CAR week 3 - ECG and blood pressure

What is an ECG?

• It’s an electrocardiogram

• it represents the electrical activity of the heart in a graph

• it monitors rate and rhythm

• electrical activity is recorded using electrodes attached to the skin surface

What does an ECG assess?

• heart rate

• heart rhythm

What is the baseline of an ECG called and what is it used for?

• it is called the isoelectric line

• used to show no voltage change

what is the shape of an ECG dependent on?

• direction of contraction and lead position

when does a positive deflection in an ECG occur

• when the wave of depolarisation travels towards the lead

when does a negative deflection occur in an ECG?

• when the wave of depolarisation travels away from the lead

What do we measure an ECG on?

• graph paper

what is paper output speed

• the rate at which the ECG machine produces a trace

what 2 paper output speeds are used for ECGs?

• 25mm.sec and 50mm/sec

what setup do we need for a 50mm/sec ECG?

• lead II

• right lateral recumbency

what does the vertical axis of an ECG show?

• amplitude of the heart’s electrical current in milivolts (mV)

• 10mm in height = 1mV

• each 1 mm square = 0.1 mV and large square is 0.5mV

what does the horizontal axis on the ECG show?

• time

• at 25mm/sec, 1mm square = 0.04sec, each large square = 0.2sec

• at 50mm/sec, 1mm square = 0.02 sec and each large square = 0.1sec

when and why would we use a 50mm/sec paper output rate

• when we have a tachycardic patient

• it’s easier to spot abnormalities that may by lost due to a fast heart rate

How do we calibrate ECG paper?

• use ECG chamber

• it’s important to make sure the ECG paper is standardised to 1mV vertically and 5mm horizontally

What causes the electrical activity in the heart?

• the myocardial muscle cells contain many ions and many ion channels which initiate depolarisation in their movement

• the abundance of ions means they can act as electrical conductors.

What leads to depolarisation of the myocardial cells?

• normally it’s interior is negative and exterior is positive, causing a potential difference across the cell membrane, forcing ions to move

• +ve ions cross the cell membrane into myocardial cell. When threshold is reached, AP is triggered and the cell depolarises

• depolarisation = contraction

• this spreads across the myocardium like a domino effect

what ions are involved in electrical conduction

• Na+

• K+

• Ca2+

What is the general idea of how antiarrhythmic drugs work? What are they used to treat?

• they work by altering Na+, Ca2+ and potassium channels, adjusting how excitable a cell is

• they may also block the sympathetic activity to the heart e.g. beta blockers

• used to treat cardiac arrhythmias

Outline the conduction pathway through the heart

• Initial impulse generated by SAN

• impulse travels through AVN and bundle of His

• then down L/R branch block into the purkinje fibres

In a healthy heart, where does normal conduction start?

• SAN

In an abnormal heart/heart with a problem, what can assume the role of pacemaker if the SAN is damaged and what is this known as?

• AVN

• escape rhythm

Outline pacemaker cells:

1. where we find them

2. what they can do

3. what enables them to do this

1. SAN

2. self generate electricity and spontaneously depolarise

3. unique ion channels provide this action potential

Outline non-pacemaker cells:

1. where do we find them

2. what do they do

3. what protects them from random excitation

1. atrial and ventricular cardiomyocytes, purkinje conduction system

2. conduct electrical impulses generated by the pacemaker cells

3. have a plateau phase where they can’t be triggered

what do that parts of an ECG represent?

• movement of these electrical signals through specific areas

What is the P-wave of the ECG

• electrical impulse generated in the SA node

• impulse is rapidly spread across the atrial muscle, causing depolarisation and contraction = atrial systole

• impulse arrives at AV node

what is the Q wave

• when the impulse travels through AV node into bundle of His → L/R bundle branch into purkinje fibres

• interventricular septum depolarises ready to spread impulse across the ventricles

• ventricles fill with blood (ventricular diastole)

what is the R wave?

