James A Magovern

Mechanical analysis of midline sternotomy wound closure

OBJECTIVE Unstable median sternotomy closure can lead to postoperative morbidity. This study tests the hypothesis that separation of the sternotomy site occurs when physiologic forces act on the closure. METHODS Median sternotomy was performed in 4 human cadavers (2 male) and closed with 7 interrupted stainless steel wires. The chest wall was instrumented to apply 4 types of distracting force: (1) lateral, (2) anterior-posterior, (3) rostral-caudal, and (4) a simulated Valsalva force. Forces were applied in each direction and were limited to physiologic levels (< 400 N). Four sets of sonomicrometry crystals were placed equidistantly along the sternum to measure separation at the closure site. RESULTS Sternal separation occurred as a result of the wires cutting through the bone. Less force was needed to achieve 2.0-mm distraction in the lateral direction (220 +/- 40 N) than in the anterior-posterior (263 +/- 74 N) and rostral-caudal (325 +/- 30 N) directions. More separation occurred at the lower end of the sternum than the upper. During lateral distraction, xiphoid and manubrial displacement averaged 1.85 +/- 0.14 and 0.35 +/- 0.12 mm, respectively. Anterior-posterior distraction caused 1.99 +/- 0.04-mm xiphoid displacement and 0.26 +/- 0.12-mm manubrial displacement. During a simulated Valsalva force, more separation occurred in the lateral (2.14 +/- 0.11 mm) than in the anterior-posterior (0.46 +/- 0.29 mm) or rostral-caudal (0.25 +/- 0.15 mm) directions. CONCLUSIONS These data suggest that sternal dehiscence can occur under physiologic loads and that improved sternal stability may be readily achieved via mechanical reinforcement near the xiphoid. Closure techniques designed to minimize wire migration into the sternum should also be developed.

Partial versus full sternotomy for aortic valve replacement.

BACKGROUND Recent trends suggest that smaller incisions reduce postoperative morbidity. This study tests the hypothesis that a partial upper sternotomy improves patient outcome for aortic valve replacement. METHODS A group of 50 patients who underwent aortic valve surgery through a partial upper sternotomy (group I) were compared to 50 patients who underwent aortic valve replacement through a median sternotomy during the same time period (group II). The mean age (60+/-2 versus 63+/-2 years; mean +/- SEM) and preoperative ejection fractions (53+/-2 versus 54+/-2) were similar. Operations were performed with central cannulation, and antegrade/retrograde blood cardioplegia. RESULTS There was one death in each group. No differences were found in aortic occlusion time, mediastinal drainage, transfusion incidence, narcotic requirement, length of stay, or cost. The incidence of pleural and pericardial effusions was increased (18.4% versus 3.9%, p < 0.03), and the need for postoperative inotropic support was higher (38.7% versus 19.6%, p < 0.03) in the partial sternotomy group. CONCLUSIONS Aortic valve replacement can be performed through a partial sternotomy with results comparable to full sternotomy. The partial sternotomy offers a cosmetic benefit, but does not significantly reduce postoperative pain, length of stay, or cost.

Lower sternal reinforcement improves the stability of sternal closure

BACKGROUND This study uses a mechanical testing system to evaluate three methods of sternal closure. METHODS Twelve sternal replicas composed of a polyurethane foam bone analogue were divided in the midline and reapproximated using three stainless steel wire techniques: six simple wires (6S), six figure-of-eight wires (6F8), or seven simple wires (7S), which included an extra wire at the lower sternum. The closures were subjected to increasing lateral distraction from 0 to 400 Newtons (N) (1 N = 0.224 lbs), and motion was measured using transducers stationed across the manubrium, midsternum, and lower sternum. RESULTS With each method of closure, the manubrium was the most stable, the lower sternum the least stable, and the midsternum intermediate between the other two. There were also differences between sternal closure methods, but only at the lower sternum. Less sternal distraction was measured with the 7S than the 6S and 6F8 methods, starting at 100 N (0.20 +/- 0.06 mm vs 0.48 +/- 0.19 and 0.39 +/- 0.10, p = 0.003), and progressively increasing until the study was stopped at 400 N (1.64 +/- 0.39 mm vs 4.92 +/- 1.73 and 5.1 +/- 1.43 mm, p = 0.003). CONCLUSIONS These data show that the lower sternum is the site of greatest instability and that reinforcement of this area with an additional wire effectively stabilizes the closure. Figure-of-eight wires are not superior to simple wires.

