MP323 - ACUTE MI Flashcards
myocardial infarction
HEART ATTACK
the interruption of blood supply to part of the heart, causing some heart cells to die
NSTEMI
non-ST-segment elevation MI
how does NSTEMI occur
occurs by developing a complete occlusion of a minor coronary artery or a partial occlusion of a major coronary artery previously by atherosclerosis.
this causes a partial thickness damage of heart muscle
STEMI
ST-segment elevation MI
how does STEMI occur
occurs by developing a complete occlusion of a major coronary artery previously affected by atherosclerosis
this causes full thickness damage of heart muscle
how many hours are required for substantial amount of myocardial tissue to be saved
6 hours
cellular changes associated with MI
- development of infarct extension (new necrosis)
- thinning and dilation of the infarct zone
- ventricular remodelling (enlarged heart)
begin within hours of MI and reach peak at 7 to 14 days
MI symptoms
- chest pain, nausea, vomiting, sweating, breathing difficulty
- substernal pain can radiate to neck, left arm, back or jaw
- not relieved by rest or GTN
- GI upsets from pain resulting in vagal stimulation
biochemical markers in MI
a rise and gradual fall (troponin) or more rapid rise and fall (creatine kinase MB) are indicators of myocardial necrosis with…
1. ischaemic symptoms
2. development of pathologic Q waves on the ECG
3. ECG changes indicative of ischaemia
creatine kinase MB
- enzyme found primarily in heart muscle cells
- used as biochemical marker
- catalyses the conversion of creatine and utilizes ATP to create phosphocreatine (PCr) and ADP
- enzyme reaction is reversible and so ATP can be generated
creatine kinase isoforms as biochemical markers
- CK-MB1 is the isoform found in plasma, CK-MB2 is found in tissues
- in MI patients, CK-MB2 levels rise, resulting in a CK-MB2 to CK-MB1 ratio greater than one
diagnostic tests - troponin
sensitive marker of myocardial necrosis
necessary for establishing the diagnosis of MI consistent with ischaemia
diagnostic tests - myoglobin
- myoglobin is an oxygen-binding protein found in skeletal and cardiac muscle
- myoglobin’s release from ischaemic muscle occurs earlier than the release of CK
- levels peak at 3 to 15 hours
- present in skeletal muscle so diagnostic value is limited
diagnostic tests - lactate dehydrogenase (LDH)
- LDH is an enzyme of the anaerobic metabolic pathway
- function of LDH is to catalyse the reversible conversion of lactate to pyruvate with reduction of NAD+ to NADH
- blood levels of LDH are usually low
- when tissues are damaged they release more LDH into the bloodstream
- made up of two sub-units; H = heart, M = skeletal muscle
electrical conduction
- sinoatrial node (SA)
- atrioventricular node (AV)
- bundle of His (common bundle)
- bundle branches
- Purkinje fibres
the sinoatrial node (SA node)
- in the right atrium
- natural pacer of the heart
- self-initiates electrical activity in the heart at the rate of 60 - 100bpm
the atrioventricular node (AV node)
- on the floor of the right atrium above the tricuspid valve
- electrical activity is delayed about 0.05sec which allows for atrial contraction and more complete filling of ventricles with blood
bundle of His
conducts electrical activity from AV node to bundle branches
Purkinje fibres
fine network that conduct the electrical impulses to the ventricular muscle
ECG components
P-wave
QRS complex
T-wave
P-wave (ECG)
represents firing of the SA node and depolarisation of the atria
PR-interval (ECG)
the delay of the electrical impulse at the AV node and the depolarisation of the atrium
PR segment (ECG)
the electrical conduction through the atria and the delay of the electrical impulse in the atrioventricular node
Q-wave (ECG)
first negative deflection
R-wave (ECG)
first positive deflection
S-wave (ECG)
second negative deflection
ST-segment (ECG)
the beginning of ventricular repolarisation, should be isoelectric (flat)
T-wave (ECG)
the ventricles repolarise and recover to their normal electrical state
what does an MI look like on an ECG
During the earliest stages, T-waves become tall and narrow - hyperacute stage
within a few hours, myocardial ischaemia results in the hyperacute T-waves inverting
inverted T-waves of ischaemia are symmetrical and relatively narrow
Q-waves (ECG)
- last stage of an MI, initial downward deflection of the QRS complex
- represent the flow of electrical forces toward the septum
- with MI usually 0.04 seconds or more in width
- Q-waves indicate tissue necrosis and are permanent
- provide ECG evidence of a previous MI
