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Cardiac Risk, Imaging, and the Cardiology Consultation


Key Points

  • 1.

    Multivariate modeling has been used to develop risk indices that focus on preoperative variables, intraoperative variables, or both.

  • 2.

    Key predictors of perioperative risk are dependent on the type of cardiac operation and the outcome of interest.

  • 3.

    New risk models have become available for valvular heart surgery and for combined coronary and valvular cardiac procedures.

  • 4.

    Perioperative cardiac morbidity is multifactorial, and understanding the predictive risk factors helps to define the risk for individual patients.

  • 5.

    Assessment of myocardial injury is based on the integration of information from myocardial imaging (eg, echocardiography), electrocardiography (ECG), and serum biomarkers, with significant variability in the diagnosis depending on the criteria selected.

  • 6.

    Echocardiography is the most widely used modality for cardiac imaging in almost any form of cardiac disease.

  • 7.

    Stress echocardiography is helpful in the assessment of inducible myocardial ischemia, myocardial viability, and certain valve disorders.

  • 8.

    Myocardial perfusion imaging can be performed using single-photon emission computed tomography (SPECT) or positron emission tomography (PET), and is useful in the evaluation of myocardial ischemia and viability.

  • 9.

    Cardiac computed tomography and cardiac magnetic resonance are increasingly used when there are conflicting results or when further information is required in the preoperative phase of care.

  • 10.

    Cardiac magnetic resonance is the gold standard for quantitative assessment of ventricular volumes, ejection fraction, and mass. It is also able to evaluate ventricular and valvular function, atherosclerosis, and plaque composition.

  • 11.

    Computed tomographic aortography is the best modality for the evaluation of aortic aneurysms and dissections. Additionally, computed tomographic coronary angiography offers an alternative to invasive coronary angiography for excluding significant coronary artery disease in patients undergoing noncoronary surgery.

The first risk-scoring scheme for cardiac surgery was introduced by . Since then, many preoperative cardiac surgery risk indices have been developed. The patient characteristics that affected the probability of specific adverse outcomes were identified and weighed, and the resultant risk indices have been used to adjust for case-mix differences among surgeons and centers where performance profiles have been compiled. In addition to comparisons among centers, the preoperative cardiac risk indices have been used to counsel patients and their families in resource planning, to identify high-risk groups for special care, to determine cost-effectiveness, to determine effectiveness of interventions, to improve provider practice, and to assess costs related to severity of disease.

Cardiac Risk Assessment and Cardiac Risk Stratification Models in Patients Undergoing Cardiac Surgery

In defining important risk factors and developing risk indices, each of the studies has used different primary outcomes. Postoperative mortality remains the most definitive outcome that is reflective of patient injury in the perioperative period. Death can be cardiac and noncardiac related, and if cardiac related, it may be ischemic or nonischemic in origin. Postoperative mortality rate is reported as either the in-hospital rate or the 30-day rate. The latter represents a more standardized definition, although it is more difficult to capture because of the difficulty inherent in assessing death rates of discharged patients who may die at home or in another facility. Risk-adjusted postoperative mortality models permit assessment of the comparative efficacy of various techniques in preventing myocardial damage, but they do not provide information that is useful in preventing the injury in real time. The postoperative mortality rate is also used as a comparative measure of equality of cardiac surgical care.

Postoperative morbidity includes acute myocardial infarction (AMI), reversible events such as congestive heart failure (CHF), and need for inotropic support. Because resource utilization has become such an important financial consideration for hospitals, the length of stay in the intensive care unit (ICU) increasingly has been used as a factor in the development of risk indices.

Predictors of Perioperative and Postoperative Morbidity and Mortality

The original risk-scoring scheme for cardiac surgery (coronary artery bypass graft [CABG] and valve) identified eight risk factors: (1) poor left ventricular (LV) function, (2) CHF, (3) unstable angina or recent myocardial infarction (MI) (within 6 weeks), (4) age greater than 65 years, (5) severe obesity (body mass index >30 kg/m 2 ), (6) reoperation, (7) emergency surgery, and (8) other significant or uncontrolled systemic disturbances. The investigators identified three classes of patients: those with none of the listed factors (normal), those presenting with one factor (increased risk), and those with more than one factor (high risk). In a study of 500 consecutive patients undergoing cardiac surgery, it was found that operative mortality increased with increasing risk score (confirming the scoring system).