Physiologic compensation is supranormal in compensated aortic stenosis: does it return to normal after aortic valve replacement or is it blunted by coexistent coronary artery disease? An intramyocardial magnetic resonance imaging study.

BACKGROUND In compensated aortic stenosis (AS), cardiac performance measured at the ventricular chamber is typically supranormal, whereas measurements at the myocardium are often impaired. We investigated intramyocardial mechanics after aortic valve replacement (AVR) and the effects relative to the presence or absence of coronary artery disease (CAD+ or CAD-), respectively. METHODS AND RESULTS Twenty-nine patients (46 to 91 years, 10 female) with late but not decompensated AS underwent cardiovascular MRI before AVR (PRE), with follow-up at 6+/-1 (EARLY) and 13+/-2 months (LATE) to determine radiofrequency tissue-tagged left ventricle (LV) transmural circumferential strain, torsion, structure, and function. At the myocardial level, concentric LV hypertrophy regressed 18% LATE (93+/-22 versus 77+/-17 g/m2; P<0.0001), whereas at the LV chamber level, ejection fraction was supranormal PRE, 67+/-6% (ranging as high as 83%) decreasing to 59+/-6% LATE (P<0.05), representing not dysfunction but a return to more normal LV physiology. Between the CAD+ and CAD- groups, intramyocardial strain was similar PRE (19+/-10 versus 20+/-10) but different LATE, with dichotomization specifically related to the CAD state. In the CAD- patients, strain increased to 23+/-10% (+20%), whereas in CAD+ patients it fell to 16+/-11% (-26%), representing a nearly 50% decline after AVR (P<0.05). This was particularly evident at the apex, where CAD- strain LATE improved 17%, whereas for CAD+ it decreased 2.5-fold. Transmural strain and myocardial torsion followed a similar pattern, critically dependent on CAD. AVR impacted LV geometry and mitral apparatus, resulting in decreased mitral regurgitation, negating the double valve consideration. CONCLUSIONS In AS patients after AVR, reverse remodeling of the supranormal systolic function parallels improvement in cardiovascular MRI-derived regression of LV hypertrophy and LV intramyocardial strain. However, discordant effects are evident after AVR, driven by CAD status, suggesting that the typical AVR benefits are experienced disproportionately by those without CAD and not by those obliged to undergo concomitant coronary artery bypass grafting/AVR.

Validation of a bone analog model for studies of sternal closure.

BACKGROUND The incidence of serious sternal wound complications may be reduced with improvements in closure methods. Biomechanical testing of median sternotomy closures in cadavers has proven useful but is limited by availability, high cost, and wide variations in the material properties of the sterna. This study tests whether artificial sterna can be used to replace whole cadavers in sternal closure testing. METHODS Two common wire closure techniques were tested using both whole cadavers and artificial sternal models formed from bone analogue material. Sternal models were molded from polyurethane foam (20 lbs/ft3) to simulate the mechanical properties observed in human cadaveric sterna. The force vector previously identified as the most detrimental to sternal cohesion (lateral traction) was used to stress the closures. Separation of the incision site was measured at the manubrium, midsternum, and xiphoid and data were compared between cadaver and bench test groups. RESULTS Sternal separations recorded in cadavers were found to be similar to bench test results for both closure types. Data variability within test groups was found to be consistently lower using artificial sterna, where peak standard deviations for sternal motion averaged less than half that measured in cadavers. CONCLUSIONS Results suggest that anatomic sternal models formed from solid polyurethane foam can be used to approximate the biomechanical properties of cadaveric sterna and that reliable information regarding sternal closure stability can be secured through this means. Moreover, bench test data were shown to be less variable than cadaveric results, thus enhancing the power to detect small differences in sternal fixation stability.

