Cardiovascular system

Published on 06/02/2015 by admin

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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

Test: Electrocardiogram (ECG)

Indication

To assess cardiac rhythm and to identify cardiovascular pathology contributing to surgical risk, e.g. previous infarcts, conduction defects, bundle branch block and strain patterns. National Institute for Clinical Excellence (NICE) guidelines indicate which patients require preoperative ECGs.

How it is done

See Fig. 3.4 for ECG lead placement.

Fig. 3.4 ECG lead placement.

Interpretation

Myocardial ischaemia

ST–elevation myocardial infarct (STEMI)

Defined by chest pain with ECG features as listed in Box 3.2 (see Fig. 3.6). In STEMI, ST elevation morphology evolves over time. Initially there are hyperacute ST changes, followed by the development of Q waves and T wave changes over days (Fig. 3.7).

Box 3.2 ECG criteria for ST elevation myocardial infarction

• Persistent ST segment elevation of ≥1 mm in two contiguous limb leads

• ST segment elevation of ≥2 mm in two contiguous chest leads

• New left bundle branch block

Fig. 3.6 An ECG showing STEMI in the anterior chest leads.

Fig. 3.7 Evolution of acute MI.

Non-STEMI

May result in ST segment depression (Fig. 3.8), transient ST elevation or T wave inversion. T wave changes are sensitive for ischaemia but less specific. T waves may become tall, flattened, inverted or biphasic.

Fig. 3.8 The various forms of ST depression: normal (A), flattened (B), planar (C) and downsloping (D).

Inverted T waves are normal in leads III, aVR and V1, in association with a predominantly negative QRS complex. T waves that are deep and symmetrically inverted strongly suggest myocardial ischaemia.

Q waves (Fig. 3.9) are pathological/abnormal if:

• >1/3 amplitude R wave (in mm)

• >1 mm (40 milliseconds) in duration and/or

• present in the right precordial leads (V1–3).

Fig. 3.9 ECG showing inferior lead ST elevation (i, iii, AVF) and the development of Q-waves.

They represent old MI/scar if in contiguous lead territories (see below).

Correlation between ECG leads and infarct territory

• Leads II, III and aVF – inferior (right coronary artery (RCA) or circumflex artery if nondominant RCA).

• Leads V1 to V3 – anteroseptal (left anterior descending artery).

• Leads I, aVL, V4–V6 – anterolateral (circumflex or dominant RCA).

Test: Exercise tolerance test (ETT)

Indications

• Noninvasive investigation for coronary ischaemia – sensitivity of ETT for detecting multivessel disease ∼81%. Most useful in patients with an intermediate likelihood of disease based on age, gender and symptoms.

• Risk stratification in patients post MI – as a predictor of the likelihood of a future cardiac event. Patients who achieve an estimated 7 METs (metabolic equivalent) or a heart rate of >130 beats/min in the absence of ischaemic ECG changes (ST depression) are considered low risk.

• Risk stratification of patients with hypertrophic cardiomyopathy – its negative predictive value for sudden death is 97% and in the young, in the absence of other risk factors, permits accurate reassurance.

Contraindications

Acute myocardial ischaemia, severe congestive cardiac failure, severe aortic stenosis, sustained ventricular arrhythmias, severe hypertension (systolic pressure >200, diastolic >110).

How it is done

Exercise on a treadmill/bicycle with simultaneous recording of heart rate, blood pressure and ECG. The Bruce protocol has seven 3-minute stages with increasing speed and incline. It aims to achieve a heart rate response of 85% predicted (220 − age).

The Modified Bruce protocol starts at a lower workload than the standard test, and is typically used for elderly or sedentary patients. The first two stages of the Modified Bruce Test are performed at a 1.7 mph and 0% grade and 1.7 mph and 5% grade, and the third stage corresponds to the first stage of the Standard Bruce Test protocol.

Data presented as

• Symptoms.

• Basic ECG interpretation.

• Reported reason for stopping test.

• Estimate of exercise capacity in METs.

• Blood pressure response.

• Presence of arrhythmias.

• ST changes (type and location).

Interpretation

Exercise test interpretation (Table 3.5)

Limitations

Diagnostic accuracy of the ETT is reduced by any abnormality in the resting 12-lead ECG – left ventricular hypertrophy, right ventricular hypertrophy, bundle branch block and pre-existing ST/T changes.

Table 3.5 Exercise test interpretation

Results	Interpretation
• Target heart rate achieved (220 − age men, 200 − age women) • Normal blood pressure response (increases) • No chest pain • No ST changes

Normal or negative

• Hypotension (>10 mmHg drop in BP in stage 1 or 2)

• ≥1 mm downsloping ST depression

• >1 mm ST elevation

• >2 mm upsloping ST depression

• U wave inversion

A positive test

Meta-analysis of the exercise test studies with angiographic correlates have demonstrated the standard ST response (1 mm depression) to have an average sensitivity of 68% and a specificity of 72% and a predictive accuracy of 69%.

Test: Biochemical markers of myocardial ischaemia

See Fig. 3.10.

Fig. 3.10 Time course of cardiac enzyme elevations.

Test: Troponin (normal range: Trop T 0.04–0.06 ng/mL, Trop I <0.01 ng/mL)

A complex made of three distinct proteins (I, T and C). In health the cardiac troponins are not detectable. They are highly sensitive markers of myocyte necrosis and are now in common usage. A blood sample is taken for monoclonal antibody testing to cardiac-specific troponin I and cardiac-specific troponin T 12 hours after onset of chest pain, and is currently the most widely used biochemical marker of myocardial injury.

