These are my notes from my 3-hour 'Introduction to ECGs' Lecture. It involves some basic physiology surrounding myocytes and the contraction the heart, electrolytes and why they are applicable. It then explains depolarisation and repolarisation of the heart and the conduction system of the heart. I...
Cardiac cells have 2 basic functions – to conduct electrical impulses and to contract.
Muscle cell (myocyte) contract at 60-100bpm. They are elongated, branching cells contain 1-2
centrally located nuclei containing actin and myosin.
Myocytes contain an abundance of
contractile proteins; actin (thin filaments)
and myosin (thick filaments). They also
have the proteins troponin and
tropomyosin.
Mitochondria make the heart resistant to
fatigue by taking energy from the food we
eat and giving it to the cells. Therefore,
someone with heart failure have a low level
of mitochondria. People who smoke also
are at risk as they damage their
mitochondria.
Intercalated discs are specialized cell-cell contacts. They allow impulses to move from one cell to the
next easily. The gap junctions within the intercalated discs, allow this impulse to move.
To see if a patient (PT) has damage to their heart, in-hospital clinicians can do a blood test which will
show levels of troponin. As paramedics you will take an in-depth history (hx) and an ECG.
Electrolytes
Electrolytes are not just in cardiac muscle
but are present throughout the whole
body. The balance of electrolytes is
essential for normal function to our cells.
Chemically speaking, they are substances
that become ions in solution and acquire
the capacity to conduct electricity (i.e.,
sodium, potassium, and calcium). Sodium
and calcium predominantly exist in the
extracellular space whereas potassium is
intracellular. They flush in and out, this
exchange in voltage is known as an action
potential.
The heart muscle cannot pump unless
there is an electrical stimulus first. The
movement of the ions is this stimulus.
For this stimulus to occur, and therefore a
heartbeat, it is dependent on 4 factors:
Automaticity (special to cardiac muscle) – cells ability to spontaneously initiate an impulse.
It is involuntary and does not require us to initiate the impulse.
Excitability – results from ion shift across cell membrane. Myocytes are very excitable.
, Introduction to ECG’s
Conductivity – ability to transmit and electrical impulse.
Contractility – how well cells contract after receiving a stimulus. This is down to the actin
and myosin.
Myocytes are divided into
cells that make up the
muscle:
Pacemaker cells –
they spontaneously
generate electrical
impulses; these cells
are found in the SA
node.
Conducting cells –
these are found in
the AV node, bundle
of HIS and Purkinje
fibres.
Cardiomyocytes - make up all the rest of the heart.
Depolarisation and repolarisation
These are electrical activities that cause
muscular activity. Once an electrical cell
generates an electrical impulse, the
impulse causes the ions to cross the cells
membrane which causes an Action
potential. This is known as depolarisation.
Depolarisation – a contraction that
moves as a wave, through the heart.
Repolarisation – a relaxation state of the
cardiac muscle.
P wave = atrial depolarisation. The
contraction of the atria.
PR interval = impulse from atria to
ventricles.
QRS complex = ventricular depolarisation. The squeezing of the ventricles. Caused by sodium rushing
in.
T wave = ventricular repolarisation
*PT’s with certain types of cancers can have high levels of calcium, which changes the morphology of
the QRS complex.
Sodium predominantly effects the QRS whereas potassium predominantly effects the T wave.
The Conducting system
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