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Medical Biochemistry > Cardiac function tests > Flashcards

Cardiac function tests Flashcards

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USMLE® Step 1 flashcards
1
Q

Enzyme activity:
Rate of change in absorbance
Give an example

A

e.g. aspartate aminotransferase.

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2
Q

e.g. aspartate aminotransferase.

A

Aspartate + 2-oxoglutarate glutamate + oxaloacetate
Oxaloacetate + NADH + H+ malate + NAD+

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3
Q

Aspartate aminotransferase activity=

A
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4
Q

What is the extinction coefficient for NADH

A

6.22mM-1CM-3

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5
Q

Amount of protein / enzyme:

A
  • Immunological methods e.g. ELISA.
  • Enzyme-linked immunosorbent assay.
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6
Q

Assay of serum biomarkers: Metabolities

A
  • Enzymatic or chemical conversion to a coloured product.
  • Spectrophotometric measurements.
  • Beer-Lambert law.
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7
Q

What is an acute Myocardial Infarction?

A
  • Imbalance between myocardial oxygen supply and demand.
  • Results in muscle injury / death.
  • Often caused by coronary thrombosis.

Vessels become blocked because of fissure / rupture of the intimal surface of an atheromatous plaque.

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8
Q

What is an acute Myocardial Infarction?

A

The reduced supply of oxygen to the heart causing tissue death.

  • Imbalance between myocardial oxygen supply and demand.
  • Results in muscle injury / death.
  • Often caused by coronary thrombosis.

Vessels become blocked because of fissure / rupture of the intimal surface of an atheromatous plaque.

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9
Q

Coronary heart disease

A
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10
Q

Diagnosis of AMI

A
  • Subjects presenting with acute onset chest pain.

May have suspected acute coronary syndrome or may have pain from another organ.

  • Acute coronary syndrome may be due to AMI or unstable angina with no accompanying tissue damage

Improved by rest or vasodilators such as glyceryl trinitrate.

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11
Q

True or False: Only 75% of patients with AMI show any diagnostic changes in ECG

A

True

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12
Q

Explain the diagnosis of AMI

A
  • Typical abrupt and total loss of coronary blood flow results in ST segment elevated MI (STEMI) with a typical ECG (electrocardiogram).
  • Partial loss of coronary perfusion can give rise to non-ST segment elevated MI (NSTEMI) with changes in T wave or ST depression or no change at all.
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13
Q

Assay of Troponins

A
  • All assays use a capture antibody and a detection antibody with some sort of label attached.

Only one Troponin T assay (Roche patent).

  • Several Troponin I assays using different antibodies.
  • Lack of primary reference troponin I makes standardisation difficult.
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14
Q

Which ones can be used as biomarkers for AMI?

A

Use of troponin I and troponin T has caused a redefinition of AMI.

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15
Q

Give an overview of ELISA

A

ELISA (enzyme-linked immunosorbent assay) is a plate-based assay technique designed for detecting and quantifying soluble substances such as peptides, proteins, antibodies, and hormones

  1. ELISA works on the principle that specific antibodies bind the target antigen and detect the presence and quantity of antigens binding. In order to increase the sensitivity and precision of the assay, the plate must be coated with antibodies with high affinity.
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16
Q

Serum troponins as biomarkers

A
17
Q

What is heart failure?

A
  • Failure of the heart to fill (diastolic) or eject (systolic) blood
  • Can use B-type naturetic peptides as biomarkers.
18
Q

NICE guidance on troponins

A
  • Recommended biochemical marker for diagnosing AMI is troponin (I or T).
  • Take at least two samples: one on admission and one 10-12 hours after the onset of symptoms.
  • One component alongside evidence from the nature and timing of clinical symptoms, ECG, or imaging tests, to make a diagnosis.
  • Other causes of troponin release include myocarditis (inflammatory disease of the myocardium) or pulmonary embolism (which leads to right ventricular dysfunction).
19
Q

Are troponins good biomarkers?

A
  • Tissue specific (i.e. only present within one tissue).
  • Rapid release in response to damage.
  • Very low in serum of normal individuals.
  • Easily measured and with clear and universal reference values.
  • Main problem is in deciding what is an abnormal result.
  • Each laboratory should ideally establish the 99th percentile reference limit.
20
Q

Is creatine kinase MB a good biomarker?

