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Introduction to medicine (Year 1) > Intro to ECG > Flashcards

Intro to ECG Flashcards

1
Q

What is an electrocardiogram?

A
  • The electro/chemical changes that cause myocardial contraction, shown by the cardiac action potential
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2
Q

How do cardiac cells contract?

A
  • The potential difference across the cellular membrane must change from negative to positive in relation to the inside of the cell (this initiates an action potential)
  • Changes in potential difference occur through the flow of ions through specialised ion channels in the cellular membrane and also relatively freely through gap junctions
  • Depolarisation initiates at SAN due to nature of SAN not having a stable resting potential, and it also depolarises very quickly
  • The myocardium (cardiac muscle) has electrical currents that sequentially depolarise each individual cell, resulting in a change in cellular morphology that allows muscle contraction
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3
Q

Describe the cardiac electrical field

A

The electric dipole (lead) consists of 2 equal and opposite charges, +q and -q, separated by a distance d. The dipole is a vector (has magnitude and direction)

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4
Q

Where are the leads placed on the frontal plane?

A

6 limb leads

  • Right arm
  • Left arm
  • Right leg
  • Left leg
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5
Q

What are the 6 limb leads in the frontal plane and state whether they are bipolar or unipolar

A
  • Lead I - Bipolar
  • Lead II - Bipolar
  • Lead III - Bipolar
  • aVR - Unipolar
  • aVL - Unipolar
  • aVF - Unipolar
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6
Q

What does it mean if a lead is bipolar or unipolar?

A
  • Unipolar leads- register activity in the heart which is directed towards, or located below the electrode
  • Bipolar leads- register the voltage between 2 electrodes
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7
Q

How are the aVR, aVL and aVF leads placed?

A

Placed to create an equilateral triangle with the heart in the middle - This is the Einthoven triangle

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8
Q

Describe the aVR lead

A

An augmented and unipolar limb lead, constructed to obtain specific information from the right upper portion of the heart, including the outflow tract of the right ventricle and the basal portion of the inter ventricular septum

Lead II travels from aVR towards aVF to become 2nd inferior lead

Placed on right wrist or shoulder

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9
Q

Describe the aVL lead

A
  • Unipolar lead placed on left wrist or shoulder, looks at upper left side of heart
  • Lead I travels towards aVL creating a second high lateral lead
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10
Q

Describe the aVF lead

A
  • Placed on left ankle or left lower abdomen, looks at bottom, or inferior wall of heart
  • Lead II travels from aVR to aVF to become a 2nd inferior lead
  • Lead III travels from the aVL towards aVF to come a 3rd inferior lead
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11
Q

What are the 6 chest leads and where are they placed?

A
  • V1 - 4th intercostal space, right sternal edge
  • V2 - 4th intercostal space, left sternal edge
  • V3 - equidistant from V2 - V4
  • V4 - 5th intercostal space, midclavicular line
  • V5 - left anterior axillary line in horizontal line with V4
  • V6 - mid axillary line, horizontal with V4 and V5
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12
Q

What is R wave progression?

A

From V1 to V6, the pattern is that of a change from the S wave being prominent to the R wave being prominent

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13
Q

What leads convey activity of the inferior surface of the heart?

A
  • II
  • III
  • aVF
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14
Q

What leads convey activity of the septal area of the heart?

A
  • V1
  • V2
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15
Q

What leads convey activity of the anterior surface of the heart?

A
  • V3
  • V4
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16
Q

What leads convey activity of the lateral surface of the heart?

A
  • V5
  • V6
  • I
  • aVL
17
Q

Describe sinus rhythm

A
  • Initiated by SAN, atria contract
  • Depolarisation spreads to AVN
  • AVN slows depolarisation to allow enough time for ventricles to fill and prevent damage to ventricles
  • Depolarisation spreads to bundle of His, which splits into purkinje fibres, which causes ventricular contraction beginning a the apex up towards the base of the heart
18
Q

Describe what sinus rhythm would look like on an ECG

A
  • 1 P wave before each QRS
  • Normal PR interval (less than 0.2 seconds)
  • Normal QRS duration
19
Q

What is the Bachmann’s bundle?

A
  • Often referred to as the intertribal bundle
  • Muscular bundle composed of parallel muscle strands connecting left and right area
  • Serves as the primary electrical connection between the right and left atria, allowing the waves of depolarisation to spread through both atria
20
Q

What does a normal ECG look like?

