The global epidemic of obesity is becoming more and more serious. It is not only a risk factor for chronic diseases such as type 2 diabetes, cardiovascular diseases, hypertension and stroke, but also considered as an independent disease . Obesity can be accompanied by diabetes, hypertension and heart failure, which can increase the morbidity and mortality of cardiovascular diseases. However, the existing clinical evidence suggests that in patients with cardiovascular diseases, the long-term death risk and the incidence of major cardiovascular events are both high with low body mass and normal body mass. This phenomenon is called "obesity paradox" in patients with myocardial infarction, coronary heart disease, heart failure, hypertension, etc. Cardiogenic shock is a common clinical critical and severe disease, a serious complication of cardiovascular diseases, with high mortality [8, 9]. So far, it is not clear whether there is also an "obesity paradox" in patients with cardiogenic shock. Swaminathan et al.  first studied 61 patients with cardiogenic shock, showing that the mortality rate of obese patients with cardiogenic shock is higher than that of non-obese patients. Since then, a multi-center German network registration study , a study of 890 patients with ST-segment elevation myocardial infarction complicated with cardiogenic shock, found that there was no evidence that the in-hospital all-cause mortality rate of obese cardiogenic shock patients was higher than that of non-obese patients. Chatterjee et al.  For 290,894 patients with cardiogenic shock, after adjusting for demography, hospital characteristics, complications and clinical manifestations, the study found that the in-hospital mortality rate of obese patients with cardiogenic shock was slightly lower than that of non-obese patients (OR = 0.89, 95% CI 0.86–0.92, p < 0.001). Shah et al.  evaluated the clinical outcomes of non-obese, obese (BMI 30.0-39.9 kg/m2) and extremely obese (BMI ≥ 40 kg/m2) cardiogenic shock patients based on Chatterjee et al. . The study found that compared with non-obesity, obesity predicted lower in-hospital mortality (OR = 0.82,95% CI 0.76–0.90), while extreme obesity predicted higher in-hospital mortality (OR = 1.17. 95% CI 1.05–1.32). We conducted a meta-analysis of the included studies and found that there was no significant difference in all-cause mortality between obese and non-obese cardiogenic shock patients in the two groups. In the subgroup analysis, it was found that the in-hospital all-cause mortality rate of patients with cardiogenic shock in obese group was lower than that in non-obese group according to the country, economic development level and research type.
At present, the exact mechanism of obesity paradox in patients with cardiogenic shock is still unclear, but it can be inferred from the results of current research: firstly, confounding factors can lead to obesity paradox, such as muscle quantity, drug therapy, exercise, age, etc. in patients with cardiogenic shock . Secondly, it is bias. Obese patients are more likely to diagnose cardiovascular diseases, which leads to misclassification bias, or obese patients' symptoms may appear earlier, which leads to lead time bias . The existence of these two biases may lead to the observation that the survival time of obese patients with cardiovascular diseases is longer than that of non-obese patients with cardiovascular diseases . Third, the marker defining obesity is not ideal. BMI is not as good as body fat, fat distribution, thin body mass and body fluid composition, which can properly reflect the important components of the body .
Our meta-analysis still has some limitations. First of all, the main limitation is the lack of randomized controlled studies. Secondly, most of the included studies are retrospective studies. There is obvious selective bias in observational research. Although we do subgroup analysis as much as possible, the potential bias still exists. Third, BMI classification was not further refined, and only the in-hospital mortality of patients with cardiogenic shock in obese and non-obese groups was compared, which may affect the accuracy of the results. Fourth, for cardiogenic shock, there is no further comparison of etiology, revascularization or conservative drug treatment. Fifth, BMI is commonly used in epidemiological investigation and population prevention to measure overall obesity, but a single indicator cannot fully and correctly reflect the relationship between obesity and diseases. Sixth, there are many prognostic indicators. We only evaluate all-cause mortality in hospital. The relationship between other prognostic indicators and BMI needs further study. Seventh, there was no long-term follow-up outside the hospital.