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Ann Thorac Surg 2001;72:1991-1996
© 2001 The Society of Thoracic Surgeons

Original article: cardiovascular

Hemodynamic changes with right lateral decubitus body positioning in the tilted porcine heart

Paul F. Gründeman, MD, PhD*a, Cornelius Borst, MD, PhDa, Cees W.J. Verlaana, Stefan Damen, MDa, Sabine Mertens, MDa

a Heart Lung Center Utrecht, University Medical Center, Utrecht, The Netherlands

Accepted for publication July 16, 2001.

* Address reprint requests to Dr Gründeman, Experimental Cardiology Laboratory, Heart Lung Institute, Utrecht University Medical Center (Room G02.523), PO Box 85500, 3508 GA Utrecht, The Netherlands
e-mail: exp.cardio{at}hli.azu.nl


   Abstract
Top
 Abstract
Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
 References
 
Background. In beating-heart coronary surgical procedures, exposure of posterior vessels through sternotomy causes cardiac function to deteriorate. We hypothesized that turning the subject to the right lateral decubitus position before cardiac retraction improves exposure of posterior vessels and preserves cardiac pump function on displacement.

Methods. Eight 80-kg open-chest pigs were instrumented with catheter-tip manometers. After a stepwise 60-degree turn to the right lateral decubitus position of the body, the heart was retracted anteriorly to 90 degrees with a suction stabilizer.

Results. Right lateral body positioning caused an approximately 45-degree right deviation of the apex, thereby exposing the left atrial groove. Stroke volume, mean arterial pressure, right atrial pressure, and right ventricular end-diastolic pressure increased to 106% ± 5% (mean ± standard error of the mean, p = 0.31), 106% ± 3% (p = 0.01), 129% ± 8% (p = 0.001), and 171% ± 14% (p = 0.002), respectively, compared with control values. In contrast, left atrial pressure decreased to 73% ± 6% (p = 0.007), whereas left ventricular preload remained unchanged (110% ± 8%, p = 0.26). Additional anterior displacement to 90 degrees fully exposed the posterior vessels, and stroke volume decreased to 90% ± 3% (p = 0.01) and mean arterial pressure to 93% ± 5% (p = 0.07) at the expense of further increased right ventricular preload (256% ± 28%, p < 0.001).

Conclusions. By placing the subject in the right lateral decubitus position, exposure through sternotomy of posterior vessels in the beating porcine heart was facilitated while mean arterial pressure was maintained.


   Introduction
Top
 Abstract
 Introduction
Material and methods
 Results
 Comment
 Acknowledgments
 References
 
In beating-heart coronary artery bypass grafting, access to the circumflex branches through sternotomy is feasible with the aid of fabric slings [1], deep pericardial retracting stitches [2], a mechanical tissue stabilizer [3, 4], or a combination of techniques. Recently, the feasibility of cardiac positioning with an apical suction device was reported [5].

Full ventral displacement of the beating heart, however, decreases stroke volume (SV) and mean arterial pressure (MAP) considerably in the pig [3, 6, 7] and in patients [8] owing to right heart deformation and impaired diastolic filling [7]. Using the Trendelenburg position [3, 4, 6–8], increasing preload by some other means [8, 9,], or administering inotropic support [8] will, in part, mitigate hemodynamic deterioration during exposure of posterior vessels. Another major problem in coronary surgical procedures on the posterior side is the lack of space as well as the unfavorable angle of view and tangential surgical approach.

The objectives of this study were (1) to investigate improvement in exposure of the posterior wall of the beating porcine heart through sternotomy by right lateral decubitus body position (RLD) (to 60 degrees, RLD position) before full retraction and (2) to monitor the accompanying cardiovascular changes.


   Material and methods
Top
 Abstract
 Introduction
 Material and methods
Results
 Comment
 Acknowledgments
 References
 
Eight Dutch Landrace 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" (NIH 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 median sternotomy to expose the heart for insertion of catheter-tip manometers, independent of body position, (Millar Instruments, Houston, TX) as previously described [3]. Before various maneuvers, pacing at a fixed rate of 80 beats/min was started after the administration of propranolol (range, 15 to 25 mg) [3]. An ultrasound transit time flow probe (size 20 or 24 mm, Transonic Inc, Ithaca, NY) was placed around the aorta for continuous measurement of the cardiac output (CO). Stroke volume was calculated by dividing CO by the heart rate. Flow probes (Transonic) were placed around the most proximal part of the main coronary vessels [3]. The animal was placed on a modified operating table that allowed a quick transition from supine horizontal body position to 60 degrees RLD position. For stepwise cardiac retraction in RLD position, the retractor-mounted Octopus2 Tissue Stabilizer (Medtronic Inc, Minneapolis, MN) was used. A personal computer-based data acquisition system stored hemodynamic variables as described previously [3].