One of the most commonly used scoring systems for CABG was developed by Parsonnet and colleagues ( Table 1.1 ). Fourteen risk factors were identified for in-hospital or 30-day mortality after univariate regression on analysis of 3500 consecutive operations. An additive model was constructed and prospectively evaluated in 1332 cardiac procedures. Five categories of risk were identified with increasing mortality rates, complication rates, and length of stay. The Parsonnet Index frequently is used as the benchmark for comparison among institutions. Since publication of the Parsonnet model, numerous technical advances now in routine use have diminished CABG mortality rates.

Table 1.1
Components of the Additive Model
From Parsonnet V, Dean D, Bernstein A. A method of uniform stratification of risk for evaluating the results of surgery in acquired adult heart disease. Circulation. 1989;79:I3, by permission.
Risk Factor Assigned Weight
Female sex 1
Morbid obesity (≥1.5 × ideal weight) 3
Diabetes (unspecified type) 3
Hypertension (systolic BP >140 mm Hg) 3
Ejection fraction (%):
Good >50) 0
Fair (30–49) 2
Poor (<30) 4
Age (y):
70–74 7
75–79 12
≥80 20
Reoperation
First 5
Second 10
Preoperative IABP 2
Left ventricular aneurysm 5
Emergency surgery after PTCA or catheterization complications 10
Dialysis dependency (PD or Hemo) 10
Catastrophic states (eg, acute structural defect, cardiogenic shock, acute renal failure) a 10–50 b
Other rare circumstances (eg, paraplegia, pacemaker dependency, congenital HD in adult, severe asthma) a 2–10 b
Valve surgery
Mitral 5
PA pressure ≥60 mm Hg 8
Aortic 5
Pressure gradient >120 mm Hg 7
CABG at the time of valve surgery 2
BP, Blood pressure; CABG, coronary artery bypass graft; HD, heart disease; Hemo, hemodialysis; IABP, intraaortic balloon pump; PA, pulmonary artery; PD, peritoneal dialysis; PTCA, percutaneous transluminal coronary angioplasty.

a On the actual worksheet, these risk factors require justification.

b Values were predictive of increased risk for operative mortality in univariate analysis.

The Society of Thoracic Surgeons (STS) National Adult Cardiac Surgery Database (NCD) ( Table 1.2 ) represents the most robust source of data for calculating risk-adjusted scoring systems. Established in 1989, the database included 892 participating hospitals in 2008 and has continued to grow. This provider-supported database, one of the largest in the world, allows participants to benchmark their risk-adjusted results against regional and national standards. New patient data are brought into the STS database on a semiannual basis.

Table 1.2
Risk Model Results
From Shroyer AL, Plomondon ME, Grover FL, et al. The 1996 coronary artery bypass risk model: the Society of Thoracic Surgeons Adult Cardiac National Database. Ann Thorac Surg. 1999;67:1205, by permission of Society of Thoracic Surgeons.
Variable Odds Ratio
Age (in 10-year increments) 1.640
Female sex 1.157
Race other than white 1.249
Ejection fraction 0.988
Diabetes 1.188
Renal failure 1.533
Serum creatinine (if renal failure is present) 1.080
Dialysis dependence (if renal failure is present) 1.381
Pulmonary hypertension 1.185
Cerebrovascular accident timing 1.198
Chronic obstructive pulmonary disease 1.296
Peripheral vascular disease 1.487
Cerebrovascular disease 1.244
Acute evolving, extending myocardial infarction 1.282
Myocardial infarction timing 1.117
Cardiogenic shock 2.211
Use of diuretics 1.122
Hemodynamic instability 1.747
Triple-vessel disease 1.155
Left main disease >50% 1.119
Preoperative intraaortic balloon pump 1.480
Status
Urgent or emergent 1.189
Emergent salvage 3.654
First reoperation 2.738
Multiple reoperations 4.282
Arrhythmias 1.099
Body surface area 0.488
Obesity 1.242
New York Heart Association class IV 1.098
Use of steroids 1.214
Congestive heart failure 1.191
PTCA within 6 h of surgery 1.332
Angiographic accident with hemodynamic instability 1.203
Use of digitalis 1.168
Use of intravenous nitrates 1.088
PTCA, Percutaneous transluminal coronary angioplasty.

