The Relationship Between the American Society of Anesthesiologists Physical Rating and Outcome Following Total Hip and Knee Arthroplasty: An Analysis of the New Zealand Joint Registry

Hooper, Gary J. FRACS; Rothwell, Alastair G. FRACS; Hooper, Nikki M. MBChB; Frampton, Chris PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.J.01681
Scientific Articles
Abstract

Background: The purpose of this study was to review the results of the first four years of use of the American Society of Anesthesiologists (ASA) physical status rating system in the New Zealand Joint Registry. Our hypothesis was that patients with a higher ASA score would have an increased mortality rate, an increased early revision arthroplasty rate, and poorer clinical outcomes at six months after total hip or knee arthroplasty.

Methods: We prospectively evaluated the preoperative ASA classes for all patients in the registry who underwent primary total hip or knee arthroplasty from 2005 to 2008 with regard to the six-month mortality rate and the Oxford Hip and Knee Scores at six months. Survival curves were constructed with use of revision joint replacement as the end point.

Results: Twenty-two thousand six hundred patients who underwent total hip arthroplasties and 18,434 patients who underwent total knee arthroplasties were recorded in the New Zealand Joint Registry. The six-month mortality rate was 0.77% following hip arthroplasty and 0.40% following knee arthroplasty. Significant differences were observed in the mortality rate between all ASA classes following hip arthroplasty (p < 0.001). Similarly, significant differences were observed in the mortality rate between ASA classes after knee arthroplasty, except between ASA classes 1 and 2 and between ASA classes 3 and 4. The mortality rate was significantly higher (p < 0.001) following hip arthroplasty compared with knee arthroplasty. A significant difference (p < 0.001) in Oxford scores was observed when ASA class 1 and ASA class 2 were compared with ASA class 3 and ASA class 4, independent of age and sex, following both hip or knee arthroplasty. A significant difference was observed in the rate of early revision (revision less than two years after the index procedure) following total hip arthroplasty when ASA class 1 (hazard ratio, 1.39 [95% confidence interval (CI), 1.04 to 1.95]; p = 0.015) and ASA class 2 (hazard ratio, 1.24 [95% CI, 1.02 to 1.55]; p = 0.030) were compared with ASA class 3, which was independent of age and sex. No significant difference was observed in the rate of early revision after total knee arthroplasty.

Conclusions: The ASA physical status score can be used as a predictor of postoperative mortality and functional status following both hip and knee arthroplasty and may predict early failure of total hip arthroplasty necessitating revision.

Level of Evidence: Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.

Author Information

1Department of Orthopaedic Surgery and Musculoskeletal Medicine, Christchurch School of Medicine and Health Sciences, University of Otago, Private Bag, Christchurch, 8140 New Zealand. E-mail address for G.J. Hooper: gary.hooper@otago.ac.nz

2Christchurch Public Hospital, Private Bag, Christchurch, 8140 New Zealand

Article Outline

The American Society of Anesthesiologists (ASA) physical status classification system was developed in 19411, and was further modified in 19632, to provide a concise summary of a patient’s preoperative medical status and to help predict which patients would have an increased mortality or serious morbidity following a major surgical procedure3,4. Initially designed for general surgical procedures, the ASA physical status classification system has also been used successfully to assess the medical outcomes following major orthopaedic procedures, particularly total hip replacement and total knee replacement5.

The New Zealand Joint Registry6 has recorded all total joint arthroplasties performed in New Zealand since 1999, and annual compliance audits via the New Zealand Health Information Service have consistently shown a >96% capture rate for all arthroplasty types, including revision arthroplasties, across New Zealand hospitals. In 2005, the New Zealand Joint Registry introduced the ASA physical status classification system to the data collected for all patients receiving total hip replacements and total knee replacements in order to provide preoperative information on a patient’s general health status. Previous studies have demonstrated that preoperative health status is an important predictor of outcomes in joint replacement7-10, with poorer functional outcomes and increased complication rates11,12, including mortality rates13,14, in those patients with poorer general medical health. The ASA rating system has been recommended as a reliable comorbidity instrument14 as it assesses the severity of disease with respect to operative risk and the ASA class should not change significantly following total joint arthroplasty. The New Zealand Joint Registry also collects patient-reported outcome measure data at six months postoperatively in the form of the Oxford Hip and Knee Scores, which has been shown to effectively assess a patient’s early functional status as well as to predict the likelihood of early revision15.

We hypothesized that the ASA rating system would identify high-risk patients undergoing primary total hip replacement and total knee replacement and that this status would be reflected in the early mortality rate following surgery. We also hypothesized that the ASA rating system would identify those patients who would be more prone to problems with the joint replacement and would be more likely either to need an early revision procedure or to have a poorer functional outcome.

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Materials and Methods

In this study, we investigated the above hypotheses by prospectively reviewing the data generated by the New Zealand Joint Registry from the commencement of the use of the ASA physical status classification system in 2005 until the end of 2008 for patients who had electively undergone a primary total hip or knee arthroplasty. Patients who had undergone a total hip replacement for a femoral neck fracture were excluded. National ethical permission was obtained and all patients signed a written consent to be included in the registry database.