Experimental protocol
Baseline cardiovascular values were recorded after stabilization in the supine horizontal position after at least 15 minutes of pacing (phase 1, supine anatomic position). Subsequently, values were taken 3 (phases 2 through 4, 6, and 8) and 15 minutes (phases 5 and 7) after stabilization after each intervention. In phase 2, the body position was stepwise changed to the 60-degree RLD position. In phase 3 (displacement), the beating heart was ventrally displaced with the Octopus2 until 90 degrees from anatomic position relative to the spine was achieved. Without the aid of other supportive tools, circumflex branches on the backside of the heart became better exposed to surgical procedures than in previous studies without RLD [3, 6, 7]. Subsequently, after stabilization of hemodynamics, the operating table was tilted 10 degrees in the head-down position (Trendelenburg position) without changing the position of the heart relative to the body (phases 4 and 5). After returning from Trendelenburg, the heart was released from the Octopus2 while the body position continued to be in RLD (phase 6 and 7). Subsequently, the body was placed back in the supine position by turning the table horizontally (phase 8). After stabilization of hemodynamic variables, the entire protocol was repeated once.

Statistical analysis
First, the duplicate data from each animal were averaged and mean values were further used for analysis. Data in Table 1 are presented as mean ± standard deviation (absolute values). Hemodynamic variables in the Results and in Figure 1 are depicted as mean ± standard error of the mean of the percentage of protocol control values (body in supine position; heart in anatomic position). Statistical analysis was performed using multivariate analysis of variance to assess the influence of changing the body position from supine into RLD position and subsequently ventral cardiac displacement. A paired Student’s t test was used to assess the modifying effect of Trendelenburg compared with control values (supine position, phase 1). A two-way nonparametric statistical test (Wilcoxon signed rank test) or unpaired two-way Student’s t test was used to compare the influence on hemodynamics after cardiac retraction in RLD position (present study) with pooled data from two previous studies [3, 6] in which cardiac retraction was performed in the supine position (n = 14). Cardiac retraction itself in the latter studies was performed in exactly the same manner as in the current study.


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  		Table 1. Hemodynamic Changes With Cardiac Displacement in Right Body Position

 


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  		Fig 1. Comparison of hemodynamics between 60 degrees right lateral body position (RLD, open bars) and supine body position (closed bars) with vertical displacement of the beating porcine heart. Mean percentage of supine baseline values (BASE) ± standard error of the mean. Open bars (this study) = displacement (DIS) in right decubitus position. Compared with baseline: *p < 0.05, **p < 0.01, #p < 0.001. Closed bars = displacement in supine position from [3, 7] (pooled data, n = 14) compared with displacement in lateral decubitus position. %p < 0.05, %%p < 0.01, $p < 0.001. (Trend = Trendelenburg maneuver.)

 

   Results
Top
 Abstract
 Introduction
 Material and methods
 Results
Comment
 Acknowledgments
 References
 
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 beating at a regular rate of 80 beats/min throughout the experiment. Although some individual animals showed minor changes in systemic and pulmonary vascular resistance, mean values were not significantly different throughout the maneuvers.

Turning body to right lateral decubitus position
The effects of turning the body to the RLD position (phase 2) are shown in Figure 2, middle panel. The position of the apex of the heart relative to the midline shifted to the right by approximately 45 degrees (arrow in middle panel of Fig 2). To some extent, the apex moved out of the pericardial cradle anteriorly. A space between the left sternal border and the left lateral side of the heart was created, thereby exposing the craniad part of the atrioventricular groove including the circumflex artery and the left atrial appendage. Visually, the contour of the outflow tract of the right ventricle was accentuated. Stroke volume and MAP increased to 106% ± 5% and 106% ± 3% (p = 0.31 and p = 0.01, respectively). Right atrial pressure and right ventricular end-diastolic pressure increased to 129% ± 8% (p = 0.001) and 171% ± 14% (p = 0.002), respectively. In contrast, left atrial pressure decreased to 73% ± 6% (p = 0.007), whereas left ventricular end-diastolic pressure increased to 110% ± 8% (p = 0.26). Coronary flows remained unchanged.



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  		Fig 2. Directional changes of the porcine heart apical position (arrow, craniad-to-caudal; top panel) in the frontal plane relative to the midline with right table tilt by 60 degrees (middle panel) and with additional anterior retraction to 90 degrees with the Octopus2 tissue stabilizer (lower panel).

