Sinus Rhythm Flashcards by Faith Taws

What factors affect the morphology of various ECG deflections?

1-Electrode postition
2-orientation of the heart within the thorax
3-thickness of heart walls
4-course of electrical activity through the myocardium
5-rate of depolarisation and repolarisation
6-pathology

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What initiates the electrical activity of the heart?

SA node- but the SA node is so small so its not possible to record the activity from that surface of the body.

Therefore P wave proceded all other electrical activity

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What does the P wave represent?

-combined electrical activity of the atria
-small rounded low amplitutde deflection is made of the RA and LA electrical components

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How does the atrial activation spread? ( p wave)

-From SA node
-towards AV node in an inferior direction
-consequences of this vector, p wave is typically upright in leads II, III aVF V5 and V6- which are orientated towards the inferior left lateral surface of the heart

-the electrical activity of the atria is moving away from aVR ( situated high on the right shoulder so P wave in typically inverted in aVR)

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Where is the SA node situated?

at the junction of the SVC and RA

-can’t see this on an ECG, can see it on an EGM

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What can P cells do?

-can automatically depolarise
-pacemaker cells
-send wave of depolarizarisation/ionic waveform through the heart

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What can we see in lead V1 in regards to the p wave?

-inital positive component immediately followed by a small negative component (equiphasic)

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How should the p wave look in lead II, V1 and aVR?

II -small, rounded and upright
V1- biphasic
aVR- negative

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What are the “normal” measurements of the p wave?

-2.5mm in amplitude
-0.08s duration
p wave axis between 0-75 degrees
-no taller than 0.25mV (2 1/2 squares)

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What happens if you can only detect a P wave in one lead?

-does not indicate an abnormality
-p waves are low amplitude and can be missing from some leads
-as long as there are p waves they are there.
-with increased HR, p waves may begin the merge with T waves

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What does the notching of p waves suggest?

-mitral valve disease, hypertension
-a tall peaked p wave suggests P pulmonale indicating RA enlargments which could be caused by RV hypertophy or lung disease

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What does the PR interval represent?

time taken for the electrical impulses to travel to tranverse the atria, the AV node, bundle of HIS, bundle branches and purkinje system within the ventricles

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How is the PR interval measured?

from the beggining of the P wave to the first positive or negative deflection

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What are “normal” PR interval ranges?

-from 0.12 sec-0.2 sec or 12omsecs-200msec
-conduction velocity is relatively slow- delay
-delay allows the atria to contract prior to ventricular contraction and contributes to 25% of the end diastolic volume

must be constant-all PR intervals must be the same in every complex,fluctuations are not normal

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Describe the electrical activity within the PR interval

-due to the AV node anatomy (few gap junctions)
-little electrical activity takes place
-PR intervals typically shortern with increased HR and may begin down-sloping with fast HR

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What is the PR segment mainly comprised of?

-activity from…
-bundle of His
-bundle branches
-purkinje fibres

PR interval is accpeted as iso-electric or baseline when connected to TP segments

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What does a shortened PR interval less than 0.12 suggest?

-suggests an accessory pathway within the conduction system- wolff-parkinson white

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What is a vector?

has force and amplitude

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What does the direction of the ECG pen mean?

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positive deflection - towards electrode
Negative deflection-away from electrode

V1- RS
V6- QR

What is R wave progression?

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trasition from the right side of the heart to the much thicker left side of the heart

-If R wave peaks in V5 it should dip in V6
-Peak in V4, Dip in V5 (its okay if V6 also dips)

What is the “normal” duration of the QRS complex?

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-should not exceed 0.12s
-otherwise it indicates a conduction abnormality
-Intrinsicoid deflection is also measured

What is intrinsicoid deflection?

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time period from the beggining of QRS to the peak of the R wave.

