Sternal instability following coronary artery bypass grafting

Physiotherapy Theory and Practice (2000) 16, 27–33 © 2000 Taylor & Francis Inc

Sternal instability following coronary artery bypass grafting Doa El-Ansary, Roger Adams, Lorelle Toms, and Mark Elkins

Several cases of sternal instability have been noted in patients following coronary artery bypass graft surgery attending our cardiac rehabilitation programme. The purpose of this prospective study was to identify factors associated with sternal instability following sternotomy involving saphenous vein grafts (SVG) and unilateral or bilateral internal mammary artery (IMA) grafts. A rating scale for quantifying sternal instability was developed and used by the physiotherapists to assess all patients. Inter-therapist and intra-therapist reliabilities for the scale were calculated and these were 0.97 and 0.98 (ICC) respectively. Twenty-four patients who underwent coronary artery bypass grafting with a sternotomy incision presented with the complication of sternal instability 6–8 weeks following surgery. They represented 16.3% of the 147 patients presenting for cardiac rehabilitation who had undergone surgery interstate over an 18-month period. Sternal symptoms reported were pain, crepitus, and/or clicking at rest or on trunk and upper limb motion. Risk factors to wound healing such as obesity, diabetes, bilateral IMA grafting, osteoporosis, repeat operations, and prolonged post-operative mechanical ventilation were noted. A significantly higher proportion of patients with bilateral IMA grafting (31.5%) as opposed to unilateral IMA grafting (14.3%) had sternal instability (P < .05). Quantifying the degree of sternal instability may play a role in identifying management options, patient progression and the point of intervention.

INTRODUCTION D. El-Ansary, Senior Cardiothoracic Physiotherapist, The Canberra Hospital, PO Box 775, Civic Square, ACT 2606, Australia R. Adams, Senior Lecturer, School of Physiotherapy, Sydney University (Cumberland Campus), Lidcombe, Australia L. Toms, Clinical Nurse Consultant (Cardiac Rehabilitation), The Canberra Hospital, ACT, Australia M. Elkins, Senior Respiratory Physiotherapist, Prince of Wales Hospital, Sydney, Australia Accepted for publication August 1999

The internal mammary artery (IMA) is currently the vessel most commonly used for coronary artery bypass grafting (CABG) (Kouchoukos et al, 1990). The left IMA is used preferentially when grafting the anterior descending coronary artery (Loop et al, 1990). The right IMA is increasingly used to graft the right coronary artery (Loop et al, 1990). In most studies, 10 year functional patency was almost 90% compared to 25–50% for vein grafts (Sabiston & Spencer, 1990). Although the use of

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PHYSIOTHERAPY THEORY AND PRACTICE

the IMA does not appear to be associated with increased risk of poor outcomes in the long term, the potential for a higher incidence of sternal wound complications as a result of devascularisation is of concern (Kouchoukos et al, 1990, Loop et al, 1990). In an effort to understand the perceived correlation between IMA harvesting and wound complications, a review of blood supply to the sternum is necessary. The dominant blood supply to the sternum was found to be periosteal and derived almost exclusively from the branches of the IMA (Knudsen et al, 1993; Moore, 1992). Three arteries have been identified to have the potential to carry blood to the sternum after harvest of the IMA. 1. Sternal/perforating branches that supply the sternum and pectoralis major. 2. Sternal/intercostal branches that supply the sternum and an adjoining intercostal space. 3. Posterior intercostal arteries that continue past the IMA to reach the sternum (De Jesus & Acland, 1995). De Jesus and Acland (1995) noted that all three arteries were found more frequently in the upper half than in the lower half of the sternum. For these vessels to function as collaterals after IMA grafting, their short common trunks of origin must remain intact (De Jesus & Acland, 1995). Several studies have measured sternal blood flow before and after median sternotomy and IMA harvest, in order to quantify the haemodynamic effects of this procedure. Seyfer, Shriver, Miller, and Graeber (1988) recorded a 90% decrease in the mean flow rate within the sternum on the harvested side. Parish et al (1992) noted tissue blood flows decreased to 46.9%, 22.1%, and 41.2% of baseline values for the manubrium, sternum, and ribs, respectively following bilateral IMA harvesting. Risk factors for poor wound healing such as obesity, diabetes, bilateral IMA grafting, prolonged mechanical ventilation (> 48 hours), and osteoporosis have been linked with a significantly higher incidence of sternal wound complications (De Jesus & Acland, 1995; Kouchoukos et al, 1990; Loop et al, 1995). Whilst several drawbacks of IMA grafting are addressed in the literature, such as a greater post-operative blood loss due to more