• electrical impulse spreads across ventricular muscle

• ventricles depolarise and contract (ventricular systole)

• blood is ejected from the ventricles

what is the S wave

• electrical impulse reaches the last remaining areas of the ventricles = late ventricular depolarisation

• ventricles then relax

what is the T wave

• ventricular muscle repolarises, resetting electrical charge after contraction in preparation for the next heartbeat

what is normal sinus rhythm

• the normal rhythm of the heart

How do we identify normal sinus rhythm

• ECG shows that the SA node is initiating electrical impulses

• electrical activity is following the normal path of conduction through the heart - this is the rhythm or pattern of the heartbeat

• ECG shows regular rate

How do we determine if the rhythm is regular or irregular

• measure distance b/w QRS complex = RR interval

• is it constant = regular rhythm

• does it differ b/w beats = irregular rhythm

what does a narrow QRS complex indicate?

• supraventricular in origin

What does a wide QRS complex indicate?

• likely to have originated in the ventricles

• if electrical activity is initiated w/in ventricles and travels by an abnormal path across the ventricular muscle, depolarisation occurs at a slower rate

What are the 3 kinds of ectopic complexes

• PACs: pre-atrial complex

• AV junctional (atrioventricular)

• VPCs - ventricular premature complex

what is the waveform and interval change of PACs?

• premature P wave

• P wave shape changes (signal isn’t from SAN)

• PR interval normal or prolonged

• QRS measurement = narrow (signal still originates from atrium)

• Pause following complex - SAN resets

what is the waveform and interval changes of an AV junctional?

• no preceding P wave or inappropriately placed P wave

• QRS complex sooner than expected

• pause following complex

What is the waveform and interval changes of VPCs

• No P wave before VPC - signalment doesn’t come from SAN

• broad QRS complex

• QRS is abnormally shaped - wide and bizarre b/c electrical activity is originating from ventricles

• pause following complex

What are the steps of checking an ECG? What are 2 additional checks we should make?

1. what is the heart rate

2. is there a P wave for every QRS?

3. is there a QRS complex for every P wave

4. what is the relationship b/w P waves and QRS complexes?

5. are QRS complexes narrow or wide?

6. Are there T waves and are their shape normal?

7. Is the rhythm regular or irregular

8. Do all complexes look the same?

Additionally:

1. amplitude of each wave

2. segment interval times

When do we use:

1. Large square method

2. Small square method

3. R wave method

For calculating heart rate?

1. for ECGs that show a regular rhythm and rate

2. useful for very fast regular rhythms, as likely to provide more accurate rate than large square method

3. useful for slow/irregular rhythms, use if RR intervals are irregular

How do we calculate HR with small square method for

a) 25mm/sec

b) 50mm/sec paper output

Firstly, count the number of small squares b/w R waves

a) 1500 / number of small squares

b) 3000 / number of small squares

How do we calculate HR using RR interval for:

a) 25mm/sec paper output

b) 50mm/sec paper output

Firstly: count the number of R waves in 30 large boxes

a) multiply that number by 10 (30 boxes = 6 seconds, 60/6=10)

b) multiply the number by 20 (30 boxes = 3 seconds, 60/3=20)

How do we do a large square method for 25mm/sec:

• what setup is needed

• what speed

• how do we calculate?

1. Lead II

2. ensure ECG is set at 25mm/sec

3. identify 2 consecutive R waves, count the number of large squares b/w each, divide this by 300.

How do we do a small square method for 25mm/sec:

• what’s setup is needed

• what speed

• how do we calculate it

1. Lead II

2. 25mm/sec

3. identify 2 consecutive R waves, count no. small squares between them, divide by 1500

what lead setup is needed for the R wave

• lead II

what lead setup do we need for a 50mm/sec

• Lead II (same as for 25mm/sec)

What is RR? what does it represent and what is it used for?

• distance b/w two R waves on an ECG

• represents the interval b/w heartbeats and regularity of rhythm

• used to calculate heart rate

what is PR and what does it reflect?

• measure from beginning of the P wave to the beginning of QRS wave

• reflects conduction through the AV node

What is an ST wave, what does it represent?

• measurement b/w end of the S wave and the beginning of the T wave

• represents the interval b/w ventricular depolarisation and repolarisation

what is important to understand about canine and feline interval times?

• they differ

  • depending on species

  • breed

  • age

  • any heart conditions

give the normal values for a dog for:

1. P wave

2. QRS complexes

3. T wave

in seconds and mV?