A
  • Enzyme assay with immuno-inactivation of the M subunit so that only the B subunit activity is measured.
  • Better is a mass immunoassay using monoclonal antibodies.
  • Tissue specific (i.e. only present within one tissue).
  • Rapid release in response to damage.
  • (Very) low in serum of normal individuals.
  • Easily measured and with clear and universal reference values.
22
Q

Could we use a biomarker of cardiac tissue damage to differentiate unstable angina, STEMI and NSTEMI?

A
22
Q

Isoforms of Creatine Kinase

A
  • Three genes: CK-M, CK-B and CK-Mt (mitochondrial enzyme)
  • Enzymes exist as dimers, which one would you use for AMI detection?
  • Creatine kinase MB may be specific for heart, but the proportion of CK-MB can increase in muscle in trauma, extreme exercise training or in disease (muscular dystrophy).
23
Q

How do we measure biomarkers of cardiovascular function?

A

Immunoassays

Usually an ELISA

24
Q

Is a BNP a good biomarker?

A
  • Serum level predominantly reflects stress on the atrial and ventricle wall.
  • Production and release induced in heart failure. BNP acts a hormone to reduce the ‘load’ on the heart.
  • Raised NT-proBNP or BNP should lead to echocardiography (ultrasound) to confirm diagnosis.
  • Increase in naturetic peptides can occur other than in heart failure (e.g. aortic stenosis).
  • A normal NT-proBNP or BNP results has strong negative predictive value for heart failure.

NB: this is a biomarker which is induced not released.

24
Q

What is the principle of ELISA?

A

An antibody is attached to a polystyrene plate which is a solid surface and is attracted or has an affinity towards bacteria, other antibodies and hormones.

A microtiter coated with antigen is filled with this antigen-antibody mixture after which free antibodies are removed by washing.

A second antibody specific to primary antibody is added which is usually conjugated with an enzyme.

Free enzyme-linked secondary antibodies are removed by washing the plate.

Finally, the substrate is added. The substrate is converted by the enzyme to form a coloured product, which can be measured by spectrophotometry.

25
Q

What are the different types of ELISA?

Which is most sensitive and specific?

A
  • Direct ELISA
  • Indirect ELISA
  • Sandwich ELISA
  • Competitive ELISA

Sandwich ELISA is most sensitive and specific

27
Q

Describe how you would find out the concentration of an unknown antigen using a standard curve

A
  1. Start with a sample of known concentration
  2. Preform a serial dilution
  3. Plot a standard curve
  4. Measure your sample and see the absorbance
  5. Read off the standard curve to find out the concentration of the unknown antigen
28
Q

How does a reduced oxygen supply lead to a myocardial infarction?

A
  • Heart muscle requires a large amount of energy
  • A lack of oxygen inhibits cellular respiration so less ATP is produced
  • Muscles of the ventricles are not able to contract to supply blood to either of the systemic and pulmonary circulation
  • Results in muscle injury and death of cardiac muscle
29
Q

Describe an ECG of someone with a heart condition (STEMI)

A

STEMI causes can include: Buildup of plaque – fat, cholesterol and other cellular waste – on artery walls

It is a compensatory mechanism. If sustained over a long period of time, results in cardiovascular hypertrophy which reduces cardiac output.

  • STEMI (ST elevated myocardial infarction)
  • ST is the point where the heart (ventricles) are contracted
  • this segment will no longer be flat but will appear abnormally elevated.

STEMI can be treated with “clot-busting” drugs called thrombolytics (also called fibrinolytics) or with a primary percutaneous coronary intervention (PCI) in a cardiac catheterization lab

30
Q

What is BNP?

A

Brain derived neurotrophic factor

Both BNP and NT (Natriuretic peptide)-proBNP are released by cardiomyocytes in response to changes in pressure and lack of oxygen delivered to the the heart. These changes can be related to heart failure and other cardiac problems. Levels goes up when heart failure develops or gets worse, and levels goes down when heart failure is stable

Compensatory mechanism:

Kidneys excrete more sodium in the urine and water follows via its osmotic potential

Medical Biochemistry (10 decks)

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