extensive chest wall dissection and sternal wound infection, little attention has been directed to the musculoskeletal complications that may occur following surgery. Selvaratnam, Matyas, and Glasgow (1994), in a study addressing brachial plexus involvement in upper limb pain, reported the existence of unilateral shoulder and upper limb pain in patients after coronary artery surgery. Stiller, McInnes, Huff, and Hall (1997) found approximately 30% of patients after cardiac surgery developed musculoskeletal complications that interfered with their level of comfort or function 8–10 weeks postoperatively. In addition to this, El-Ansary (1995) investigated the incidence of musculoskeletal problems in 97 patients attending the cardiac rehabilitation programme at The Canberra Hospital 3–6 weeks after CABG interstate (New South Wales or Victoria) and found that 45% of patients with saphenous vein grafts (SVG) and 71% of patients with IMA grafts had musculoskeletal complications post-operatively. El-Ansary (1995) attributed the higher incidence to the inclusion of problems of the anterior chest wall in the study and it was argued that median sternotomy primarily affects this region The existence of musculoskeletal problems following CABG may be due to the mechanical demands placed on the patient during the surgical procedure (Selvaratnam et al, 1994; El-Ansary, 1995). These demands include extension of the cervical spine during intubation, rotation of the cervical spine during central line insertion, retraction and elevation of the ribs during IMA harvesting, and dissection of the IMA (El-Ansary, 1995). However, there is also an obvious issue with sternal blood supply after surgical harvesting of the IMA. No studies to date have examined the effects of devascularisation on bony union and the factors associated with sternal instability following CABG. These issues need to be addressed prior to establishing the role of physiotherapy in the management of sternal instability.

MATERIALS AND METHODS All patients over an 18-month period having CABG performed interstate (New South Wales or


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Victoria, via a median sternotomy) attending the cardiac rehabilitation programme who gave their written, informed consent were eligible for inclusion (n = 147). The sample included 24 patients who developed sternal wound complications such as infection (6), bony instability (24), and osteomyelitis (6) in the 6–8 weeks following surgery. According to the nature of the graft, patients with sternal wound complications were allocated to one of the following groups:

l Bilateral IMA graft alone or combined with vein

l Saphenous vein grafts alone (SVG) (n = 40). l One IMA graft alone, or combined with vein

The presence of other risk factors to wound healing such as prolonged mechanical ventilation and osteoporosis was confirmed for each patient by

grafts (unilateral IMAG) (n = 91).

grafts (bilateral IMAG) (n = 16). Data were collected for those patients without sternal wound complications and for the sample group (Tables 1 and 2). This included age, sex, graft type as well as risk factors to wound healing such as obesity and diabetes.

Measurements

Table 1 Descriptive data and risk factors to wound healing for the 123 patients without sternal wound complications according to the three operative groups

Mean age (years) Number of males/females Obesity Diabetes Prolonged mechanical ventilation Osteoporosis Chronic airflow limitation Hemiplegia (pre-existing) Broken wires Sternal infection (deep) Sternal infection with complete sternectomy Removal of wires secondary to infection ±plastic surgery

SVG (n = 34/40)

Unilateral IMAG (n = 78/91)

Bilateral IMAG (n = 11/16)

62.7 28/6 6 (18%) 4 (12%) 2 (6%) 3 (9%) 3 (9%) 2 (6%) 0 0 0 0

61.5 64/14 12 (15%) 9 (11.5%) 0 3 (4%) 4 (5%) 1 (4%) 0 0 0 0

58 8/3 3 (27%) 0 0 0 0 0 0 0 0 0

Table 2 Descriptive data and risk factors to wound healing for the 24 patients with sternal wound complications according to the three operative groups

Mean age (years) Number of males/females Obesity Diabetes Prolonged mechanical ventilation Osteoporosis Chronic airflow limitation Hemiplegia (pre-existing) Broken wires Sternal infection (deep) Sternal infection with complete sternectomy Removal of wires secondary to infection ±plastic surgery

SVG (n = 6/40)

Unilateral IMAG (n = 13/91)

Bilateral IMAG (n = 5/16)

60.3 3/3 1 (17%) 3 (50%) 0 0 1 (4%) 0 0 1 (4%) 0 0

65 7/6 7 (54%) 3 (23%) 0 2 (15%) 2 (8%) 1 (4%) 1 (4%) 3 (12%) 0 3 (12%)

59.4 5/0 4 (80%) 1 (20%) 0 0 0 0 0 2 (8%) 1 (4%) 2 (8%)

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their general practitioner, physician, or from the patient’s medical record. Other factors that are suspected to affect sternal union and wound healing (e.g. broken wires, chronic airflow limitation) were also recorded. Sternal infection was considered present in patients in whom sternal instability developed in association with positive wound culture and in whom a second surgical procedure (incision and drainage, removal of the sternal wires with debridemen t and secondary closure) was required (Kouchoukos et al, 1990). Superficial wound infection responding to conservative treatment, sterile wound dehiscence, and wounds demonstrating delayed healing were not considered in this study.

