Results

ACCESS TO CABG SURGERY AND SES

Table 1 shows the hospital admission rates (per 10 000) for IHD and CABG surgery by gender and SES level (level I = highest SES; level IV = lowest SES). In both genders, hospitalisation for IHD increased steadily with decreased SES. CABG hospitalisation rates among men were relatively constant among SES levels (RR 1.07, 95% CI 0.84, 1.35 when comparing level IV to level I). For women with the lowest SES (level IV), an increase in the CABG hospitalisation rate was observed (RR 2.05, 95% CI 1.22, 3.45 with respect to SES level I). However, the overall hospitalisation rate was lower among women than among men.

The rate of CABG per 100 IHD hospitalisations was lower among men with the lowest SES (RR 0.63, 95% CI 0.44, 0.89 when comparing level IV to level I). Among women, no statistically significant differences in this indicator with respect to SES level were observed.

PATIENTS CHARACTERISTICS AND MORTALITY WITHIN 30 DAYS AFTER CABG

Most patients who had received a CABG surgery during the study were men (83.4%); the proportion of women was higher in SES level IV compared with the other levels. Mean age was highest for those in SES level I (64.9 years versus 62.3 years for level IV). As expected, the percentage of patients treated in private hospitals was highest in level I (9.4% compared with 0.6% for level IV) (table 2).

Patients with the lowest SES (level IV) showed a higher severity of illness upon hospital admission compared with patients with the highest SES (level I) (44.7% of comorbidities versus 37.9%). Overall, hypertension and diabetes were the most frequently reported comorbidities (18.3% and 13.5%, respectively); diabetes was more common in SES level IV compared with level I (16.3% versus 9.7%) (table 3).

The overall crude mortality within 30 days after CABG surgery was 6.1%, with excess among patients in SES level IV (8.1%) compared with those in level I (4.8%). After adjusting for gender, age, type of IHD, comorbidities, and other surgical procedures during the same admission, we found a statistically significant association between SES and 30 day mortality (OR= 2.89; 95% CI 1.44, 5.80 when comparing level IV with level I) (table 4).

Variability in mortality was observed across hospitals: 5.0% for teaching hospitals, 9.5% for non-teaching hospitals, 2.3% for private hospitals; taking teaching hospitals as reference, and considering all the potential confounders included in model 1, non-teaching hospitals showed an excess in mortality risk (OR= 2.10, 95% CI 1.32, 3.34). The association between SES level and mortality was confirmed, though slightly reduced (OR= 2.45; 95% CI 1.21, 4.96), when we included in the model, as potential confounder, the type of hospital, a variable possibly related to the quality of care, thus representing an intermediate factor in the association under study (model 2 in table4).

The possible effect modification of gender, age, and type of hospital on the relation between SES and 30 day mortality was tested by forcing interaction terms in the logistic model. However, no significant contribution was observed.

Discussion

In Italy, as in the United Kingdom, the NHS theoretically provides universal coverage to any citizen in any part of the country without economic barriers to medical care. We found that, even though people with the lowest SES showed an excess in the age adjusted IHD hospitalisation rates compared with those with the highest SES (an excess of 57% among men and of 94% among women), the rate of CABG procedures per 100 IHD hospitalisations among men was lower in the most socially disadvantaged group. These data suggest that this sector of the population was receiving fewer operations than needed. Moreover, in our study, the most socially disadvantaged socioeconomic group experienced a higher risk of 30 day mortality after CABG surgery, even after taking into account differences in the initial severity of illness.

The medical decision to refer a patient for CABG surgery is complex. The physician's choice of whether a patient should be managed conservatively or surgically mainly depends on the patient's clinical characteristics (symptoms, exercise tolerance, angiographic evidence of coronary artery disease, and myocardial functional status).17 Medium and high risk patients with stable coronary artery disease are eligible for CABG surgery.18 A SES related difference in clinical indications for CABG surgery is an obvious explanation for the observed differences in rates of utilisation of CABG procedures in our study. We did not obtain information on all IHD hospitalised patients' clinical characteristics to correctly estimate the need for surgery and the criteria used by clinicians. However, it is probable that IHD patients in the lower social class level had less favourable coronary profiles than people in the upper social class. In fact, disadvantaged patients tend to have more risk factors and more severe disease than those in the highest SES,7 18 19 and an association between SES and IHD mortality was observed in Rome.14 This could produce a possible underestimation of the association between access to CABG surgery and SES. Furthermore, if CABG surgery is contraindicated for lower SES patients because of their more severe clinical conditions, this may imply a further discrimination, given that diagnostic and therapeutic measures for poor patients with IHD tend to be delayed compared with well off people.5

Another possible explanation of our results is that patients with low SES could have received percutaneous transuminal coronary angioplasty (PTCA), an alternative revascularisation procedure for IHD patients. PTCA is generally used for treatment of proximal and single vessel disease lesions and only recently it has been proposed as an effective alternative to CABG surgery in selected populations of patients with multivessel coronary artery disease.20 Therefore, patients who are referred for PTCA have better medical baseline characteristics than those undergoing CABG.21 However, there is no evidence of the use of one revascularisation procedure for IHD patients compensating the lack of use of the other.22 Male age adjusted hospital admission rates for PTCA in Rome (1997; 580 hospitalisations) were not associated with SES level (level I 4.2 per 10 000; level IV 4.4 per 10 000).