1. 0.02-0.04 = 0-0.4mV

2. 0.03-0.05 = <_ 3 mV.

3. 0.04-0.08 = 0.5 mV

give the normal values for a cat for:

1. P wave

2. QRS complexes

3. T wave

in seconds and mV?

1. <0.04s = 0-0.2mV

2. 0.03-0.04s = -0.1→0.8mV

3. 0-0.04s = 0-0.3mV

what are the normal interval times for:

1. RR

2. PR

3. ST

   in a dog?

1. 0.35-0.8s

2. 0.08-0.12s

3. 0.04-0.1s

what are the normal interval times for:

1. RR

2. PR

3. ST

   in cats?

1. 0.06-0.13s

2. 0.05-0.07s

3. 0.06-0.08s

Using ECG rules, assess this ECG

• the black line = 6s

• assess what you can see

• is it normal sinus rhythm

1. HR = 200bpm = tachyarrhythmia

2. no P wave

3. QRS complexes are narrow and upright

4. RR interval irregular

5. not normal sinus rhythm

Using ECG rules, assess this ECG

• assess what you can see

• is it normal sinus rhythm

• HR = 300bpm = tachyarrhythmia

• no P waves

• QRS complexes wide

• RR interval regular

• not normal sinus rhythm

Using ECG rules, assess this ECG

• assess what you can see

• is it normal sinus rhythm

• HR = 120bpm = normal

• several P waves occur without a following QRS complex

• QRS complexes are narrow and upright

• RR interval irregular

• not a normal sinus rhythm

Using ECG rules, assess this ECG

• assess what you can see

• is it normal sinus rhythm

1. HR = 30bpm = bradyarrhythmia

2. P waves are present, no association with QRS complexes - no P wave for every QRS and no QRS for every P wave

3. QRS complexes are normal width/height

4. RR interval is regular

5. not a normal sinus rhythm

What questions do we need to ask ourselves when assessing arrhythmias? (3)

1. do we have brady or tachyarrhythmia?

2. is the rhythm, sinus, supraventricular or ventricular

3. where did the heartbeat originate from ^ (SAN, AVN/atria, ventricles?)

what is meant by an ectopic complex?

• premature or extra beats caused by unusual impulses

Outline sinus arrhythmia:

1. what is its rhythm

2. what causes it to increase/decresae

3. how would we describe the rate

4. what is common in dogs with this?

• normal sinus rhythm

• rate increases on respiration and decreases during pause b/w breaths

• change in rate = variation in RR intervals = regularly irregular

• common dogs - high resting vagal tone

Outline sinus tachycardia:

1. where does it originate from

2. what waves are present and what does this make the rhythm

3. how would we describe it on an ECG

4. what can cause it

• sinus = normal beat originating from the SAN

• P wave, QRS complex and T wave present = normal sinus rhythm

• fast but regular rhythm

• causes = increased sympathetic tone, medication, systemic

Outline sinus bradycardia:

1. where does it originate from

2. what waves are present and what does this mean

3. how would we describe it on an ECG

4. what can cause it?

1. SAN = sinus

2. P wave, QRS complex, T wave = normal sinus rhythm

3. slow but regular rhythm

4. medications, electrolyte disturbances, vagal stimulation, hypothermia, hypoadrenocorticism, hypothyroidism, raised ICP

Outline supraventricular tachycardia:

1. what does this mean

2. what can the rhythm be?

3. what waves are present/absent

4. what causes it?

1. tachycardia - fast

2. can be regular or irregular

3. P waves absent/hidden in another part of the wave

   • P waves can be inverted below the baseline

   • narrow QRS complexes, varying heights

   • T waves follow QRS complexes

4. Cause = electrical signals re-enter atria (go along an abnormal pathway which constantly stimulates atrial systole)

   • the green circle represents this

Outline atrioventricular block:

1. what is it

2. what kinds are there and how do we diagnose

3. what does an ECG change imply?

   • what waves are present and not present?

