Introduction

Low cardiac output syndrome (LCOS), a clinical manifestation of decreased cardiac output and peripheral tissue perfusion, was first proposed by Rao et al. ¹ Previous studies have shown that all-cause mortality in LCOS ranges from 14.8 to 62.5 % in the short term (one month after its onset) and from 21.4 to 36.6 % in the long term (two months to one year after its onset) ². LCOS after heart surgery leads not only to poor tissue perfusion but also to multiple organ dysfunction of the brain, lungs, liver, kidneys and gastrointestinal tract, which increases the use of health care resources and associated costs ³. More importantly, LCOS may cause reversible decrease in cardiac output (CO) after heart surgery. The early detection and appropriate treatment of LCOS can prevent its progression to refractory cardiogenic shock and improve clinical outcomes, so its early detection is extremely important ⁴.

According to studies conducted in North America (USA and Canada), 45 % of worldwide surgical activity decreased between March and April 2020 compared to that performed in 2019. There was an increase of up to 7 % in coronary artery surgeries, probably at the expense of urgent surgeries, with significant decreases in heart valve surgeries of up to 7.5 % ⁵.

In 2020 ⁶, in Spain, heart valve replacement and coronary artery surgeries showed a mortality rate of 13.61 %. Polyvalvular and coronary artery surgeries also showed similar figures (12.07 %), which is associated with intraand postoperative complications such as low cardiac output (LCO).

According to Vera-Rivero et al. ⁷, Cuba is one of the few developing countries capable of cooperating in the cardiovascular surgery field, and the specialty got started with the creation of heart centers in different regions of the country.

Statistical data ⁸ shows that between 2019 and 2020 a total of 3,004 patients underwent surgical treatment due to a cardiovascular pathology.

Bertini et al. ⁹ state that perioperative morbidity and mortality remain two of the main limitations of heart surgery with extracorporeal circulation, despite they have progressively been reduced over the last decades.

Based on the foregoing, this research was conceived to determine the possible clinical and electrocardiographic predictors associated with the onset of LCOS and how they can influence on its progression.

Materials and methods

Results

Table 1 shows the prevalence of patients under 65 years of age (51.81 %), a larger number of patients in the case group (60.71 %) and a p value = 0.008, which corresponds to a highly significant association. Furthermore, it shows that 96 (85.68 %) patients had atrial fibrillation, with a smaller number in both study groups, though an extremely high statistical association (p = 0.007) was found. Concerning the echocardiographic parameters, a high prevalence of patients with LVEF > 50 % (91.10 % [p = 0.047]) was observed, with a larger number in both groups of patients. Moreover, a larger number of patients with RVEF > 17 mm (88.40 % [p = 0.008]) was evidenced. Both variables were statistically significant; other variables such as sex, DM, COPD, ischemic heart disease and PASP were not statistically significant (Table 1).

Emergency surgery was one of the parameters evaluated in this research. As shown in Table 2, a large number of patients (45.24 % [p ≤ 0.05; OR: 2.14]) underwent emergency surgery, thus showing statistical significance, and LCO posed a risk factor for those patients.

Measures of public health impact such as ARPE and PARP showed that 33 % of the cases, i.e., patients with LCO, were the result of non-elective surgeries. In the event that adequate elective surgeries were performed, these sequelae would be reduced by 51.7 % (Table 2).

As for parameters such as extracorporeal circulation time, data from 50 patients (29.76 % [p = 0.000; OR: 1.54; LL: 0.73; UL: 3.24]) demonstrated that it is a predisposing factor in patients whose time exceeds 90 minutes. Concerning the perioperative bleeding, a small number of patients (12.50 % [p = 0.013; OR: 3.12; LL: 1.22; UL: 7.93]) showed an extremely high statistical significance; moreover, bleeding more than 500 ml posed a risk factor for the onset of LCOS (Table 3).

A logistic regression analysis was used to determine how the clinical and echocardiographic variables affect the probability of presenting LCOS. A total of 112 cases were used in the analysis. The model explained 78.9 % of LCOS cases in surgical patients. Variables such as age, RVEF, EEC and PASP showed p values ≤ 0.05, which allows us to conclude that they have a statistically significant association with complications. Variables such as LVEF and perioperative bleeding did not have statistical significance (p > 0.05) (Table 4).

When analyzing the area under the curve (AUC) (Table 5) and the receiver operating characteristics (ROC) curve (Figure 1), based on sensitivity and specificity, it was found that the AUC had a range of 0.452−0.762. Thus, variables such as LVEF, extracorporeal circulation time and perioperative bleeding can predict LCOS risk.

Figure 1 AUC of predictor variables 

Discussion

LOCS is a frequent and serious complication in heart surgery patients. However, both the incidence and response to treatment and later progression have been highly variable in different publications to date.

Age is an independent cardiovascular risk factor; therefore, the probability of death increases with age, which has been validated in different risk scales. Although the risk of death increases exponentially, some studies show that the probability of dying triples in people over 75 years of age ¹¹.

Lorenzo ¹², in a review article, claimed that being older than 65 years is one predisposing factor for complications and LCO in heart surgery patients.