 
Cardiac retraction in right lateral decubitus body position
The effects of cardiac retraction in the RLD body position (phase 3) are shown in Figure 2, lower panel. Additional displacement of the heart by the suction device resulted in further deflection of the apex to the right from approximately 45 degrees to 90 degrees relative to the midline (arrow in Fig 2, lower panel) and, relative to the spine, a full anterior displacement to 90 degrees. The posterior aspect of the heart became fully exposed. Stroke volume and MAP decreased to 90% ± 3% (p = 0.008) and 93% ± 5% (p = 0.15), respectively, at the expense of (further) increased preloads. Circumflex arterial flow decreased to 88% ± 4% (p = 0.05).

Statistical comparison of hemodynamic data after cardiac retraction in RLD position with those in supine body position [3, 7] revealed that, in supine position, SV and MAP decreased more (by 37% ± 5% and 37% ± 5%, p < 0.001 and p = 0.002, respectively) and ventricular preload pressures increased less (right ventricular end-diastolic pressure rose to 171% ± 12% versus 256% ± 20% and left ventricular end-diastolic pressure rose to 112% ± 9% versus 142% ± 8%, p = 0.003 and p = 0.04, respectively).

Tilting whole-body head-down 10 degrees (Trendelenburg) in right lateral decubitus body position
The effects of tilting whole-body head-down 10 degrees (Trendelenburg) in RLD body position (phases 4 and 5) are given in Table 1. Stroke volume and MAP (over)normalized at the expense of further increased ventricular preloads (right ventricular end-diastolic pressure increased to 298% ± 36% [p < 0.001] and left ventricular end-diastolic pressure increased to 157% ± 4% [p < 0.001]). At 15 minutes of Trendelenburg position (phase 5), no further changes in circulatory status had occurred.

Return of operating table to right lateral decubitus position and release from retractor
The effects of returning the operating table to the RLD position and releasing the heart from the retractor (phases 6 and 7) are shown in Table 1. Stroke volume and MAP were normalized.

Return to supine body position
The effects of returning to supine body position (phase 8) are given in Table 1. Stroke volume and MAP decreased to 82% ± 8% (p = 0.06) and 72% ± 6% (p = 0.008), respectively.


   Comment
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
Acknowledgments
 References
 
The principal findings of the study were, first, in 60 degrees RLD body position, partial exposure of circumflex branches was created while SV and MAP had increased. Second, additional retraction of the beating heart to a full 90 degrees of anterior displacement achieved full exposure without causing a major decrease in SV and MAP.

Turning body to right lateral decubitus position
The position of the apex of the heart relative to the spine shifted to a right lateral and a ventral position, both by approximately 45 degrees owing to the directional change in gravitational force (Fig 2, middle panel). Without major changes in MAP, ample space was created between the left sternal border and the left lateral side of the heart. Already part of the atrioventricular groove including the circumflex artery and the left atrial appendage was visible and exposed. The maneuver caused no ventricular arrhythmias. Visually, the contour of the outflow tract of the right ventricle was enlarged, which corresponded to the increased right ventricular end-diastolic pressure. Some right ventricular dysfunction was apparently induced because the increase in preload produced only a marginal augmentation of SV. The impaired right ventricular pumping function is attributed to insufficient diastolic expansion owing to mechanical deformation of the thin-walled right ventricle by its lateral-to-ventral position, ie, half out of the thorax. The decreased left atrial pressure is attributed to a decrease in hydrostatic pressure. Left ventricular end-diastolic pressure, however, remained unchanged. By turning the body to the RLD position, diverging changes in hydrostatic pressure were induced in each of the four heart chambers.

Cardiac retraction in right lateral decubitus body position
Compared with our previous studies [3, 6, 7] without RLD position, retraction of the heart by the suction device to 90 degrees ventral position resulted in better exposure of the backside of the heart with only a 10% decrease in SV and MAP. However, right ventricular preload was markedly enhanced at the same time. The change of position of the apex of the heart from RLD body position to 90 degrees in the right lateral-to-ventral direction was well tolerated because the heart was only additionally displaced 45 degrees instead of the full 90 degrees retraction in supine body position relative to the spine.

Tilting whole-body head-down 10 degrees (Trendelenburg) in right lateral decubitus body position
A moderate Trendelenburg position produced an increase in arterial pressure compared with baseline values (phase 1) at the expense of very high right ventricular preload pressure. With 60 degrees right table tilt, the Trendelenburg position was unnecessarily performed (to correct for diminished diastolic function in the displaced heart) and, maintained for 15 minutes, was probably harmful because of volume overload. After return to the supine body and anatomic heart positions, MAP was decreased and did not return within 3 minutes to baseline values (Table 1).