The ASA physical status classification system consists of five classes to assess preoperative medical status (Table I). However, ASA class 5 has not been included in the New Zealand Joint Registry as it applies to patients having life-saving surgery who have a high expected mortality rate.

The ASA rating was assigned by the arthroplasty anesthetist and recorded on the New Zealand Joint Registry data sheet, which was returned to the New Zealand Joint Registry.

Functional outcomes were obtained at six months postoperatively from the New Zealand Joint Registry, which distributed the Oxford 12 questionnaire to a randomly selected sample of 28% of patients who had undergone primary hip or knee total joint arthroplasty, which, with a 75% return rate, ensured a score for approximately 20% of primary hip and knee replacements. The Oxford Hip Score and the Oxford Knee Score are patient-generated scores ranging from 0 points (the worst score) to 48 points (the best score). The Oxford Hip Score and the Oxford Knee Score were then compared with each of the ASA classes.

We arbitrarily chose six months as an appropriate cutoff for mortality that was likely to be secondary to complications of the joint replacement and then compared these results with each of the ASA classes. The New Zealand Joint Registry regularly updates patient mortality data via access to the New Zealand National Register of Births, Deaths and Marriages.

Finally, we compared the revision rates following primary total hip replacement and total knee replacement against each ASA category. Revision was defined as the addition, removal, or replacement of a prosthetic component for any reason.

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Statistical Analyses

The mean ages and Oxford scores were compared among ASA classes with use of analysis of variance (ANOVA). Areas in which this analysis indicated significant differences were further explored with use of the Fisher protected least significant difference tests. The mortality rate within six months after surgery was compared between ASA classes with use of chi-square or Fisher exact tests. The time to first revision was compared between ASA classes with use of a log-rank test and was portrayed with use of Kaplan-Meier curves, with patients censored at death if they had not had a revision. Analyses of time to first revision allowing for age and sex were undertaken with use of the Cox proportional hazards models. As the rerevision rates are very low and rerevision times are likely dependent on concurrent patient and operation factors associated with the revision, we have not specifically explored rerevision rates. A two-tailed p value of <0.05 was considered to indicate significance.

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Source of Funding

No funds were received by the authors of this study from any external source. The New Zealand Joint Registry is funded by the New Zealand Orthopaedic Association, individual orthopaedic surgeons, the Accident Compensation Corporation, the Ministry of Health, and the Southern Cross Hospitals Trust.

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Results

Primary Total Hip Replacement

Twenty-two thousand six hundred patients who underwent total hip replacement from 2005 to December 2008 were recorded in the New Zealand Joint Registry; the majority (17,169 [76%]) of these patients were in ASA class 1 or ASA class 2 (Table II). An increasing mean age was observed with increasing ASA class (p < 0.001) (Table II). The mortality rate within six months was 0.77% (174 patients). A significant difference (p < 0.001) was observed between all categories, ranging from ASA class 1 (0.12%) to ASA class 4 (10.06%) (Table II).

Survival curves comparing the ASA classes with regard to the rate of revision of the prosthesis for any reason showed no significant difference among the classes, except between ASA class 1 and ASA class 3 (p = 0.040), with a range from 1.5% to 2.1% at two years (Fig. 1). However, correction for age and sex demonstrated a significant difference in revision rates between ASA class 1 and ASA class 3 (hazard ratio, 1.39 [95% confidence interval (CI), 1.04 to 1.95]; p = 0.015) and between ASA class 2 and ASA class 3 (hazard ratio, 1.24 [95% CI, 1.02 to 1.55]; p = 0.030).

The mean Oxford Hip Scores at six months showed a steady decline from ASA class 1 to ASA class 4, with significant differences when ASA class 1 was compared with the other classes and when ASA class 2 was compared with ASA class 3 and ASA class 4. No significant difference was observed when ASA class 3 was compared with ASA class 4. These results remained significant when corrected for age or sex (Table III).

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Primary Total Knee Replacement

Eighteen thousand four hundred and thirty-four patients underwent total knee replacement in the same time period, of which 74% were in ASA class 1 or ASA class 2 (Table IV). The mean age at surgery also increased with a higher ASA status with a ten-year spread between ASA class 1 and ASA class 4 (Table IV). The mortality rate within six months was 0.40% (seventy-four patients). A significant difference was observed among all ASA classes except when ASA class 1 was compared with ASA class 2 and when ASA class 3 was compared with ASA class 4 (Table IV). The mortality rate was significantly higher (p < 0.001) for patients who had undergone total hip replacement and were in ASA class 3 and ASA class 4 compared with that for patients who had undergone total knee arthroplasty and were in the same classes.

Survival curves showed an even revision rate for patients in ASA classes 1, 2, and 3, ranging from 1.2% to 1.6% at two years. The low numbers of patients in ASA class 4 were too small for statistical evaluation (Fig. 2). Correction for age and sex did not change these results.