Sternal instability A scale for quantifying sternal instability was developed and utilised by the physiotherapists to assess all patients at 6 and 12 weeks following surgery, whilst in the cardiac rehabilitation programme. The testing procedure consisted of palpation of the sternum by placing three fingers along the median sternal ridge and noting motion and separation during shoulder flexion and abduction, trunk rotation and lateral flexion, deep inspiration and coughing. Results were recorded as a single grade. The grading system from 0 to 4 is shown in Table 3. Sternal regions were assessed (e.g upper, middle, and lower segments) and graded separately if

Table 3 Sternal instability grades of motion 0 = No detectable movement (Normal) 1 = Slight increase in movement upon special testing (i.e. upper limb, trunk) with no significant bony separation 2 = Moderate increase in movement upon special testing and with activities of daily living (i.e. walking). Minimal bony separation 1 finger space. 4 = Complete instability >1. 5 finger spaces * 1 finger space = 1.0–1.25 (cm wide)


used to examine the significance of diabetes and obesity as risk factors to wound healing for each of the three groups. To test the hypothesis that harvesting both mammary arteries would lead to most sternal problems, directional z-tests were carried out to determine differences between the proportion of patients with sternal instability after bilateral IMA surgery, and unilateral IMA or saphenous vein graft surgery. Sternal instability grades for the three groups at 6 and 12 weeks were analysed using the Wilcoxon test. Probability values of less than .05 were deemed significant.

RESULTS Twenty-four subjects presented to the Canberra Hospital with sternal wound complications and sternal instability. This represented 16.3% of our total of 147 patients. The sample included six patients with SVG grafts, 13 with a unilateral (R/L) IMA graft, and five patients with bilateral IMA grafts. In addition, three patients received repeat grafts, with one patient being represented in each group. All patients reported pain, crepitus, and an “unstable feeling” which interfered with their level of function or comfort 6–8 weeks post-operatively. Table 1 contains descriptive data for those patients who did not present with any sternal wound complications (n = 123). Table 2 gives descriptive data for the 24 subjects with sternal wound complications and sternal instability. Inspection of data in both tables indicates that the control and the sample groups were fairly homogeneous with respect to mean age and presence of obesity, diabetes, and other perceived risk factors for wound healing. A closer examination of the data for the sample group reveals that the three operative groups of

patients were also reasonably homogeneous. Each surgical group had more males than females, with the bilateral IMAG group having no female subjects. With respect to the two major factors identified in the literature, obesity and diabetes, there was no significant difference in the distribution across the three surgical groups as shown by the Chi-square test. Other perceived risk factors to wound healing in Tables 1 and 2 were also not significantly different among the three surgical groups. It remains possible that data with respect to osteoporosis and prolonged mechanical ventilation are conservative, as this information was obtained from medical records or from the treating general practitioner/physician. Post-operative factors that may have affected sternal union such as broken wires and sternal infection as indicated in Table 2 were not peculiar to any surgical group. Although the literature seems to suggest a higher incidence of sternal infection following a bilateral IMAG procedure this finding was not confirmed by this study. However, a larger sample size is needed to evaluate the proposed association between devascularisation of the sternum and sternal infection (Kouchoukos et al, 1990). Table 5 summarises the number of patients with the various grades of sternal instability. Of the six patients who presented with sternal infection, 50% had an obesity index of 30kg/m2 or greater, and four subjects (66%) had diabetes. Although subject numbers are small this seems to support findings by Kouchoukos et al (1990) who demonstrated an association between bilateral IMA grafting, diabetes, obesity, and a significantly higher incidence of sternal infection. The proportion of patients with sternal instability in each of the three surgical groups was also

Table 5 Incidence of sternal instability for all patients according to operative groups at 6 weeks following surgery Sternal instability grade 0 1 2 3 4

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SVG (n = 40)

Unilateral IMAG (n = 91)

Bilateral IMAG (n = 16)

0 2 (5%) 3 (7%) 1 (2%) 0

0 2 5 (5%) 5 (5%) 0

0 0 1 (6%) 2 (12%) 2 (12%)