Limitations
The pig’s chest cavity conformation is carinate compared with the barrel-shaped human chest wall. Despite these anatomic differences, inferences made from previous animal studies [3, 6, 7] were largely applicable to humans [4, 8–11]. Cardiac retraction was performed in the normal healthy pig heart with its apex slightly pointing rightward. It remains to be determined whether, in patients, rotation to, say, 30 to 45 degrees right oblique decubitus position may also produce enhanced exposure as observed in this study, as well as favorable hemodynamic effects on full cardiac retraction. Also, the assisting person located opposite to the surgeon may have difficulties in viewing the circumflex territory shielded by the left sternal border. Patients with impaired left ventricular function may not tolerate 90 degrees vertical displacement of the beating heart.

Conclusions
Exposure of posterior vessels through sternotomy was facilitated by placing the pig in the RLD position. At the expense of enhanced right ventricular preload, CO and MAP decreased by only 10% on 90 degrees anterior displacement of the beating porcine heart. The potential benefit of the RLD position for off-pump coronary surgical procedures of posterior branches remains to be demonstrated in patients.


   Acknowledgments
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
References
 
The authors acknowledge the contribution of Michel A. Verlaan (biotechnician) and Hans P. van der Brugge (data acquisition). For assistance in statistical analysis, we thank Joop A. J. Faber, McS, PhD, and colleagues of the Center of Biostatistics, University of Utrecht.


   References
Top
 Abstract
 Introduction
 Material and methods
 Results
 Comment
 Acknowledgments
 References
 

  1. Kazama S., Ishihara A. Fabric heart retractor for coronary artery bypass operations. Ann Thorac Surg 1993;55:1582-1583.[Abstract/Free Full Text]
  2. Splittgerber F.H., Friedrich I., Falk B., Kroncke G., Talbert J.G. Exposing the circumflex artery: the heartflip technique. Ann Thorac Surg 1996;61:1019-1020.[Abstract/Free Full Text]
  3. Gründeman P.F., Borst C., van Herwaarden J.A., Verlaan C.W.J., Jansen E.W.L. Vertical displacement of the beating heart by the Octopus stabilizer: influence on coronary flow. Ann Thorac Surg 1998;65:1348-1352.[Abstract/Free Full Text]
  4. Jansen W.L., Borst C., Lahpor J.R., et al. Coronary artery bypass grafting using the Octopus method: results in the first hundred patients. J Thorac Cardiovasc Surg 1998;116:60-67.[Abstract/Free Full Text]
  5. Dullem K.C., Resano F.G. Xpose. A new device that provides reproducible and easy access for multivessel beating heart bypass grafting. Heart Surg Forum 2000;3:113-118.[Medline]
  6. Gründeman P.F., Borst C., van Herwaarden J.A., Mansvelt Beck H.J., Jansen E.W.L. Hemodynamic changes during displacement of the beating heart by the Utrecht Octopus method. Ann Thorac Surg 1997;63:S88-S92.
  7. Gründeman P.F., Borst C., Verlaan C.W.J., Meijburg H., Mouës C.M., Jansen E.W.L. Exposure of circumflex branches in the tilted beating porcine heart: echocardiographic evidence of right ventricular deformation and the effect of right or left heart bypass. J Thorac Cardiovasc Surg 1999;118:316-323.[Abstract/Free Full Text]
  8. Nierich A.P., Diephuis J., Jansen E.W.L., et al. Embracing the heart: perioperative management of patients undergoing off-pump coronary artery bypass grafting using the Octopus tissue stabilizer. J Cardiothorac Vasc Anesth 1999;13:123-129.[Medline]
  9. Hart J.C., Spooner T., Edgerton J., Milsteen S.A. Off-pump multivessel coronary artery bypass utilizing the Octopus Tissue Stabilization system: initial experience in 374 patients from three separate centers. Heart Surg Forum 1999;1:15-28.
  10. Hart J.C., Spooner T.H., Pym J., et al. A review of 1,582 consecutive Octopus off-pump coronary bypass patients. Ann Thorac Surg 2000;70:1017-1020.[Abstract/Free Full Text]
  11. Spooner T.H., Dyrud P.E., Monson B.K., Dixon G.E., Robinson L.D. Coronary artery bypass on the beating heart with the Octopus: a North American experience. Ann Thorac Surg 1998;66:1032-1035.[Abstract/Free Full Text]



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