ACEF II Score for cardiac surgery

Clinical background

Preoperative risk assessment before cardiac surgery requires an estimate of operative mortality that can be weighed against the natural course of the disease and against alternatives such as percutaneous coronary intervention (PCI) or medical treatment. EuroSCORE II is the most widely used model but requires up to 18 variables and presupposes access to complete preoperative data. In emergencies or in resource-limited settings it is not always possible to collect all the variables in time.

The ACEF II risk score was developed to fill precisely that gap: a parsimonious risk model that can be calculated at the bedside from five variables always available before cardiac surgery. The score features in the ESC/EACTS guidelines on myocardial revascularisation as an alternative to EuroSCORE II, particularly for coronary bypass surgery [1].

Calculating the ACEF II risk score

The formula is:

$$\text{ACEF II} = \frac{\text{age}}{\text{EF},$$

The five variables are: age in whole years, the left ventricular ejection fraction as a percentage, the serum creatinine with a threshold of 2 mg/dL (corresponding to approximately 177 µmol/L, the unit used in Swedish clinical practice), the haematocrit as a percentage, and emergency surgery (defined as surgery that must be performed before the next working day, in line with the EuroSCORE II definition of an emergency).

The derivation cohort consisted of 7,011 consecutive cardiac surgical patients operated on at a single centre (IRCCS Policlinico San Donato, Milan) [1]. The validation cohort comprised 1,687 consecutive patients at another Italian centre (Siena). The outcome modelled was operative mortality, defined as death in hospital or within 30 days of surgery. The variables were chosen by multivariable logistic regression, in which age divided by ejection fraction formed the core term, to which creatinine, emergency status and anaemia were added as binary or continuous terms.

Interpretation in practice

ACEF II is a continuous score in which a higher value corresponds to a higher operative mortality risk. The calculator defines no fixed risk bands, and the derivation study presents no universal thresholds for low, moderate or high risk [1]. The score should therefore be read as a relative measure of risk rather than as an absolute mortality percentage, and preferably in relation to the patient's alternative treatment options.

In clinical use the score serves to identify quickly the patients at increased risk in whom more careful preoperative optimisation or further discussion of the treatment strategy is warranted. In hybrid coronary revascularisation a cut-off of 1.35 has been proposed for risk stratification with respect to major adverse cardiac and cerebrovascular events (MACCE), but this was derived in a specific population and for a different outcome from operative mortality and should not be generalised [2].

A patient with a high score should not automatically be refused surgery. The score flags risk; it does not determine the indication for treatment. At very high values, however, the discussion of operative risk should include an assessment of alternative strategies and of whether the preoperative risk factors can be modified, for example by correcting anaemia or optimising renal function.

Validation and performance

In the derivation cohort the c-statistic was 0.814, significantly better than the original ACEF score (c-statistic 0.773, P = 0.041) and comparable to EuroSCORE II [1]. In the external validation cohort, ACEF II retained better discrimination than ACEF and showed good calibration [1].

In a large external validation of 14,804 patients from eight Italian cardiac surgical centres (2009 to 2019), EuroSCORE II was, however, significantly more accurate than ACEF II in the population as a whole [3]. The difference was marked in isolated aortic surgery, in which ACEF II performed considerably less well. In isolated bypass surgery, by contrast, there was no significant difference between the two models [3]. In isolated valve surgery, EuroSCORE II was superior [3].

In a Chinese cohort of 120 patients undergoing hybrid coronary revascularisation, ACEF II was an independent prognostic marker for MACCE with an AUC of 0.740 and a hazard ratio of 2.24, comparable to EuroSCORE II (AUC 0.703) and better than the SYNTAX score II CABG (AUC 0.621) [2].

An Indonesian validation of the original ACEF score (not ACEF II) in 1,833 patients showed poorer discrimination, with an AUC of 0.638 and poor calibration (Hosmer–Lemeshow P < 0.001) [4]. EuroSCORE II performed better (AUC 0.774) but was also poorly calibrated in the same population. Both models underestimated mortality in all risk groups [4]. The result illustrates that risk models developed in Western cohorts may have limited reliability in populations with different demographics and different resources.

Limitations

ACEF II comprises only five variables. It omits established risk predictors such as chronic lung disease, diabetes, previous cardiac surgery, endocarditis and a critical preoperative state. In complex procedures, or in patients with several risk factors that the model does not capture, the score may underestimate the true risk.

The score performs worst in aortic surgery, in which five variables do not capture the complexity that selectively affects the outcome of these procedures [3]. It performs best in isolated coronary bypass surgery, in which the three core variables of age, renal function and ejection fraction carry most of the prognostic information.

The original ACEF score showed poor discrimination and calibration in an Indonesian cohort, and although ACEF II has not been validated specifically in Scandinavian practice, there is a systematic risk that the model calibrates less well in populations that differ from the Italian derivation cohort [4].

The score estimates operative mortality, not long-term survival or MACCE. For decisions concerning long-term outcomes, additional assessments are required.

The serum creatinine is entered in mg/dL in the formula. In Swedish clinical practice, serum creatinine is reported in µmol/L, so the threshold of 2 mg/dL corresponds to approximately 177 µmol/L. The calculator handles the conversion internally, but in a manual calculation the correct unit must be ensured.

References

1. Ranucci M, Pistuddi V, Scolletta S, et al. The ACEF II Risk Score for cardiac surgery: updated but still parsimonious. Eur Heart J 2018;39(23):2183-2189. PMID: 28498904
2. Li Y, Li C, Feng D, et al. Predictive value of ACEF II score in patients with multi-vessel coronary artery disease undergoing one-stop hybrid coronary revascularization. BMC Cardiovasc Disord 2021;21:489. PMID: 34629062
3. Santarpino G, Nasso G, Peivandi AD, et al. Comparison between the age, creatinine and ejection fraction II score and the European System for Cardiac Operative Risk Evaluation II: which score for which patient? Eur J Cardiothorac Surg 2022;61(5):1118-1122. PMID: 35134895
4. Widyastuti Y, Boom CE, Parmana IMA, et al. Validation in Indonesia of two published scores for mortality prediction after cardiac surgery. Ann Card Anaesth 2023;26(1):23-28. PMID: 36722584