We observed that women hospitalised for IHD undergo CABG surgery far less frequently than men. Such a sex bias in the delivery of CABG surgery has been previously reported, but the reasons of this evidence are still a matter of debate.23 24 Sex related differences in natural history of disease, behavioural and life style risk factors, and benefits from surgery have been advocated to explain this difference. We failed to show differences by SES among women. We cannot exclude that this is attributable to the limited sample size of our study among women (given α=0.05, we had a power of 80% to detect RR⩽0.59 or RR⩾1.7), but we can also presume that gender differences were so strong that inequalities in provision of CABG surgery could not be detected.

Sociodemographic factors may also interact with clinical characteristics to both directly and indirectly influence the physician's choice in diagnostic and therapeutic procedures. Hannanet al 25 recently found that African-American and Hispanic candidates were less likely to undergo CABG surgery than white patients despite the fact that CABG surgery was clinically judged to be appropriate and necessary. Moreover, given that those patients who smoke have poorer results after revascularisation procedures,26 many clinicians are reluctant to perform these procedures unless patients have stopped smoking.27Finally, a strong inverse relation between income and rates of use and waiting times for invasive cardiac procedures has been recently found in a cohort of acute myocardial infarction patients in Canada, after controlling for patients' clinical characteristics.5

The decision of whether or not a procedure will be performed also depends on the patient's preferences for care, yet the issue of patient's preferences versus physician's judgement has received attention only recently.28 29 In our study, no information was available to determine the importance of patient's awareness of their health status in explaining the observed differences among SES levels. It should also be pointed out that in Italy many surgeons belonging to the NHS also have private outpatient practices, though strict regulations to this regard have been issued. Thus, a patient who privately approaches a cardiologist employed by the NHS has a greater probability of receiving CABG surgery.

Mortality after CABG surgery is a function of the rate of occurrence of complications, and both mortality and complications have been shown to be closely related to the preoperative risk as predicted by different illness severity scores.30 Differences in the level of preoperative risk, including clinical symptoms, anatomic compromising of coronary vessels, adequacy of previous treatment, severity of comorbidities, immunological status, stress, depression, and social isolation could explain the finding that mortality after CABG surgery was highest among the people with the lowest SES. In fact, although people with low SES were significantly younger in our study, they had a higher prevalence of mixed forms of IHD (a presumable index of more severe disease), more comorbidities (diabetes, COPD), and more postoperative complications (table 3).

Differential standards of medical care and management, measured as percentage of medical injuries attributable to negligence, have been observed in New York.31 Egbert and Rothman have shown that African-Americans undergoing surgery are more likely to receive care from a physician in training than from a senior staff surgeon.32 The possibility that people with the lowest SES level have the worst prognosis as a consequence of inadequate medical care during CABG hospitalisation is difficult to evaluate in our study. We lack detailed information on the process of care (that is, technical aspects of operations, preoperative and postoperative care programmes) and other possible factors influencing poor prognosis after CABG surgery such as the ability to adapt lifestyle (smoking, diet) and compliance to treatment.

The interpretation of our results is potentially limited by several factors. People were classified using a small area index: although this index strongly predicts socioeconomic inequalities in mortality in Rome,14 it is still an aggregate index and individual data on various components of SES are lacking. However, aggregate indices of SES have been widely proposed in the US33 and in the UK34 as a valid mean of overcoming lack of individual information. The definition of the severity of illness was derived from the ICD-9 diagnosis and procedure codes, as assigned in the HIS database. Administrative data have been widely used to assess the effectiveness and quality of care provided by various services, especially in cardiac surgery.35 Although clinical datasets seem to be more accurate in detecting clinical and functional myocardial impairment, code-based measures have been shown to have good statistical performance in predicting inhospital deaths from CABG surgery.36 Moreover, we empirically attempted to distinguish comorbidities from complications and death related conditions, as suggested by other investigators,37 to improve the clinical validity of our model. We have no reason to suspect that the accuracy of coding may be biased toward a specific SES level.

Other studies have reported that access to a resource consuming procedure such as CABG surgery may be affected by SES.4 38 Inequalities in provision have been observed not only in the USA, where no NHS programme exists and where the patient's insurance status could thus influence access to care,39but also in Canada40 and the UK,41 countries with universal health coverage. Our study suggests that interventions to promote equal availability and effectiveness of CABG procedures will be crucial to reduce inequalities among social groups, even in countries with a NHS.