1. delay or disturbance in the transmission of electrical impulses from the atria to the ventricles

2. numerous types - assess the relationship b/w P waves and QRS complexes to diagnose

3. AVN issues - P waves present but not associated with QRS complexes, prolonged PR interval

Outline ventricular fibrillation:

1. what does it look like on an ECG:

   • what waves are present/absent

2. what is causing it

1. irregular deformed wave

   • no distinct P wave, QRS complex or T wave anywhere

2. ventricular myocardium depolarising erratically (not one smooth motion)

   • re-entry, triggered activity, automaticity

Outline ventricular tachycardia (VT):

1. what does it look like on an ECG

2. what is the heart rate (normally)

3. what does a rapid rate result in?

4. what can cause it?

1. wide and bizarre QRS complexes, no associated P waves

2. tachycardia = >120/180bpm (dog/cat)

3. low cardiac output = reduced preload and stroke volume

4. re-entry, triggered activity, automaticity

Outline atrial fibrillation:

1. what does it look like on an ECG

2. what other symptoms may be present

3. what causes it?

1. narrow, upright QRS complexes

   • irregular ventricular rate

   • absence of P wave

   • irregular RR interval

   • undulating baseline

2. tachycardic, pulse deficits

3. re-entry, triggered activity

Outline hyperkalaemia:

1. what does it look like on an ECG

2. what causes it?

1. peaked/spiked T waves

   • flat/absent P waves

   • prolonged QT interval

   • widened QRS complex

2. cause = high potassium disrupts electrical signals

Outline ventricular premature complexes (VPCs):

1. where do the signals originate from?

2. what does it look like on an ECG

3. what does the signal interrupt?

4. what is it an example of?

1. ventricles - no P wave

2. QRS are wide and bizarre

3. signal interrupts sinus (normal rhythm)

4. ectopic beat - premature impulse that starts in ventricles

these may look different in each ECG, the wave may e inverted in some and not in others - this is dependent on where the signal originates in the ventricles

what are the 3 pieces of equipment we can use to measure blood pressure and are they direct or indirect?

1. arterial catheter and pressure transducer = direct

2. doppler ultrasonography = indirect

3. oscillometric = indirect

what do each of these methods read pressure wide:

1. arterial catheter and pressure transducer

2. doppler ultrasonography

3. oscillometric

1. systolic, diastolic, mean

2. systolic

3. mean, then systolic and diastolic is calculated through algorithms

what are 2 advantages to arterial catheter and pressure transducer

1. ‘real time’ monitoring to detect trends and allow for immediate intervention if necessary

2. restraint only needed for initial placement, so falsely elevated readings due to stress from handling are less likely

what are 4 disadvantages to arterial catheter and pressure transducer

1. equipment is expensive

2. invasive procedure which is difficult

3. risk of complications such as bleeding from site

4. need close and continuous monitoring

what are 5 advantages of doppler ultrasonography

1. easy to use

2. affordable

3. readily available in most clinical settings so can be utilised in conscious patients and when monitoring anaesthesia

4. usually well tolerated

5. appropriate for patients with hypertensions and cardiac arrhythmias

what are 4 disadvantages to doppler ultrasonography

1. absolute values may not be identical, therefore as accurate, as direct monitoring

2. correct cuff size is imperative

3. patient may need restraining which could cause an increase in BP (if incorrect). Position of patient can affect reading

4. difficult to obtain readings in patients with severe peripheral vasoconstriction and if patient is stressed

what are 3 advantages to oscillometric BP reading

1. automated process, therefore less technical skill is required

2. monitors can be programmed to measure at timed intervals

3. little/no restrain of patient

what are 3 disadvantages to oscillometric BP reading?

1. not accurate in patients <5kg

2. patient must be still, so good for anaesthesia monitoring but not always in conscious patients

3. less accurate if patient has cardiovascular arrhythmias, significant tachycardia or bradycardia, vasoconstriction or hypothermia

When do we measure BP?

1. during anaesthesia

2. in patients with heart disease

3. in diabetic patients

4. in patients with renal disease

5. in patients with thyroid disease (hyper (cats) hypo (dogs))

6. in obese patients

7. in geriatric patients

8. in patients undergoing medical treatment where blood pressure allows us to monitor stability → blood transfusion

What equipment do we need for doppler BP reading?

• headphones

• ultrasound gel

• probe

• doppler

• surgical spirit ± clippers

• cuff

• sphygmomanometer

• pump to inflate the cuff

How does a doppler work?