There are currently three general STS risk models: CABG, valve (aortic or mitral), and valve plus CABG. These three models comprise seven specific, precisely defined procedures: the CABG model refers to an isolated CABG; the valve model includes isolated aortic or mitral valve replacement and mitral valve repair; and the valve plus CABG model includes aortic valve replacement with CABG, mitral valve replacement with CABG, and mitral valve repair with CABG. Besides operative mortality, these models were developed for eight additional end points: reoperation, permanent stroke, renal failure, deep sternal wound infection, prolonged (>24 hours) ventilation, major morbidity, operative death, and finally short (<6 days) and long (>14 days) postoperative length of stay. These models are updated every few years and are calibrated annually to provide an immediate and accurate tool for regional and national benchmarking, and they have been proposed for public reporting. The calibration of the risk factors is based on the ratio between observed and expected result (O/E ratio), and calibration factors are updated quarterly. The expected mortality (E) is calibrated to obtain a national O/E ratio.

The European System for Cardiac Operative Risk Evaluation (EuroSCORE) is another widely used model for cardiac operative risk evaluation. It was constructed from an analysis of 19,030 patients undergoing a diverse group of cardiac surgical procedures from 128 centers across Europe ( Tables 1.3 and 1.4 ). The following risk factors were associated with increased mortality: age, female sex, elevated serum creatinine level, extracardiac arteriopathy, chronic airway disease, severe neurologic dysfunction, previous cardiac surgery, recent MI, reduced left ventricular ejection fraction (LVEF), chronic CHF, pulmonary hypertension, active endocarditis, unstable angina, procedure urgency, critical preoperative condition, ventricular septal rupture, noncoronary surgery, and thoracic aortic surgery. For a given individual, each of these risk factors is assigned a score, and the sum total of these is used to predict surgical risk. In 2003 a more sophisticated logistic version of EuroSCORE was released to permit more accurate risk assessment in individuals deemed to be at very high risk.

Table 1.3
Risk Factors, Definitions, and Weights (Score)
From Nashef SA, Roques F, Michel P, et al. European system for cardiac operative risk evaluation (EuroSCORE). Eur J Cardiothorac Surg. 1999;16:9.
Risk Factors Definition Score
Patient-Related Factors
Age Per 5 years or part thereof over 60 years 1
Sex Female 1
Chronic pulmonary disease Long-term use of bronchodilators or steroids for lung disease 1
Extracardiac arteriopathy One or more of the following: claudication; carotid occlusion or >50% stenosis; previous or planned intervention on the abdominal aorta, limb arteries, or carotids 2
Neurologic dysfunction Disease severely affecting ambulation or day-to-day functioning 2
Previous cardiac surgery Requiring opening of the pericardium 3
Serum creatinine >200 µmol/L before surgery 2
Active endocarditis Patient still under antibiotic treatment for endocarditis at the time of surgery 3
Critical preoperative state One or more of the following: ventricular tachycardia or fibrillation or aborted sudden death, preoperative cardiac massage, preoperative ventilation before arrival in the anesthesia room, preoperative inotropic support, intraaortic balloon counterpulsation or preoperative acute renal failure (anuria or oliguria <10 mL/h) 3
Cardiac-Related Factors
Unstable angina Rest angina requiring IV nitrates until arrival in the anesthesia room 2
Left ventricular dysfunction Moderate or LVEF 30–50% 1
Poor or LVEF <30% 3
Recent myocardial infarct (<90 days) 2
Pulmonary hypertension Systolic pulmonary artery pressure >60 mm Hg 2
Surgery-Related Factors
Emergency Carried out on referral before the beginning of the next working day 2
Other than isolated CABG Major cardiac procedure other than or in addition to CABG 2
Surgery on thoracic aorta For disorder of the ascending aorta, arch, or descending aorta 3
Postinfarct septal rupture 4
CABG, Coronary artery bypass graft surgery; LVEF, left ventricular ejection fraction.

Table 1.4
Application of EuroSCORE Scoring System
From Nashef SA, Roques F, Michel P, et al. European system for cardiac operative risk evaluation (EuroSCORE). Eur J Cardiothorac Surg. 1999;16:9, by permission.
95% Confidence Limits for Mortality
EuroSCORE Patients (N) Deaths (N) Observed Expected
0–2 (low risk) 4529 36 (0.8%) 0.56–1.10 1.27–1.29
3–5 (medium risk) 5977 182 (3.0%) 2.62–3.51 2.90–2.94
≥6 (high risk) 4293 480 (11.2%) 10.25–12.16 10.93–11.54
Total 14,799 698 (4.7%) 4.37–5.06 4.72–4.95
EuroSCORE, European System for Cardiac Operative Risk Evaluation.

