CARDIAC MARKERS

IN ACS AND AMI

G M KELLERMAN PATHOLOGY NORTH HUNTER SERVICE

TISSUE MARKERS

• Purpose – “What organ is the problem?” “How bad is it?”

• Sensitivity – what proportion of the affected patients do we find?

• Specificity - what proportion of unaffected patients do we identify

correctly?

• Need – high sensitivity (miss very few) and high specificity (not too

many false positive results)

• There are very few tests that fulfil this ideal character

• Second purpose – having identified the organ and the degree of

malfunction, does the test result predict which of the many possible

pathological processes is occurring in the specific patient?

• There are even fewer tests that fulfil this wish

• THERE IS NO MAGIC SOLUTION – YOU MUST CORRELATE

THE CLINICAL PICTURE WITH THE TEST RESULTS

• UNFORTUNATELY MANY DOCTORS BELIEVE IN MAGIC

WHY DO CARDIAC MARKERS?

• ORIGINALLY USED AS A GUIDE TO THE DIAGNOSIS

OF MODERATE TO BIG MYOCARDIAL INFARCTION

• GRADUALLY EXTENDED TO HELP WITH SMALLER

MYOCARDIAL INFARCTS AND DELAYED DIAGNOSIS

• TECHNOLOGY TO MEASURE TROPONINS ENABLED

MUCH SMALLER INFARCTS TO BE IDENTIFIED

• IMPROVING TECHNOLOGY ENABLES RECOGNITION

OF OTHER CAUSES OF TROPONIN LIBERATION

SMALL TROPONIN INCREASES NOW NEED MUCH

THOUGHT AS THEY INDICATE MYOCARDIAL

DAMAGE, BUT DO NOT IDENTIFY CAUSE

OLD MARKERS

• 1950’s – aspartate transaminase (AST). Also found in liver and

skeletal muscle so not specific, and sensitive only for big infarcts

• 1960’s – lactate dehydrogenase (LD). Found in all tissues so very

non specific.

• 1960’s – creatine kinase (CK). Far more in skeletal muscle, small

amounts in other tissues, so not specific.

• 1970’s – creatine kinase cardiac isoenzyme (CK-MB). More specific

as only trace amounts in skeletal muscle and other tissues but

sensitivity limited for small lesions as tests not reliable

• 1980’s – troponins, constituents of the muscle fibre identified and

became measurable. 2 of them have different detailed amino acid

composition in cardiac and skeletal muscle so the condition of tissue

specificity is satisfied.

TROPONINS (1)

All muscle moves by having myosin slide over actin filaments

using the energy of ATP to drive the movement

This has to be controlled to happen only when needed

The control is exerted by allowing Ca++ ions to access the appropriate site on the enzymatic part of the myosin when contraction occurs

The Ca++ binds to a receptor protein (calmodulin) which is part of a three-unit complex called TROPONIN

Troponin C = calmodulin, which binds the Ca++

Troponin I inhibits the myosin ATPase until Ca++ binds to

the Troponin C component, which changes protein interactions

and permits contraction to occur

Troponin T anchors the complex to Tropomyosin

Tropomyosin is a long thin structural protein linked to the actin filaments

MUSCLE CONTRACTION

• A C T I N

• M Y O S I N

• ATPASE

• TROPONIN I

• C Ca++

• T

• ……...T R O P O M Y O S I N………

TROPONINS (2)

The especial value of troponins is that the genetic coding for both

Troponin T and Troponin I is different in heart from other tissues

resulting in differences in their polypeptide chains

It has therefore been possible to manufacture antibodies that are completely specific for cardiac Troponins T or I, thus making

specific immunologically based tests available

The additional advantage is that there is a extremely small

concentration of either of these troponins in normal plasma so that

any significant elevation of cardiac troponin T or I is pathological

Recent research with high sensitivity methods using improved antibodies in the assay shows that there is a minute but measurable

amount of Troponin in plasma of normal people (<16 ng/L in females, <26 ng/L in males)

TROPONINS (3)

The maximum troponin level that I have seen in an untreated patient

who has survived an AMI is of the order of 400,000 ng/L

whereas the normal level is not > 16 ng/L (F) or 26 ng/L (M)

These values are probably very similar for both Troponin T and I

So tiny bits of cardiac myocyte necrosis are detectable (<0.1 g)

In our laboratory we measure Troponin I because Troponin T

requires a special machine for which we have no room!

