ECG Flashcards
What does ECG stand for?
Electrocardiography
What does an ECG do and how?
- Monitors and records biopotentials produced by the heart during operation
- A non-invasive externally recorded signal from electrodes placed on the skin
What are the 2 biggest causes of death in men aged 25-64?
Malignant neoplasms (cancers) - 34%
and
Heart and other circulatory diseases - 28%
Total Blood Volume
4.5-5.5L
What are the 2 circulation types of the circulatory system?
Systemic
Pulmonary
How much blood is contained in the veins?
80%
What is the CMV?
Cardiac minute volume
- 5.6L/min
What is the CMV divided between?
Brain and other body organs according to importance and immediate need
What systems are responsible for function of the heart?
- autonomous pulse generator
- conducting system
- contractile system
How does the heart conduct?
- electrical impulse originates in the SAN (sino-atrial node)
- signal passed onto the AVN (atrioventricular node)
- propagation delayed so that blood flows into ventricles first
- pulse spreads quickly via Bundle of His (A-V bundles) and Purkinje fibres through ventricles
Where is the SAN?
Top of the right atrium
What are other names for the sinoatrial node?
Sinuatrial node
SA node
SAN
Sinus node
What is the role of the SAN?
- generates sinus rhythm
- group of cells
- wall of right atrium
- near SVC entrance
- modified cardiac myocytes
- have some contractile filaments but do not contract
- lack fast sodium current component
- this means slower calcium currents are responsible for sino-atrial upstroke
“Funny current” If
- means the membrane potential of the SAN never reaches a resting potential
- responsible for the pacing activity of the SAN
- called funny because it has opposite effects to those of most other heart currents
Who published the first human electrocardiogram?
1887
Augustus D. Waller
British physiologist
St Maru’s Medical School
Who invented the first practical electrocardiogram?
1903
Willem Einthoven
Dutch physiologist
What is the aim of an ECG?
- record electrical signals from the heart at body surface
- describe a unique voltage generator in the chest
- look at heart activity from different view points by placing electrodes in different places
Why may ECG results vary from subject to subject?
- body shape
- position of heart in chest
- posture and respiration
How can the electrical activity of the heart be examined?
- a projection onto 3 orthogonal planes (sagittal, frontal, transversal)
Frontal Plane electrode placement
LIMB LEADS
- attached to ends of limbs (wrists and ankles) OR origins of limbs (upper thigh and shoulders)
- makes no difference as limb viewed as a long wire conductor originating from a point on the trunk of the body
Transverse Plane electrode placement
CHEST LEADS
- defined locations
- V1-V6
What does Einthoven’s Triangle state?
I + (-II) + III = 0
he reversed polarity of lead II in the triangle
What does Einthoven’s triangle assume?
- heart is located in an infinite homogeneous volume conductor (or at center of homogenous sphere representing the torso)
Define a lead
The stretch between 2 limb electrodes
What are the limb leads?
Standard leads + augmented leads
Standard Leads
- 3 standard (I, II, III)
- BIPOLAR LEADS
- form a triangle
- heart electrically constitutes the null point
- reference electrode on the right foot
- utilise a single positive and single negative electrode between electrical points are measured
Augmented Leads
- measure from 1 electrode to central point of other 2 electrodes
- UNIPOLAR LEADS
What do the chest (precordial) leads do?
- UNIPOLAR LEADS
- each lead records electrical variations that occur almost directly under electrode
- unipolar leads (measured against reference of Wilson Central Terminal)
Wilson’s Central Terminal
- central terminal gives reference for precordial unipolar
- formed by connecting a 5Kohm resistor from each terminal of he limb leads to a common point called the central terminal
- = (I + II + III)/3
Unipolar Leads
- augmented and precordial
- have a single positive recording electrode
- augmented leads utilise central point of other 2 electrodes as a composite negative electrode
- precordial leads utilise combination of 3 bipolar leads points to give a virtual zero potential as a composite negative electrode
- Wilson’s central terminal
Bipolar leads
- three standard
- utilise single positive and single negative electrode between which electrical potentials are measured
What are 3 and 5 lead ECGs?
- for continuous monitoring and viewed only on the screen of an appropriate monitoring device (e.g. during an operation or whilst being transported in an ambulance)
Recording depolarisation
- Position deflection = a wave of depolarisation heading toward towards the positive electrode
- Maximal deflection = if wave of depol travels parallel to lead axis
- EG = wave of depol travels towards the left leg produces a positive deflection in both leads II and III because positive electrode for both leads is on left leg
QRS axis
- represents net overall direction of heart’s electrical activity
- average all electrical signals from heart
- direction of activity based on axial reference system
- can inform reader of changes in sequence of ventricular activation
QRS abnormalities
- ventricular enlargement
- myocardial damage
- conduction blocks (hemiblocks)
Range of QRS axis
-30 to +90
LAD
-30 to -90
RAD
+90 to +180
How to determine the QRS axis?
- in the lab use leads I and III
- measure height of R wave and QRS complex and plot of grid paper
- use intersect to determine heart axis