A
  • Wave - A positive or negative deflection from baseline that indicates a specific electrical event. The waves on an ECG are P, QRS, T
  • Interval - Time between two specific ECG events. Intervals commonly measured on an ECG include the PR interval, QRS interval, QT interval and RS interval
  • Segment - The length between 2 specific points on an ECG that are supposed to be at the baseline amplitude (neither negative nor positive). The segments on an ECG include the PR segment, ST segment and TP segment
  • Complex - The combinations of multiple waves grouped together. This is the QRS complex.
  • Point - There’s only one point on an ECG and this is the J point, which is where the QRS complex ends and the ST segment begins
21
Q

How is rate calculated from a 12-lead ECG?

A
  • When cardiac rhythm is regular, heart rate can be determined by the interval between 2 successive QRS complexes
  • On standard paper with the most common tracing settings, the heart rate is calculated by dividing the number of large boxes (5mm or 0.2 seconds) between two successive QRS complexes into 300
22
Q

What is tachycardia?

A
  • Increased heart rate
  • Heart rate >100bpm
23
Q

What is bradycardia?

A
  • Decreased heart rate
  • Heart rate <50bpm
24
Q

How is rhythm calculated from a 12-lead ECG?

A
  • Rhythm refers to the pattern of electrical impulses that causes the heartbeat
  • Normal cardiac rhythm is a sinus rhythm that follows a predictable patter
  • When cardiac rhythm is regular, heart rate can be determined by the interval between 2 successive QRS complexes
  • On standard paper with the most common tracing settings, the heart rate is calculated by dividing the number of large boxes (5mm or 0.2 seconds) between two successive QRS complexes into 300
25
Q

What does axis mean?

A

Axis is the average direction of electrical movement through the heart during a depolarisation

26
Q

How is the axis calculated from a 12-lead ECG?

A
  • The most efficient way to estimate axis is to look at LEAD I and LEAD aVF.
  • A positive QRS in lead I puts these in roughly the same direction as lead I
  • A positive QRS in lead aVF similarly aligns the axis with lead aVF. Combining both coloured area- the quadrant of overlap determines the axis
27
Q

What does the P wave show?

A
  • Represents atrial depolarisation
  • In healthy individuals, a P wave precedes each QRS complex
  • The height of the P wave should be no more than 3mm (ideally 2.5mm)
28
Q

What does the PR interval show?

A
  • PR interval begins at the start of the P wave and ends at the beginning of the Q wave
  • It represents the time taken for electrical activity to move between the atria and the ventricles, and includes the time the electrical impulse is being held at the AVN
29
Q

What does the Q wave show?

A
  • The first negative deflection (current flowing away from the lead)
  • Septal depolarisation - this is when depolarisation is being distributed at the bundle of His, it’s distributed between the left and right branches
  • It’s is distributed to the left ventricle first, and the movement moves from left to right
  • The positive charge moves away from the positive electrode
30
Q

What does the R wave show?

A
  • The first positive deflection (current flowing towards lead)
  • Action potentials spread through the purkinje fibres so movement is through to the apex, and then moves up to the ventricles - This is the R wave
  • The left ventricle has thicker muscles and a greater number of myocytes, so generates a greater number of action potentials and therefore has a larger positive deflection
  • The right ventricle has a smaller electrical charge due to fewer action potentials being generated
  • The left side is bigger, therefore mean movement is towards the left -> towards the electrode = positive wave
31
Q

What does the S wave show?

A
  • Second negative deflection
  • Ventricular depolarisation - Action potentials begin moving superiorly towards the base of the heart
  • It moves away from the lead = negative deflection
32
Q

What does the QRS complex show?

A
  • Depolarisation of the ventricles
  • Appears as 3 closely related waves on the ECG (Q,R and S waves)
33
Q

What does the QT interval show?

A
  • Total time from ventricular depolarisation to complete repolarisation
  • This begins at the start of the Q wave and extends to the ends of the T wave
  • If there’s no Q wave, then the starting point would be the beginning of the R wave
34
Q

What does the ST interval show?

A
  • Isoelectric (flat) segment between the end of QRS and start of T wave
  • Ventricular repolarisation
  • Depression in flat line can indicate ischaemia
  • Elevation in flat line can indicate myocardial infarction
35
Q

What does the T wave show?

A
  • Ventricular repolarisation
  • Negative charge travels upwards due to repolarisation
  • The flow of charge is negative - negative charge flow towards negative electrode = positive deflection
36
Q

How can an ECG show evidence of hypertrophy?

A
  • More tissue = higher voltage
  • Will cause a taller R wave or deeper S wave
37
Q

Why would a P wave not be present on an ECG?

A
  • SA node fails to initiate impulse resulting in no P waves/QRS complex

Introduction to medicine (Year 1) (59 decks)

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