Pérez Vela et al., in a multicenter study published in 2018, stated that 68.3 ± 9.3 years of age was one of the factors mostly associated with the onset of postoperative low output and that 65.2 % were males ¹³. A study conducted by Fernández Mesa et al. ¹⁴, which included 156 patients who underwent surgery due to left valvular heart diseases in 2018, reported that 46 of them had LCO and patients over 65 prevailed in this group.

The abovementioned authors’ statements agree with what is shown in this study.

Zhao et al. ¹⁵, in a study conducted at Guangdong Provincial People’s Hospital, showed the prevalence of males (69.8 %) compared to females (30.2 %). Males did not have the highest scores in the research results, which disagrees with the literature and this research results.

The presence of comorbidities such as DM, high blood pressure and COPD, among others, is also associated with complications, including LCO. According to Cubides Núñez, the most frequent comorbidity in his study was high blood pressure, followed by COPD ¹⁶.

In the previously mentioned study conducted by Fernández Mesa et al. ¹⁴, COPD and chronic atrial fibrillation were the most frequent comorbidities in the group of patients with LCO.

Pérez Vela et al. ¹³ reported in their study that high blood pressure and dyslipidemia were found in patients that developed LCOS in their series, with 70.8 % and 63.5 %, respectively.

The research on atrial fibrillation conducted by Gómez Núñez et al. ¹⁷ showed that one of the most frequent complications was LCO, which was observed in 23 % of the patients.

When comparing such evidence to this research, there are similar aspects in the description of atrial fibrillation by the aforementioned authors. In this research, fibrillation had high statistical significance, which agrees with what has been stated.

Perioperative left ventricular systolic dysfunction has been considered a predisposing factor for postoperative LCOS, as shown in the literature. Sepúlveda et al. ¹⁸ reported segmental contraction abnormalities in 273 patients (58.6 %), out of which 208 (59.4 %) required intraoperative inotropic support. Moreover, they stated that 35 (83.3 %) out of 42 patients with reduced ejection fraction or ejection fraction under 40 % had LCO. This agrees with a study conducted by Li et al. ¹⁹, where an ejection fraction under 40 % was considered a predictor. Burstein et al. ²⁰ found ejection fraction values of approximately 47.3 % with a standard deviation of ± 16.2 %. Our study showed the onset of LCO in patients with an ejection fraction > 50 %, which demonstrates the varying nature of this condition.

The evaluation of the right ventricular (RV) function is a key aspect in heart surgery patients, not only before surgery but also in the early postoperative period. This is evidenced in studies such as those conducted by Fernández et al. ¹⁴, who claimed that patients with LCO had lower RVEF, higher mean pulmonary arterial and right atrial pressure, and higher pulmonary arterial systolic pressure, all of them showing significant differences (p < 0.0001). This is consistent with the findings of this research.

Fernández Mesa et al. ¹⁴, in their multivariate analysis, found that the main predictors of the onset of LCO were prolonged extracorporeal circulation time (OR: 4.89; p = 0.001), age over 65 years, sex and LVEF ≤ 40 %, which agrees with this research.

Additionally, González Kadashinskaia et al. ²¹ mentioned that extracorporeal circulation was one of the most common procedures in surgical operations, with almost 55 %, which indicates that this is a widely used technique. Likewise, Fernández Mesa et al. ¹⁴ pointed out in their study that prolonged extracorporeal circulation time is associated with a higher probability of suffering such complication, which is consistent to the results of this study.

In another article by Fernández Mesa et al. ²², they concluded that patients with LCO were 3.5 times more likely to die in the five-year period after surgery than those who did not had this condition.

Lorenzo ¹², in his review article, mentioned that a singlecenter study published by Ding et al. included 1,524 heart surgery patients, out of which 205 developed LCOS. Then, a multivariate analysis led to conclude that age over 65 years, LVEF < 50 % and surgery with extracorporeal circulation were independent factors for LCO. In this regard, Jiménez Hernández et al. ²³ drew similar conclusions.

Aslan et al. ²⁴ reported that markedly prolonged aortic clamp time and total cardiopulmonary bypass time are predictors of the onset of LCO.

Maganti et al. ²⁵ claimed in a study that 7 % of the cases presented postoperative LCO, which was associated with urgency surgery and low ejection fraction.

Kochar et al. ²⁶ stated in a multivariate and multicenter research that the factors mostly associated with a higher risk of LCO were preoperative ejection fraction and age, thus resulting in patients with the worst results. In this regard, Lui et al. ²⁷ showed evidence of a predictive model based on variables such as age, and preoperative factors such as low LVEF and aortic clamp time.

Elhenawy et al. ²⁸ associated mitral valve surgery with a higher prevalence of postoperative LCOS when factors such as age, urgency surgery and preoperative left ventricular function were involved. In turn, Seguel et al. ²⁹ reported low incidence of LCO.

Additionally, De Lima et al. ³⁰ described in their research that the conditions that exacerbate complications during the immediate postoperative period after heart surgery are perioperative bleeding and atrial fibrillation, the former with an incidence of up to 50 %.

The main limitation of this research was basically not having an adequate sample size to determine the behavior of postsurgical complications. It should be noted that this data was collected from a center of cardiovascular medicine, which allows-in any case-reflecting its strengths and obvious limitations.

In conclusion, clinical and echocardiographic elements such as age, atrial fibrillation, LVEF and PASP were independently associated with the onset of LCO.