This study examined the association between alcohol consumption and mortality among CABG patients. The primary result was that among CABG patients, only heavy drinkers (>21 units/week) were significantly associated with an increased risk of all-cause mortality. The risk of dying did not differ between the abstainers and moderate consumers.
Interpretation
Our results are comparable with those of previous studies that investigated the influence of alcohol consumption on composite outcomes, including mortality, among CABG patients [
14,
15]. Likewise, these studies showed no risk of increased mortality with moderate alcohol consumption. In addition, our results aligned with those from other studies, that did not target CABG patients; in these studies, moderate alcohol consumption did not adversely affect all-cause mortality and cardiovascular mortality [
2‐
8].
In contrast to comparable studies [
14,
15], our results showed an increased risk of all-cause mortality for heavy drinkers. One of these studies was not able to distinguish between heavy drinking and moderate alcohol consumption, and the other study used a lower threshold to define the highest alcohol use (≥14 units/week) than our study, thus limiting knowledge concerning heavy drinkers (>21 units/week). The different definition of heavy alcohol consumption and the lack of examining heavy drinkers might explain why our results reveal an increased risk for all-cause mortality for this specific patient group. The results of an increased risk of all-cause mortality for heavy drinkers aligned with the results of other studies not targeting CABG patients [
2,
3]. Thus, our study extends this observation to include CABG patients who, consequently, do not seem to differ from the general population in terms of alcohol consumption and mortality, even though the burden of co-morbidities for CABG patients has increased over time [
12,
13].
The fully adjusted model examining the association of alcohol consumption and cardiovascular mortality showed an increased cardiovascular mortality rate among heavy drinkers, but this result was not statistically significant. As shown in the forest plots (Figs.
3 and
4), the results from model 1 and model 2 for all-cause mortality and cardiovascular mortality illustrate J-shaped curves, which indicate higher mortality rates for the heavy drinkers, as reported elsewhere [
3,
7,
32,
42]; thus, these results indicate an increased mortality rate among heavy drinkers. The pattern for all-cause mortality and cardiovascular mortality is nearly identical, as 86 % of all deaths are ascribed to cardiovascular causes. This majority of deaths attributable to cardiovascular mortality calls for separate investigation of this specific cause of death. However, as alcohol consumption may be related to other not-cardiovascular related causes of death (e.g. cancer), all-cause mortality is the most important outcome and the cardiovascular mortality results should be interpreted with caution. Conclusively, this result does not alter the finding that alcohol consumption seems to have little influence on mortality among CABG patients, except for heavy drinkers.
Other studies have found beneficial effects of moderate alcohol consumption on all-cause mortality and cardiovascular mortality [
2‐
8]. The beneficial effect of alcohol is generally ascribed to increased plasma high-density lipoprotein (HDL) [
43] and antithrombotic effects [
44], where increased HDL levels are best established [
30]. No beneficial effects of moderate alcohol consumption were observed in our study. For the sensitivity analysis, analyses were also conducted on the population with complete data. Conclusively, no major differences were observed between the analyses on the imputed data and the population with complete data; however, the risk estimates were generally slightly attenuated. Our results showed no harm for moderate-heavy drinkers regardless of whether the analysis was performed as complete cases or with multivariate imputation, thereby indicating that CABG patients do not need stricter advice to abstain from alcohol consumption, other than heavy drinking.
Previously comparable studies [
14,
15] have examined alcohol consumption and composite outcomes, including mortality, for CABG patients. Therefore, our results most likely reflect a fairer examination between alcohol consumption and mortality for CABG patients because we did not use composite outcomes.
Strength and limitations
A major limitation of our study is the observational design, which does not eliminate unmeasured confounders. Residual confounding may, therefore, be present. Due to the study design, our results should only be considered as associations and not as causal relationships.
Although this study had missing data on alcohol consumption for 1622 patients, there was no difference in relation to educational level, diabetes mellitus, stroke, renal failure, liver disease, COPD and AF between the population with complete data and the group with missing data, which decreases the possibility that the missing data introduced selection bias in the study. Of the 1622 patients, 466 underwent acute CABG, which might explain the missing information on alcohol consumption (i.e., the condition of the patient may have been too severe to report the amount of weekly alcohol consumption). Multivariate imputation was performed on the missing covariates. This technique has the advantage of increasing the size of the study population and is typically more efficient than complete cases analysis [
40,
41]. Analysis with complete cases only could lead to over-or underestimation of the effect sizes [
41]; thus, multivariate imputation is seen as a strength. However, the multivariate imputation did only increase the study population with 155 patients with missing values on covariates, and the high number of missing data should be recognized as a limitation.
We have adjusted for a wide range of potential confounders, which have been possible due to national registers and increases the strength of the analysis. In general, the information of the robust data obtained from the registers is regarded as high quality, and it is one of the main strengths of this study [
21,
22,
27,
28,
38]. The data obtained from the national registers are collected independently of this study and, therefore, decrease the possibility of both selection and information bias. The use of the registers further adds to the strength of the study because complete follow-up data are available for all CABG patients.
The amount of alcohol consumption reported tends to be underestimated, particularly among heavy drinkers [
45], and the validity of the amount of moderate alcohol consumption is more stable than heavy drinking [
42]. An underestimation where some heavy drinkers by mistake is classified as moderate-heavy drinkers would probably lead to a lower association between alcohol consumption and mortality for heavy drinkers. This could also lead to an elevated risk for moderate-heavy drinkers. However, no increased risk was found for this group. Thus, underestimation of alcohol consumption for moderate-heavy drinkers would probably have little impact on the results. Nevertheless, newer studies consider the measurement of alcohol consumption as valid [
46,
47]. In this study, alcohol consumption was measured as a part of the anaesthesiologist anamnesis prior to the operation because alcohol consumption can impact the administration of anaesthesia. The measurement of alcohol consumption obtained in connection with the comprehensive CABG procedure is, therefore, expected to be reliable.
We were not able to identify the different types of alcohol that could have potentially influenced the results if one type of alcohol is more harmful or beneficial than another. Compared to a moderate intake of beer or liquor, moderate wine drinking has been associated with reduced mortality [
48] and higher socioeconomic position [
49]. We have adjusted for income and educational level as a proxy for socioeconomic position. The reduced association between moderate wine drinking and mortality may, however, be ascribed to more favourable drinking habits and socioeconomic position than wine
per se [
49]. Nevertheless, it would have been favourable to have access to separate information on wine, beer and liquor. It has been proposed that it is alcohol
per se that has a protective effect on mortality rather than different types of alcohol [
5,
30,
50]. In this study, we found no beneficial effect of moderate alcohol consumption, and the lack of separating different types of alcohol should, therefore, not affect the results crucially.
The use of all-cause mortality as an outcome had the advantage that it requires no further ascertainment than the time of death, and therefore, it will tend to eliminate the possibility of information bias for the outcomes. However, this issue could occur depending on the cardiovascular mortality because the correctness of cause-specific mortality depends on an individual physician’s registration of the death certificate, which might lead to misclassification and possible bias if the specific cause of death registration was dependent on alcohol consumption. Nevertheless, there seems to be a high validity of causes of death compared to clinical records [
25].