Improvement of sternal closure stability with reinforced steel wires

BACKGROUND Sternal dehiscence occurs when steel wires pull through sternal bone. This study tests the hypothesis that closure stability can be improved by jacketing sternal wires with stainless steel coils, which distribute the force exerted on the bone over a larger area. METHODS Midline sternotomies were performed in 6 human cadavers (4 male). Two sternal closure techniques were tested: (1) approximation with six interrupted wires, and (2) the same closure technique reinforced with 3.0-mm-diameter stainless steel coils that jacket wires at the lateral and posterior aspects of the sternum. Intrathoracic pressure was increased with an inflatable rubber bladder placed beneath the anterior chest wall, and sternal separation was measured by means of sonomicrometry crystals. In each trial, intrathoracic pressure was increased until 2.0 mm of motion was detected. Differences in displacement pressures between groups were examined at 0.25-mm intervals using the paired Students t test. RESULTS The use of coil-reinforced closures produced significant improvement in sternal stability at all eight displacement levels examined (p < 0.03). Mean pressure required to cause displacement increased 140% (15.5 to 37.3 mm Hg) at 0.25 mm of separation, 103% (34.3 to 69.8 mm Hg) at 1.0 mm of separation, and 122% (46.8 to 103.8 mm Hg) at 2.0 mm of separation. CONCLUSIONS Reinforcement of sternal wires with stainless steel coils substantially improves stability of sternotomy closure in a human cadaver model.

Reduced incidence of atrial fibrillation with minimally invasive direct coronary artery bypass

BACKGROUND Atrial fibrillation (AF) is a frequent complication after coronary artery bypass graft (CABG) surgery. The purpose of this study was to determine the incidence of postoperative AF after minimally invasive direct coronary artery bypass (MIDCAB) in comparison with CABG. METHODS Between November 1995 and May 1997, 96 MIDCAB procedures were performed. During the same period, 42 patients underwent traditional single CABG using the left internal mammary artery graft (S-CABG). The incidence of in-hospital AF, defined as a sustained episode requiring treatment, was compared between the two groups. RESULTS There was no difference in age, ejection fraction, or preoperative risk score between the groups. The use of beta-blockers before or after surgery was not different. The incidence of postoperative AF in the first 6 weeks after surgery was 4% (4 of 96) for MIDCAB and 28% (12 of 42) for S-CABG (p = 0.003). Patients with postoperati

Left pleural effusion after coronary artery bypass decreases with a supplemental pleural drain

BACKGROUND This prospective study was undertaken to determine the incidence of symptomatic left pleural effusion after coronary artery bypass grafting, and to determine if routine drainage of the pleural cavity with a supplemental flexible drain reduces this incidence. METHODS The clinical course of study patients was prospectively recorded during the initial hospitalization and at 6-weeks after surgery. All patients had a mediastinal and a left pleural tube, which were removed on the 1st postoperative day. The supplemental drain system was implanted in a subset of patients and remained in place for 3 to 5 days. A symptomatic effusion was defined as one that required thoracentesis, tube thoracostomy, or readmission for treatment. RESULTS A total of 460 patients were studied, of whom 115 had a supplemental drain. The two groups (supplemental drain versus control) were equivalent with respect to age, gender distribution, and comorbid diseases. The incidence of symptomatic left pleural effusion for the entire group was 9.8% (45 of 460). Symptomatic left pleural effusion occurred in 11.9% (41 of 345) patients when only chest tubes were used, and in 3.5% (4 of 115) when a supplemental drain was placed. This difference was significant (F ratio 7.583, p < 0.005). There were no complications from the supplemental drain. CONCLUSIONS The incidence of symptomatic left pleural effusion can be greatly reduced with the use of a supplemental pleural drain that remains in place for several days after surgery.