The mean Oxford Knee Scores at six months also showed a steady decline from ASA class 1 to ASA class 4, with significant differences observed when both ASA class 1 (p < 0.001) and ASA class 2 (p < 0.001) were compared with ASA class 3. Significance remained with correction for age and sex. As for patients with total hip replacement, there was no difference when ASA class 3 and ASA class 4 were compared (Table V). The range of scores for the total knee replacement group (34.6 to 38.5 points) was smaller than that for the total hip replacement group (35.2 to 42.1 points). Among patients in ASA class 1, the mean score for the total hip replacement group was 4 points better than that for the total knee replacement group; however, among patients in ASA class 4, the scores were similar in both groups.

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Discussion

In this study, we have confirmed the importance of the ASA physical rating score as a predictor of the early mortality rate within six months after both elective total hip and total knee replacement; this score also offers clinicians important information when discussing outcomes with patients preoperatively. Patients in ASA class 4 had a greater increase in mortality rate within six months after total hip replacement as compared with patients in ASA class 1. Similarly, patients in ASA class 4 had a profound increase in mortality rate after total knee replacement. These results were independent of age or sex, but we cannot rule out the possibility that other predictors, not measured, were partially responsible for this effect. However, there was no other obvious explanation for the large difference in the mortality rate following total knee replacement compared with total hip replacement. Other studies have shown that the ASA physical status is predictive of mortality in patients in ASA class 3 or ASA class 4 who have undergone either revision arthroplasty16 or bilateral arthroplasty procedures17. The age of the patient also has been shown to be relevant, with octogenarians18 having a significant mortality risk if they were in ASA class 3.

In-hospital mortality rates have been associated with ASA class19, but, as the in-hospital mortality rate for elective primary total joint arthroplasty is very low, we used the six-month mortality rate as a better reflection of the relative postoperative risk for these patients. It is likely that a percentage of the deaths within the first six months after surgery was unrelated to the surgery, but we have no evidence to support this supposition.

The ASA physical rating score can be predictive of the early functional status as measured by the Oxford scores, with ASA class 1 performing significantly better than the other classes following both total hip replacement and total knee replacement. This result was independent of age or sex. We believe that our results validate the use of the ASA physical rating score as a preoperative assessment, which can then be closely correlated with the postoperative Oxford score and used in joint registries as a measure of preoperative function for international data comparison.

There was a larger spread in ASA classes from ASA class 1 to ASA class 4 following total hip replacement compared with total knee replacement, but there was a similar mean score for patients in ASA class 4. According to the Kalairajah20 classification of Oxford scores, all patients in ASA class 1 who had undergone total hip replacement had an excellent outcome (>41 points) at six months, whereas no patient who had undergone total knee replacement had an excellent outcome at six months. These results are consistent with other studies15 that have shown a poorer early functional status, as measured by the Oxford Knee and Hip Scores, for patients who had undergone total knee replacement compared with those following total hip replacement. Both groups had achieved at least a good result (>34 points) across all ASA classes at six months. The overall functional difference, as measured by the Oxford scores, was small for some patients, and although the large number of patients registered made the result significant, the question arose as to whether this change in Oxford score contributed to a clinically significant improvement. A change of 3 points on the Oxford Knee Score has been related to a clinically significant improvement21,22, confirming that the difference is likely to be clinically relevant between ASA class 1 and ASA classes 3 and 4 following total hip replacement and between ASA class 1 and ASA class 4 following total knee replacement.

The overall data showed that the ASA classes were not a predictor of the likelihood of early revision after either total hip replacement or total knee replacement, but, when the data were corrected for age and sex, there was a difference, with the outcome for patients in ASA class 1 and ASA class 2 being significantly better than that for patients in ASA class 3 following total hip replacement. This difference was higher within the first two years. However, by three years the results were similar across all classes. Other studies have shown an increase in perioperative complications23-26 with increasing ASA class for patients who had undergone total hip replacement, including the risks of dislocation and infection. These complications have been shown to be the commonest reason for early revision following total hip replacement27 in New Zealand. In the present study, there was no association between ASA class and revision for either dislocation or infection.

One weakness of the present study was the lack of a preoperative score. It is possible that each group started with a different functional status, which could have resulted in a different overall functional improvement among patients undergoing hip or knee arthroplasty.

Another weakness was that a major criticism of the ASA physical rating class has been the subjective nature of the assessment with poor interobserver correlation28,29. Despite this criticism, the ASA physical status classification system has remained the most commonly used anesthetic preoperative assessment and, because of its simplicity and ease of use, continues to be widely reported in clinical studies and joint registries. We accept that there may be poor interobserver reliability when determining between ASA class 1 and ASA class 2, but the difference between ASA class 1 and ASA class 3 is so profound (a normal healthy patient compared with a patient with severe systemic disease) that we believe that the significance of our results, when comparing ASA class 1 with ASA class 3 and ASA class 4, was unlikely to be affected by this potential error. The use of the ASA physical rating has been supported by recent reports that continue to recommend its use as a reliable comorbidity tool14,29.

Investigation performed at the University of Otago, Christchurch, New Zealand

A commentary by Edward Y. Cheng, MD, is linked to the online version of this article at jbjs.org.

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Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. None of the authors, or their institution(s), have had any financial relationship, in the thirty-six months prior to submission of this work, with any entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. One or more of the authors has had another relationship, or has engaged in another activity, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.

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