Represents the time period from the endocardial surface of the heart to the epicardial surface located beneath the electrode

Describe maximum aplitutde of the QRS complex

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-amplitude increases until age 30 and gradually decreases
-Amplitude of 3.0mV can be seen, view with caution
-low amplitude complexes less than 0.5mV limb leads and 1.0mV chest leads

What is cardiac rotation of the QRS complex?

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-complexes change morphology
( e.g being an RS complex to a QR complex, usually between V3 and 4.)
- If it changes between V2 and 3 counter clockwise rotation.
-Between V5 and 6 clockwise rotation

How do we use the limb leads to derive the mean electrical axis of the heart at a specific point in time?

1-Find the most biphasic/smallest QRS- indicates electrical activity is perpendicular to the most dense current density 2-find the lead that is right angles to the original lead 3-is the lead positive or negative? -method derived an accuracy of 15 degrees -go back to the original lead and look either side of the perpendicular lead

How do ECG machines with an exact axis figure work?

-derived from lead I and III -QRS in each lead is added algerbraically and plotted on the equilateral triangle, where the two lines intersect, a line is drawn from the centre of the triangle- this is the electrical axis

What factors affect the mean axis of the heart?

-position of heart within the chest -thickness of ventricular walls -rotation of ventricles around their longitudinal axis -A horizontal heartcan cause left axis deviation

What are the current values indicating left axis deviation?

-30-90 degrees- normal -30-45-moderate -45 to 90- left axis deviation

What are the current values for right axis deviation?

90-120-moderate 120-180-right axis deviation

What is the ST segment?

J point to the beggining of the T wave

What is the J point?

where the S wave meets the isoelectric line

What does the J wave represent?

period between the end of ventricular depolarisation and the beggining of repolarisation and roughly the length of the ST segment corresponds to the plateau phase of the action potential

What are some "normal" measurements for the ST segment?

-should be in the same horizontal plane as the TP segment i.e isoelectirc -the ST segment should always be within 1mm of baseline (in V2 and 3 this may be up to 2mm) -J points can appear elevated or depressed

What does ST displacememnt suggest?

above or below the isoelectric line can indicate ischaemia but ST changes could also be altered by drugs such as digoxin,electrolyte balance, conduction abnormalities

Where is the QT interval measured?

-from beggining of QRS to the end of the T wave

What does the QT interval represent?

total time taken for depolarisation and repolarisation - good representation of the duration of the action potential

What are the changes to the QT interval with HR?

QT interval lengthens as HR slows QT interval shorterns as HR increases

What is the equation for Bazett's correction factor?

QTc= QT/square root of R-R interval in seconds

Normal bazetts correction values in males and females?

females-0.41s males-0.43s

What things affect the QT interval?

-electrolyte imbalance -congenital disease -drugs -prolongation of the QT interval can predispose to potentially life threatening arrythmias -age,sex,structural heart disease

What difficulties are encountered when measuring the QT interval?

-T wave morphology -noisy baseline and atrial fibrillation -can exacerbate intra and inter-observer error -QT intervals should be measured in V2 and V3

What does the T wave usually look like?

-asymmetrical -ascending limb is usually more gradual than the descending limb -polarity of T wave normally corresponds with the QRS complex -no more than 2/3 of the height of the R wave- rarely exceeds 10mm of height

What can the T wave indicate?

-prescence of coronary heart disease as well as aquired structural eart disease -inverted T waves in V4-V6 is always abnormal -T wave may be inverted in V1 upright but a change in polarity is not normal -Positive T wave in V1 and a negative T wave in V2 is not normal

What is the vunerable period for R on T?

The terminal phase of the T wave

What does a U wave represent?

-Slow repolarisation of papillary muscles or late repolarisation of mid myocardial cells- cause is still vague -oftern seen with slow HR, especially in V2 and V3 typically in the same polarity as the T wave

What does each square represent when calculating rate?

small square-0.04s large square-0.2s

How do you calculate the R-R interval for small squares? How do you then calculate the HR?

number of small squares x 0.04 HR=60/R-R interval

Sinus Rhythm Flashcards

(47 cards)