Objective To analyse the relationship between the degree of frailty and the risk of presenting poor short-term outcomes. Methods observational cohort study of the population ≥ 70 years of age undergoing elective and high-priority major cardiac surgery at our hospital. A total of 232 consecutive patients were enrolled in Salamanca University Hospital from October 2017-December 2019 This cohort study of 232 patients prospectively compared the results of the FRAIL questionnaire and the Fried Phenotype Criteria (FPC) and retrospectively adapted these tools based on the characteristics and confounding factors found in our sample. The individual items comprising the multi-item scales were then independently analysed using logistic regressions. Results Frailty was associated with increased mortality, although the differences were not significant. Standardizing the FPC improved its ability to identify frail patients (p = 0.027). Scores with both original tools were associated with a prolonged postoperative stay (p ≤ 0.05). Additionally, a positive result on the FRAIL questionnaire was associated with a higher number of complications (p = 0.025). In our study, the predictive capacity emerged from specific items: grip strength, gait speed, illness, and resistance. We united these items with the severity of pulmonary hypertension to create a specific frailty scale for cardiac surgery, and the scores were significantly associated with a combined endpoint, containing death, prolonged stay, and/or presenting ≥ 3 complications (p = 0.011). Conclusions The results indicated that frailty determined by either of the original tools was associated with worse results after cardiac surgery. Likewise, milder degrees of frailty, which we call pre-frailty, can also anticipate poor outcomes, in some cases. Standardization of the FPC improved its predictive capacity. In our study, we designed a tool for cardiovascular patients that is predictive of poor postoperative outcomes. Adapting frailty tools is necessary to avoid confusion with cardiovascular disease.