Cardiovascular system

Published on 06/02/2015 by admin

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Last modified 06/02/2015

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TOPIC 3 Cardiovascular system

Perioperative cardiac risk assessment

Indications

• Identify those at significant risk of developing perioperative cardiac complications.

• Prioritize investigation and treatment prior to surgery.

Test: Risk assessment scoring

In 1977 Goldman and colleagues developed a preoperative cardiac risk index (Table 3.1) based on nine clinical factors to give a cumulative risk score, predicting outcome after noncardiac surgery.

Table 3.1 Goldman risk prediction index

Risk factor	Score
Third heart sound (S3)	11	Score >25 56% risk of death 22% risk severe CVS complications <25 4% risk of death 17% risk of severe CVS complications. <6 0.2% risk of death 0.7% risk of severe CVS complications

Elevated jugular venous pulse 11 Myocardial infarct within 6/12 months of surgery 10 Heart rhythm other than sinus rhythm 7 ECG with >5 premature ventricular beats 7 Age >70 5 Emergency surgery 4 Intrathoracic/intra-abdominal or aortic aneurysm surgery 3 Poor general health status or bed ridden 3

In 1986 this was modified by Detsky (Table 3.2) to include angina, suspected aortic valve disease and pulmonary oedema. Based on this model patients are stratified as low, intermediate or high risk for a cardiac event.

Table 3.2 Detsky’s modified cardiac risk index

Factor	Risk
Age older than 70 years	5
Myocardial infection within 6 months	10
Myocardial infection after 6 months	5
Canadian Cardiovascular Society Angina Classification^*
Class III	10
Class IV	20
Unstable angina within 6 months	10
Alveolar pulmonary oedema
Within 1 week	10
Any history of pulmonary oedema	5
Suspected critical aortic stenosis	20
Arrhythmia
Rhythm other than sinus plus atrial premature beats	5
More than five premature ventricular beats	5
Emergency operation	10
Poor general medical status	5

Class	Points	Cardiac risk
I	0–15	Low
II	20–30
III	31+	High

* The Canadian Cardiovascular Society Angina Grading Scale is commonly used for the classification of severity of angina: Class I – angina only during strenuous or prolonged physical activity; Class II – slight limitation, with angina only during vigorous physical activity; Class III – symptoms with everyday living activities, i.e., moderate limitation; Class IV – inability to perform any activity without angina or angina at rest, i.e., severe limitation.

The American College of Cardiology (ACC)/American Heart Association (AHA) provide a structured evidence-based approach to perioperative cardiovascular risk evaluation, which incorporates clinical predictors, functional capacity (see below) and surgery-specific risks.

Metabolic equivalent task (MET)

METs are a measure of functional capacity, which estimate the energy requirement to carry out activities of daily living (Table 3.3). One MET is defined as the average resting oxygen uptake for a 70-kg male and is equal to approximately 3.5 mL/kg/min. Assessment predicts a patient’s exercise capacity, which may contribute to patient risk assessment.

Table 3.3 MET (metabolic equivalent) values

No of METs	Activity
2 METs	Eat, dress or use the toilet. Walk indoors around the house. Walk on level ground at 2–3 mph or 3.2–4.8 km/h
4 METs	Light work around the house like dusting or washing dishes. Climb a flight of stairs or walk up a hill. Walk on level ground at 4 mph or 6.4 km/h. Participate in moderate recreational activities like golf, bowling
>10 METs	Participate in strenuous sports like swimming, singles tennis, football, basketball or skiing

Adapted from the Duke Activity Status Index and AHA Exercise Standards.

The AHA/ACC guidelines suggest that patients unable to meet a 4-MET demand are at increased perioperative and long-term risk.

Test: Cardiopulmonary exercise testing (CPEX)

Indications

• Allows a functional capacity assessment of the cardiopulmonary unit and determines its ability to deliver oxygen (DO₂) during exercise.

• Enables identification of high-risk populations who might benefit from invasive monitoring and cardiac optimization to improve DO₂ and surgical outcome.

How it is done

The subject breathes into a mouthpiece whilst exercising on a bicycle or treadmill to a predefined protocol. Total oxygen uptake (VO₂), minute ventilation (VE) and carbon dioxide production (VCO₂) are measured by continuous respiratory gas analysis, as are blood pressure and ECG.

Interpretation

VO₂max (Fig. 3.1): Represents maximal oxygen uptake during exercise of increasing intensity. Expressed in mL/kg/min, VO₂max is a function of both the maximal cardiac output and the maximal tissue extraction of O₂. Under exercise conditions, oxygen consumption becomes a linear function of cardiac output. This measurement is therefore an indirect measure of ventricular function.

Fig. 3.1 Maximal oxygen uptake (VO₂) during exercise of increasing intensity as a measure of ventricular function and oxygen delivery (STPD = standard temperature (0°C), barometric pressure at sea level (101.3 kPa) and dry gas: standard temperature and pressure, dry).

Anaerobic threshold (AT) (Figs 3.2 and 3.3): This is the point during exercise at which anaerobic metabolism is used to supplement aerobic metabolism as a source of energy. In exercise, when lactate is produced it is buffered by bicarbonate, leading to increased production of CO₂. This causes a rise in VCO₂, which exceeds the rise in VO_2, therefore the VCO₂/VO₂ ratio increases.

Fig. 3.2 Anaerobic threshold can be measured as the point at which the patient’s gas analysis (red line) during exercise fails to track the normal relationship between oxygen consumption and carbon dioxide production during increasing aerobic metabolism (brown line).

Fig. 3.3 Implications of anaerobic threshold (AT) with respect to perioperative cardiovascular risk.

(Adapted from Older P et al. Chest 1999. 116(2) 355–62 Cardiopulmonary exercise testing as a screening test for perioperative management of major surgery in the elderly.)

An AT of >11 mL/min/kg predicted postoperative survival with a high sensitivity and specificity. Cardiovascular death was virtually confined to patients with an AT <11 mL/min/kg. Older P. Chest 1999. 116(2)355–62