Indications

To aid diagnosis of myocardial infarction. The troponin assay has prognostic information that can determine mortality risk in acute coronary syndromes (ACS) and guides urgency of angiographic intervention.

Limitations

False Positives – troponin and CK may be raised after prolonged arrhythmia, myocarditis, significant LVF and in patients with renal failure and pulmonary embolism.

Test: Creatinine-kinase-MB(CKMB) (Normal range 32–190 IU/L)

Until recently CKMB was the most commonly used iso-enzyme to detect myocardial injury. It remains a useful measure to determine re-infarction, as levels fall to normal only after 36–72 hours.

Echocardiography

Test: Transthoracic echocardiography (TTE) and transoesophageal echocardiography (TOE)

Indications

Structural and functional assessment of the heart and great vessels (Table 3.6).

Table 3.6 Echo parameters and clinical significance

Normal values	Comment
Left atrial diameter 3–4 cm	Atrial dilatation can be due to atrioventricular valvular pathology, diastolic dysfunction, interatrial shunts (consider if ventricular function is normal)
Left ventricular (LV) internal diameter (cm): diastole 3.5–5.9, systole 2.4–4.0	LV diastolic dimensions are increased if there is volume loading e.g AR, MR or cardiomyopathy.
LV thickness (cm): septum 0.8–1.3 males, 0.7–1.0 females	Septal thickening is seen in hypertrophic cardiomyopathy (usually 4–6 mm thicker)
LV ejection fraction ≥65%	50–65% mild impairment, 40–50% moderate impairment, <35% severe impairment
Fractional shortening 28–44%	Another quantitative assessment of contractility.

How it is done

• TOE – NBM, sedated, patient supine flat.

• TTE – awake/asleep, semirecumbant.

Physical principles

High-frequency vibration (1–10 MHz) emitted and received from a probe containing a series of piezo-electric crystals. Wave reflection occurs at interfaces between tissues of varying acoustic density and as the speed of ultrasound waves is known, the depth to the reflected surface can be calculated, when the time delay between emission and reception is known. This is displayed as a point on a screen, the magnitude of the point reflecting the strength of reflected signal and is known as B-mode scanning.

There are several imaging modalities but 2D and Doppler have the greatest utility in clinical medicine:

• 2D scanning (Fig. 3.11) refers to real-time imaging in two-dimensional views with multiple B-mode lines.

• Doppler scanning (Fig. 3.12) utilizes the change in frequency observed when ultrasound waves are reflected from a moving target (red blood cells). Change in wavelength is proportional to velocity. Blood flow can be measured at a precise distance from the ultrasound probe with pulsed wave Doppler (PWD), or at all points along the ultrasound beam, without localization known as continuous wave Doppler (CWD).

• Colour flow Doppler combines 2D images with PWD to produce a map of blood flow velocities and directions.

Fig. 3.11 Parasternal long axis view.

Fig. 3.12 Apical four chamber view.

Normal	Peak AVG <16 mmHg	nil
Mild	Peak AVG 20–30 mmHg	Mean F AVG <20 mmHg
Moderate	Peak AVG 31–50 mmHg	Mean AVG 20–30 mmHg
Severe	Peak AVG >70 mmHg	Mean AVG 40 mmHg

AVG, aortic valve gradient.

Mitral stenosis (Table 3.8)

Based on calculation of mitral valve gradient (MVG mmHg) or mitral valve area (MVA cm²).

Table 3.8 Severity of mitral valve disease based on valve pressure gradient

Mild	Mean MVG <5 mmHg
Moderate	Mean MVG 5–10 mmHg
Severe	Mean MVG >10 mmHg

MVG, mitral valve gradient.

Mitral regurgitation (Table 3.9)

Based on analysis of colour flow Doppler (CFD) and Continuous wave Doppler (CWD) profile. Graded from 1 (mild) to 4 (severe). Please see Table 3.9.

Table 3.9 Grading of regurgitation for mitral and aortic valve

0/4	None	No regurgitation
1/4	Mild	Regurgitant flow limited to immediately around the valve area
2/4	Moderate	Regurgitant flow occupies up to a third of the chamber
3/4	Moderate to severe	Regurgitant flow occupies up to 2/3 of the chamber
4/4	Severe	Regurgitant flow in most of the receiving chamber and flow reversal in pulmonary veins

Aortic regurgitation

Assessed in a similar way to MR, by CFD and CWD (see above):

• the pressure half time of the CWD regurgitant jet is measured and the shorter the half time the more severe the regurgitation

• <240 milliseconds is considered severe.

Limitations (Table 3.10)

Echocardiography is operator dependent and with TOE there is a 1–2% risk of significant complication including respiratory compromise, emesis, agitation, oesophageal rupture, haemorrhage and cardiac dysrhythmia.

Table 3.10 Comparison of TTE and TOE

Clinical indication	TTE useful	TOE useful
Atria and ventricle size, shape and function	+	+
Pericardial effusion	−	+
Myocardial thickness	+	+
Valve structure and function	+	++
Aortic arch	+	Limited
Left atrial appendage	−	+
Superior vena cava	Limited	+
Thoracic aorta	−	+
Clinical utility
Noninvasive	+	−
Image quality operator dependent	+	−
Can be performed in sitting position	+	−
Sedation required	−	+