• ultrasound gel is placed over crystal surface and placed distal to the cuff

• doppler uses the crystal to detect pulsatile flow and converts it to an audible sound

• cuff is inflated to a P greater than that of BP to occlude the artery, so audible sound of pulsatile flow is lost

• as P is released, audible sound of flow returns and the number on the sphygmomanometer in which it does, is the systolic pressure

• repeat 2-3 times to obtain several readings (fully deflating the cuff b/w each reading) and take the average value

How do we choose the correct cuff size? What happens if cuff is too large/too small?

• width of cuff needs to be 30-40% of total circumference of the limb (or tail) where we place the cuff

• too large = reading will be falsely low

• too small = reading will be falsely elevated

What is Einthoven’s triangle?

• place 3 electrodes on the body to make an equilateral triangle

• shows how leads I, II and III surround the heart like a circuit.

• Useful for identifying correct/incorrect lead placement which may lead to incorrect diagnosis.

why do we get 6 electrical views of the heart from 4 electrodes?

• each electrode records electrical activity of the heart in relation to itself and in combination with other leads

What are 9 steps we can take to reduce artifacts on an ECG

• switch off non-essential electrical devices and equipment nearby

• is the patient close to the machine power source? Try using battery power if possible

• ensure cables aren’t tangled or in contact with metal objects/tables

• all leads should NOT run across the machine

• inspect wires and cables for cracks/damage

• is filter on/off? - filter can help reduce interference from muscle, turn off for lead II (can lead to distortion of the waves)

• ensure all connectors are attached to the machine

• ensure all electrodes are attached properly

  Bottom image - caused by poor respiration of patient and poor electrode connection 

what are 6 things to consider about the patient when preparing for an ECG?

1. reduce movement as much as possible

2. don’t cross the limbs

3. patient should be calm, not panting/purring/trembling

4. fur/skinn should be free from excessive oil/water - clean with surgical spirit and dry

5. fur can be clipped to improve contact

6. right lateral recumbency

what are 3 points to consider about electrode application?

1. use adequate surgical spirit or ultrasound gel to improve contact/signal

2. maximise surface area of attachment

3. consider if the electrodes are placed over a joint or significant muscle mass - interference from movement/contraction more likely, aim for fleshy areas around olecranon and stifle

Give the standard time for dogs for:

1. P wave

2. PQ interval

3. QRS

4. QT interval

1. <0.04s

2. 0.06-0.13s

3. <0.05 or <0.06 (giant)s

4. 0.15-0.25s

Give the standard time for cats for:

1. P wave

2. PQ interval

3. QRS

4. QT interval

1. <0.04s

2. 0.05-0.09s

3. <0.04s

4. 0.12-0.18s

Give the standard amplitude (upper limit) for these waves in a dog:

1. P

2. R

3. T

1. <0.4mV

2. <3.0mV

3. <1/4 R wave mV

Give the standard amplitude (upper limit) for these waves in a cat:

1. P

2. R

3. T

1. <0.2mV

2. <0.9mV

3. <1/4 R wavemV

what is the general function of the cardiovascular system?

• deliver oxygenated blood to the tissues and to remove CO2

What should we always do when interpreting clinical findings/readings

• consider all of them - never just one

what would we like to measure in the cardiovascular system vs what can we measure?

1. tissue oxygen delivery

2. tissue perfusion and blood oxygen content

what drives tissue perfusion

• mean arterial pressure (MAP)

how do we measure MAP?

• arterial blood pressure - invasively: canulated peripheral arterty

• arterial blood pressure - non-invasive: sphygmomanometry, oscillometric and doppler

• ‘feeling with fingers’ - not ideal

what is the gold standard method of measure MAP?

• cannulated artery

Outline the direct/invasive method of taking a blood pressure

• real time - beat by beat

• gives more information from trace

• more accurate

• insert a cannula into a peripheral artery

• connect a cannula to transducer

• reported as systolic/diastolic and (mean)

what pressure readings can a doppler obtain and what is it dependant on?

• diastolic, systolic and mean

• method of reading it

what does CVP measure and why is this relevant clinically?

• it measures the volume of blood returning to the RHS of the heart

• reflects the volume of blood returning to the heart - if P increases, indicates heart may be failing. if it falls - may have haemorrhage in the body