aspect of the heart became fully exposed, which included excellent view of the circumflex artery and its branches, distal RCA, the great cadiac vein, coronary sinus, inferior caval vein, and left lower pulmonary vein. In diastole, normal radial expansion of the left ventricle was observed. The head of the device rotated counterclockwise in synchrony with ventricular contraction. After stabilization, SV had decreased to 94% ± 4% (p = 0.135) and MAP remained unchanged relative to phase 2 (95% ± 4%, p = 0.09 vs base line) at the expense of (further) increased preloads. Coronary flows remained unchanged.
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Phase 5 to 6: return of table to horizontal position, replacement of the heart in the anatomical position, and release from the retractor
The SV and MAP normalized. After detachment of the device, a superficial mashed dark discoloration was observed that outlined the contour of the suction device (total time attached to the heart was approximately 10 minutes). After 15 minutes (phase 6), the color of the suction lesion had turned to light red.
Displacement of the heart with Starfish versus Octopus retraction
Exposure of the posterior wall by the apical suction device largely prevented the substantial drop in SV and MAP observed when similar exposure was achieved by the octopus stabilizer: SV diminished to 94% ± 13% (Starfish) versus 71% ± 6% (Octopus, mean ± SD) and 95% ± 13% (Starfish) versus 77% ± 8% (Octopus), respectively [ 4].
Comment
The principal findings of the study were, first, that displacement of the beating heart by the apical suction positioner was easy and safe. Second, fixing the positioner to the apex hardly influenced heart function. Third, 90-degree anterior displacement provided full exposure of the circumflex territory and caused only a 6% or less decrease in SV and MAP. Fourth, the Trendelenburg maneuver resulted in an overshoot in SV and MAP at the expense of significantly elevated ventricular preload pressures.
Cardiac retraction with the suction cardiac positioner
The major advantage of displacing the heart with the apical suction positioner is the separation of exposure and stabilization of the target coronary artery as well as the improvement in cardiac function. In 1995, in addition to cardiac displacement with the Octopus [3, 6, 11], we explored in the pig model two alternative ways to expose the back side of the heart, one using a rigid prototype apical suction device and one using apically placed traction sutures (unpublished results). It was noted that when the heart was vertically displaced, using either approach, by pulling at the apex, MAP was surprisingly well preserved. In the present study, guided by the on-line cardiac output readings from the ultrasound aortic probe, a substantial drop in SV by overstretching the heart too much in the long-axis direction was avoided and the optimal position with respect to cardiac output for posterior wall exposure could be selected. The crucial observation was that a 1- to 2-cm change in apical position could improve cardiac output by up to 10% to 15%. The lateral and posterior aspects of the left ventricle became well exposed and were readily accessible for placement of a local wall stabilizer (Fig 2). Visually, the right ventricle was displayed unhindered, filling down to its apex. The left ventricle showed proper radial diastolic expansion circumferentially. The positioner moved in synchrony with the natural motion of the apex with a counterclockwise rotation.
Compared with our previous studies [3–5] using the suction-based Octopus tissue stabilizer for retraction of the heart, obtaining 90-degree ventral position with the Starfish resulted in easier exposure of the back side of the heart with a significantly smaller decrease in stroke volume (< 6% compared with 29% [4]) and arterial pressure (< 5% compared with 23% [4]). Right ventricular preload, however, was again enhanced, suggesting some right heart dysfunction. It is likely that compression of the thin-walled right ventricle by surrounding tissue contributed to the marginally decreased right ventricular function. It is inferred that keeping the left ventricle in its normal anatomical shape by gently pulling at the apex when it points ventrically largely preserved left ventricular diastolic and systolic function. As a result, the Trendelenburg maneuver caused an overshoot in SV and arterial pressure. It is conceivable that according to Laplace's law, midventricular enddiastolic wall tension was reduced owing to its smaller short-axis luminal diameter achieved by pulling at the apex.
Recently, George and associates [10] provided clinical evidence for another mechanism of the SV decrease upon vertical displacement. The authors demonstrated with three-dimensional echocardiography in patients undergoing off-pump coronary surgery that mitral valve annulus distortion may significantly contribute to hemodynamic instability during positioning for posterior wall grafting. In that study, however, no apical suction retraction was employed and compression stabilization may have contributed to the loss of cardiac output. The patient's heart may have become temporarily ischemic when the target coronary vessel was occluded, inducing papillary muscle dysfunction in addition to mechanical interference with valve funtion. In the present pig model, ischemic valve dysfunction originating from the papillary muscle is highly unlikely because coronary flow was not interrupted for coronary surgery and it followed the changes in arterial pressure. Previously, in both patients [1, 6] and animal [4], no valvular incompetence was observed that could explain the substantial drop in arterial pressure seen after full cardiac retraction for exposure of the posterior and lateral wall.