Quality control is better for Troponin T because Troponin I exists in

several different molecular forms in plasma and breakdown products

occur, so that different brands of reagents give different results

Therefore you must consult the laboratory reference range

However, Troponin I results are probably at least 99% as reliable as

Troponin T in the clinical arena

TIME RELATIONS (1) When and for how long is each test positive?

Cardiac muscle stops contracting about 1 minute after blood flow stops, but it takes about 30 minutes of fatal ischaemia for the muscle to

start dying. This is followed by release of components

Smaller proteins are released earlier than larger ones

It is important to realise that no biochemical test becomes positive under 2-3 hours after onset of severe enough vascular blockage

Revascularisation strategies – thrombolysis or angioplasty – must be started earlier than that to get maximal effect, although some

benefits are found up to about 6 hours post-infarction

So need to rely on clinical features and ECG for early diagnosis

The cardiologist’s motto:

TIME IS MUSCLE (Compare the neurologist treating stroke by thrombolysis – TIME IS BRAIN)

0 1 2 3 4 5 6 7 10 0

5

10

15

20

Multiples of upper

reference limit

Days after onset of AMI

Source: A. H. Wu,

Journal of Clinical Immunoassay (1994) 17, 45-48.

Kinetics of AMI markers

Myoglobin

LD

Troponin T

Troponin I

CKMB

TIME RELATIONS (3)

Note that AST, CK, CK-MB, and both Troponins all

rise at more or less the same time, starting at

about 3-6 hours and peaking at 24-36 hours

Recent work suggests that with the supersensitive new methods, Troponin rise may be reliably detectable in 3

hours if the infarct is of moderate or large extent

After that, CK-MB falls a little faster than CK total, but troponins fall far more slowly, taking up to 2 weeks to

return to baseline, as does H-type LD

So some days after an AMI there is still evidence available to show that it happened in Troponin levels – LD is now

obsolete

IMPORTANCE OF SMALL INFARCTS

Research has shown that the occurrence of further cardiac episodes during the following year or more is much more frequent if there has

been ANY rise in troponin than if there has been no increase

The arbitrary time of 8 hours post onset was the time interval allowed to elapse before the test in these cases, which used to be referred to

as “Unstable Angina Pectoris” or UAP, but financial pressures in ED departments exert pressure to make an earlier decision

This is the basis of risk stratification and of decisions on the intensity of therapy to be instituted in patients in whom an AMI is suspected

(and also valuable in the differential diagnosis of chest pain)

In principle (but doctor’s judgment can override this)

No rise = low risk, can send home on aspirin, return for follow up tests (e.g. stress test, echocardiography, nuclear imaging) as outpatient

Any rise = high risk, admit for at least anticoagulation

WHY DO AMI PATIENTS DIE?

• Sudden death is usually due to cardiac arrest – possibly ventricular asystole

following AV node ischaemia, more often ventricular fibrillation due to

disturbed conduction pathways.

• With large parts of ventricles not contracting, cardiac output falls and the

body is inadequately perfused. If severe, this can lead to death in

“cardiogenic shock” with lactic acidosis, often within hours.

• Less severe output failure is followed by severe sympathetic nerve activity

and vasoconstriction, especially in kidneys and splanchnic region, leading to

necrosis of kidney and/or liver, which may not recover or which may lead to

long and slow convalescence.

• Large dead areas and/or severe remodelling changes following ongoing

inadequate perfusion may lead to ongoing congestive cardiac failure with

inadequate body perfusion, oedema etc and gradual decline and death.

• Finally, where there has been one episode resulting from atherosclerosis,

there can always be a new one – and any clot can always extend.

TROPONINS (4) - SUMMARY

• CAUSES OF TROPONIN RISE • 1. Classical ST elevation MI – much necrosis. Big rise

• 2. Non ST elevation MI – various sizes, with or without ECG changes

• 3. Acute coronary syndrome, no other demonstrable changes, but it has been shown that even a small Troponin rise means a worse prognosis

• 4. Non ischaemic disease of heart – acute failure with large pulmonary embolus viral or other myocarditis

myopathy, e.g. catecholamine or cocaine

renal failure – probably from hypertension

5. Reasonably severe disease elsewhere – e.g. sepsis

6. There is increasing evidence that silent, gradual myocyte death in patients with conditions that lead to reduced myocardial reserve, without any acute symptoms at any time, may also give rise to small irregular increases in Troponin. We find too many such small rises in asymptomatic patients to be explained by the other listed causes. There has recently been some supporting evidence from Sweden, that patients whose “normal healthy” troponin levels are in the upper part of the “normal range” have an increased risk over a 10-year period of follow up

7. Remember that after thrombolysis the washout of Troponin and other cardiac enzymes is rapid and leads to higher values in plasma, often of shorter duration.