LV reverse remodeling imparted by aortic valve replacement for severe aortic stenosis; is it durable? A cardiovascular MRI study sponsored by the American Heart Association

BackgroundIn patients with severe aortic stenosis (AS), long-term data tracking surgically induced effects of afterload reduction on reverse LV remodeling are not available. Echocardiographic data is available short term, but in limited fashion beyond one year. Cardiovascular MRI (CMR) offers the ability to serially track changes in LV metrics with small numbers due to its inherent high spatial resolution and low variability.HypothesisWe hypothesize that changes in LV structure and function following aortic valve replacement (AVR) are detectable by CMR and once triggered by AVR, continue for an extended period.MethodsTweny-four patients of which ten (67 ± 12 years, 6 female) with severe, but compensated AS underwent CMR pre-AVR, 6 months, 1 year and up to 4 years post-AVR. 3D LV mass index, volumetrics, LV geometry, and EF were measured.ResultsAll patients survived AVR and underwent CMR 4 serial CMRs. LVMI markedly decreased by 6 months (157 ± 42 to 134 ± 32 g/m2, p < 0.005) and continued trending downwards through 4 years (127 ± 32 g/m2). Similarly, EF increased pre to post-AVR (55 ± 22 to 65 ± 11%,(p < 0.05)) and continued trending upwards, remaining stable through years 1-4 (66 ± 11 vs. 65 ± 9%). LVEDVI, initially high pre-AVR, decreased post-AVR (83 ± 30 to 68 ± 11 ml/m2, p < 0.05) trending even lower by year 4 (66 ± 10 ml/m2). LV stroke volume increased rapidly from pre to post-AVR (40 ± 11 to 44 ± 7 ml, p < 0.05) continuing to increase non-significantly through 4 years (49 ± 14 ml) with these LV metrics paralleling improvements in NYHA. However, LVmass/volume, a 3D measure of LV geometry, remained unchanged over 4 years.ConclusionAfter initial beneficial effects imparted by AVR in severe AS patients, there are, as expected, marked improvements in LV reverse remodeling. Via CMR, surgically induced benefits to LV structure and function are durable and, unexpectedly express continued, albeit markedly incomplete improvement through 4 years post-AVR concordant with sustained improved clinical status. This supports down-regulation of both mRNA and MMP activity acutely with robust suppression long term.

Preoperative statin is associated with decreased operative mortality in high risk coronary artery bypass patients

BackgroundStatins are widely prescribed to patients with atherosclerosis. A retrospective database analysis was used to examine the role of preoperative statin use in hospital mortality, for patients undergoing isolated coronary artery bypass grafting (CABG.)MethodsThe study population comprised 2377 patients who had isolated CABG at Allegheny General Hospital between 2000 and 2004. Mean age of the patients was 65 ± 11 years (range 27 to 92 years). 1594 (67%) were male, 5% had previous open heart procedures, and 4% had emergency surgery. 1004 patients (42%) were being treated with a statin at the time of admission. Univariate, bivariate (Chi2, Fishers Exact and Students t-tests) and multivariate (stepwise linear regression) analyses were used to evaluate the association of statin use with mortality following CABG.ResultsAnnual prevalence of preoperative statin use was similar over the study period and averaged 40%. Preoperative clinical risk assessment demonstrated a 2% risk of mortality in both the statin and non-statin groups. Operative mortality was 2.4% for all patients, 1.7% for statin users and 2.8% for non-statin users (p < 0.07). Using multivariate analysis, lack of statin use was found to be an independent predictor of mortality in high-risk patients (n = 245, 12.9% vs. 5.6%, p < 0.05).ConclusionsBetween 2000 and 2004 less than 50% of patients at this institution were receiving statins before admission for isolated CABG. A retrospective analysis of this cohort provides evidence that preoperative statin use is associated with lower operative mortality in high-risk patients.