In 2011 the EuroSCORE was recalibrated to keep up with the new evidence. The revised EuroSCORE, known as EuroSCORE II , permits more accurate risk estimation yet preserves the powerful discrimination of the original model. The EuroSCORE II is currently the recommended model for assessment of cardiac surgical risk. It can be accessed online ( www.euroscore.org/calc.html ) or downloaded as a smartphone application.

Consistency Among Risk Indices

Many different variables have been found to be associated with the increased risk during cardiac surgery, but only a few have consistently been found to be major risk factors across multiple and very diverse study settings. Age, female sex, LV function, body habitus, reoperation, type of surgery, and urgency of surgery were among the variables consistently present in most of the models ( Box 1.1 ).

Box 1.1
Common Variables Associated With Increased Risk for Cardiac Surgery

  • Age

  • Female sex

  • Left ventricular function

  • Body habitus

  • Reoperation

  • Type of surgery

  • Urgency of surgery

Although a variety of investigators have found various comorbid diseases to be significant risk factors, no diseases have been shown to be consistent risk factors, with the possible exception of renal dysfunction and diabetes. These two comorbidities were shown to be important risk factors in a majority of the studies ( Box 1.2 ).

Box 1.2
Medical Conditions Associated With Increased Risk

  • Renal dysfunction

  • Diabetes (inconsistent)

  • Recent acute coronary syndrome

Sources of Perioperative Myocardial Injury in Cardiac Surgery

Myocardial injury, manifested as transient cardiac contractile dysfunction (“stunning”), or AMI, or both, is the most frequent complication after cardiac surgery and the most important cause of hospital complications and death. Furthermore, patients who experience a perioperative MI have a poor long-term prognosis; only 51% of such patients remain free from adverse cardiac events after 2 years, compared with 96% of patients without perioperative MI.

Myocardial necrosis is the result of progressive pathologic ischemic changes that start to occur in the myocardium within minutes after interruption of its blood flow (eg, during cardiac surgery) ( Box 1.3 ). The duration of the interruption of blood flow, either partial or complete, determines the extent of myocardial necrosis, and both the duration of the period of aortic cross-clamping (AXC) and the duration of cardiopulmonary bypass (CPB) have consistently been shown to be the main determinants of postoperative outcomes.

Box 1.3
Determinations of Perioperative Myocardial Injury

  • Disruption of blood flow

  • Reperfusion of ischemic myocardium

  • Adverse systemic effects of cardiopulmonary bypass

Reperfusion of Ischemic Myocardium

Surgical interventions requiring interruption of blood flow to the heart must be followed by restoration of perfusion. Reperfusion, although essential for tissue and organ survival, is not without risk because of the potential extension of cell damage as a result of reperfusion itself. Myocardial ischemia of limited duration (<20 minutes) that is followed by reperfusion leads to functional recovery without evidence of structural injury or biochemical evidence of tissue injury. However, reperfusion of cardiac tissue that has been subjected to an extended period of ischemia results in a phenomenon known as myocardial reperfusion injury.

Myocardial reperfusion injury is defined as the death of myocytes, which were alive at the time of reperfusion, as a direct result of one or more events initiated by reperfusion. Myocardial cell damage results from restoration of blood flow to the previously ischemic heart and extends the region of irreversible injury beyond that caused by the ischemic insult alone. The cellular damage that results from reperfusion can be reversible or irreversible, depending on the duration of the ischemic insult. If reperfusion is initiated within 20 minutes after the onset of ischemia, the resulting myocardial injury is reversible and is characterized functionally by depressed myocardial contractility, which eventually recovers completely. Myocardial tissue necrosis is not detectable in the previously ischemic region, although functional impairment of contractility may persist for a variable period, a phenomenon known as myocardial stunning. Initiation of reperfusion after longer than 20 minutes, however, results in escalating degrees of irreversible myocardial injury or cellular necrosis. The extent of tissue necrosis that develops during reperfusion is directly related to the duration of the ischemic event. Tissue necrosis originates in the subendocardial region of the ischemic myocardium and extends to the subepicardial region of the area at risk; this is often referred to as the wavefront phenomenon. The cell death that occurs during reperfusion can be characterized microscopically by explosive swelling, which includes disruption of the tissue lattice, contraction bands, mitochondrial swelling, and calcium phosphate deposition within mitochondria.

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