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recorded. Patients who underwent bilateral IMAG demonstrated a significantly higher incidence of sternal instability (31.3%) compared to those with a unilateral IMA graft (4.3%) (z 1.67, P < .05), but not compared to those with SVG (15%) (z 1.38, P = .08). Although the smaller proportions were similar in both tests, the lower numbers in the combined SVG-bilateral IMA test would have reduced the power to detect significant differences. Mean sternal instability grades are depicted in Fig. 1. There was a higher incidence of sternal instability at 6 weeks than at 12 weeks for both the SVG and the unilateral IMA groups. The Wilcoxon test revealed a significant improvement in sternal instability between 6 and 12 weeks in the SVG (P = .01) and unilateral IMAG groups (P = .03). There was no significant change in the sternal instability score over the same time frame in the bilateral IMAG group (P = .08).

DISCUSSION Sternal wound complications have been previously addressed in the literature and managed surgically by cardiothoracic and plastic surgeons,

where significant infection or loose/broken wires are present. The present study is the first to have aimed to quantify sternal instability. It is suggested here that quantification of sternal instability is important as it may provide a tool for objectively determining the following: l Guidelines for exercise prescription in cardiac l l l l

rehabilitation. Patient progress/clinical outcome. Point of intervention. Treatment options (surgical/conservative). Guidelines for exercise and activity at home.

It is our clinical impression that management of the post-operative cardiac surgical patient should encompass pre- and post-operative education regarding preventative strategies to minimise undue stress on the healing sternum (e.g. encouraging the use of both arms to perform activities, the wearing of a support bra in females). It is also felt that management of the patient with sternal instability should include regular monitoring of sternal union as well as comprehensive guidelines for activity modification and exercise prescription in the cardiac rehabilitation programme.

Fig. 1 Mean sternal instability grades and standard deviations for the 24 patients with sternal wound complications according to the three operative groups (Note: A higher sternal instability grade represents a more unstable sternum).


Individual outpatient physiotherapy follow-up as well as orthopaedic stabilisation (e.g. taping, orthopaedic brace) to further reduce stress on the unstable sternum may be relevant in some patients. Future research should address the accuracy and reliability of the rating scale developed for quantifing sternal instability as well as identifying best practice and establishing outcome measures for management of the unstable sternum. In conclusion, this study demonstrated that 16.3% of patients undergoing CABG surgery via a median sternotomy developed sternal instability which interfered with their level of comfort or function in the post-operative period. A significantly higher proportion of patients with bilateral rather than unilateral IMAG had sternal instability. A higher sternal instability score in patients following bilateral IMAG further supported this finding. Since risk factors to wound healing such as diabetes and obesity were not significantly different in any groups, this delayed healing and variation as represented by no significant change in the sternal instability score (over 12 weeks) may be secondary to marked and bilateral devascularisation of the sternum. As use of the IMA, particularly bilaterally, is becoming more frequent and the patient population presents more risk factors associated with wound healing, sternal instability may become an increasing problem. Physiotherapists can play an integral role in the assessment and management of sternal instability following CABG surgery and sternotomy incision. However, in order to provide comprehensive patient care, intervention must take place within a “team setting” encompassing the cardiothoracic surgeon, cardiologist, cardiac rehabilitation nurse, occupational therapist, and general practitioner.

Acknowledgements The authors wish to acknowledge the support of Dr Howard Peak (Cardiologist and Executive Co-ordinator of the CRP), June Gunning (Director of Physiotherapy), the Private Practice Trust Fund, the Cardiac Rehabilitation Team, Dr Peter Bissaker (Director of Cardiac Surgery) and the Cardiologists at the Canberra Hospital. In addition, we wish to acknowledge the following people who assisted in reviewing earlier drafts of the paper: Dr Timothy McKenzie (Cardiothoracic Surgeon), Elizabeth Trickett (Physiotherapy Ward Supervisor), Nari Strange

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(Senior Physiotherapist), and Damien McKay (Physiotherapist) at the Canberra Hospital; Dr Ros Bohringer (Senior Lecturer, School of Biological Sciences) and Dr Elizabeth Ellis (Senior Lecturer, School of Physiotherapy) at the University of Sydney. Thank you to Amanda Thwaite (Physiotherapy Administration) and the patients who participated in this study. The material in this paper was presented at the 5th National Conference of the Cardiothoracic Special Group of the Australian Physiotherapy Association, Perth, 1997. This study was conducted prior to the commencement of cardiac surgery at the Canberra Hospital.

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