8

SUPERSENSITIVE TROPONIN

• IN RECENT YEARS MORE AND MORE SENSITIVE TROPONIN ASSAYS HAVE BEEN DEVELOPED SO THAT WE CAN NOW DETECT TRACES OF TROPONINS IN NORMAL PEOPLE, AND ESSENTIALLY ALL LABORATORIES ARE NOW ADOPTING THESE METHODS

• BY CUSTOM, CURRENT “ABNORMAL” VALUE STARTS AT THE UPPER 99TH PERCENTILE OF NORMAL, ABOUT 16 NG/L (F), 26 NG/L (M)

• ACUTE CORONARY SYNDROME STARTS AT TROPONIN > THIS LEVEL, + APPROPRIATE SYMPTOMS, ± ECG CHANGE, + INCREASE IN TROPONIN OVER NEXT 6-8 HOURS OF > 50%

• THERE IS STILL ARGUMENT ABOUT CUTOFFS!

• THERE IS ALSO A LOT OF ANXIETY IN ED’S AND CARDIOLOGY UNITS ABOUT THE NEED TO ACCEPT THAT SMALL TROPONIN RISES DO NOT EQUATE TO THE ACUTE CORONARY SYNDROME, AND THAT THOUGHT IS ESSENTIAL

BNP (brain natriuretic peptide)

• There is a group of peptides which stimulate the kidney to excrete more Na and

water, to reduce intravascular overload

• ANP – atrial natriuretic peptide – is secreted mainly by atria, when stretched by

excessive blood volume within them

• BNP – brain natriuretic peptide - was first found in pig brain, but it is now known that

there is a lot in the ventricular wall, released when the ventricular muscle is

overstretched (e.g. with increased end-diastolic volume in congestive cardiac failure)

• CNP – no ideas on its real role yet

• BNP is synthesised as a precursor protein, pro-BNP, which loses its N-terminal part

(NTpro-BNP) to release BNP. The inactive other fragment NTpro-BNP is also

released into the circulation and can be measured

• There is no real agreement yet as to whether BNP or NTpro-BNP is the better marker

for disturbance of ventricular function, probably BNP especially in old people

• Our laboratory currently measures BNP, but we must batch it and do it 3 times a

week so it is not available on an urgency basis. This may change if the number of

requests increases

BNP

• In health the plasma level is <100 ng/L

• In severe congestive cardiac failure, levels of several hundred are

found, and >800 ng/L is virtually diagnostic

• Values between 100 ng/L and 800 ng/L must be interpreted in the

clinical scenario, the higher the value the more likely that the

ventricles are in failure

VALUE OF TEST

• To sort out congestive cardiac failure from other causes of dyspnoea

in patients with multiple pathologies especially lung problems

• To monitor the outcome of therapy for congestive failure – if it

decreases significantly and steadily it suggests that the condition is

responding to the treatment

• To date it is not a reimbursable test and is expensive to the patient

C-REACTIVE PROTEIN (CRP)

• A marker of inflammatory processes

• An acute phase reactant whose concentration in plasma is usually somewhere between 0.1 to 10 mg/L (median level about 1.5 mg/L)

• Concentration increases rapidly and manyfold with inflammation (up to > 400 mg/L) depending on intensity and severity

• WHEN ALL OTHER CAUSES OF INFLAMMATION CAN BE EXCLUDED, the risk of cardiac events over the next several years increases with increased baseline CRP levels in the range of about 0.1-4 mg/L

• No value in acute episodes of suspected AMI

• It appears to be an independent risk factor for cardiac disease, additional to Cholesterol and other known risk factors

MY FAVORITE STRESS TEST

• A CLIMB OF 300 METRES IN HEIGHT IN

3 KILOMETRES OF WALKING AT A

BAROMETRIC PRESSURE OF 580-600

mm Hg

• PaO2 approx 60 mm Hg

• CRITERION FOR NORMALITY – NO

ANGINA AND ABILITY TO USE CAMERA