Mean coronary flow in the three proximal major branches varied according to changes in MAP (ie, with afterload). In the Trendelenburg position, for instance, when arterial pressure showed an overshoot to 115%, coronary flow increased to 117% to 125%. From this observation, we conclude that coronary flow was unimpeded throughout all maneuvers. In contrast to an earlier study [5], where lateral wall exposure was performed with the aid of a suction stabilizer, blood flow in the proximal circumflex coronary artery was slightly more reduced compared with coronary flow in the right and left anterior descending coronary artery when afterload was reduced, but the comparison did not reach level of significance. We have no indication that circumflex flow to the postero-lateral territory was adversely influenced by gently stretching the heart in the long-axis direction.
Clinical implications
Cardiac displacement by the Starfish positioner will facilitate exposure of posterior arteries. Provided that cardiac displacement is guided by cardiac output or MAP, full exposure of posterior arteries will be feasible with limited adverse hemodynamic consequenses.
Because the exposure of the base of the heart was excellent, it is conceivable that other cardiac procedures can also be facilitated by the use of the cardiac positioner (eg, atrial tissue ablation on the beating heart, retraction of the right ventricle for access to the proximal aorta, limited access pacemaker lead placement, and, eventually, endoscopic positioning for closed-chest coronary surgery).
Limitations
Cardiac retraction was carried out in the normal healthy pig heart with its apex slightly pointing rightwards. The pig's chest cavity conformation is "carinad"-shaped compared with the "barrel"-shaped human chest cavity. Despite this anatomic difference, inferences made from previous porcine heart displacement studies [3–5, 9] turn out to be fairly well applicable to the human [1, 2, 6–8].
In the present study, no stabilizer was placed along a coronary artery in addition to the use of the suction positioner. Provided that forcefully pushing the myocardium laterally was avoided, in three experiments (unpublished), additional local wall stabilization by the Octopus suction stabilizer did not have a negative impact on stroke volume and arterial pressure.
Conclusions
Exposure of posterior vessels through sternotomy was greatly facilitated by lifting the porcine heart with the Starfish apical suction positioner. At the expense of marginally augmented right and left ventricular preload, SV and MAP decreased on average by only 6% or less upon ninety-degree anterior displacement of the beating heart.
Disclosures and freedom of investigation
The University Medical Center Utrecht (UMCU) receives royalties from Medtronic for this invention; however, this research was performed independently, funded with academic grants. The present study was funded by a grant from the UMCU to Dr Borst. The UMCU receives royalties from the sales of the Octopus stabilizer and the Starfish positioner. As inventors, Drs Grundeman and Borst have received compensation from the UMCU. The authors have performed a free and in-depth evaluation of this new technology.
The authors acknowledge the technical contributions by Merel Schurink and Ricardo Budde.
Footnotes
The Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
References
This article has been cited by other articles:
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  A. Fukumoto, M. Yamagishi, K. Doi, M. Ogawa, T. Inoue, and H. Yaku Off-pump Pericardiectomy Using an Ultrasonic Scalpel and a Heart Positioner Asian Cardiovasc Thorac Ann, December 1, 2007; 15(6): e69 - e71. [Abstract] [Full Text] [PDF]
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 M. Tabata, S. Takanashi, T. Horai, T. Fukui, and Y. Hosoda Emergency conversion in off-pump coronary artery bypass grafting Interactive CardioVascular and Thoracic Surgery, October 1, 2006; 5(5): 555 - 559. [Abstract] [Full Text] [PDF]
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 A. L. Fernandez, J. B. Garcia-Bengochea, J. Alvarez, L. F. Perez, J. M. Suarez, J. Forteza, J. Cortes, and A. Martinez Apical suction leads to severe ischemia of the ventricular apex. Eur. J. Cardiothorac. Surg., April 1, 2006; 29(4): 506 - 510. [Abstract] [Full Text] [PDF]
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 J. F. Beek and J. A.P. van der Sloot Invited commentary Ann. Thorac. Surg., October 1, 2005; 80(4): 1369 - 1370. [Full Text] [PDF]
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