302 Physiology of tachycardia

Question 1

Why is tachycardia worrying?

Answer 1

It can be a reflection of a normal response or an indication that something more serious is going on

Question 2

What is the equation for cardiac output?

Answer 2

CO = SV x HR

Question 3

What are the red flags signs when someone is tachycardic?

Answer 3

Red Flags in awake patients:
SOB
Lightheadedness
Chest pain
Fainting (syncope)

Red Flags in asleep patients:
ECG changes: ST changes, rhythm changes; ectopics
Hypotension
Change in oxygen saturations
Change in end-tidal CO2

Question 4

Why are perioperative arrhythmias serious?

Answer 4

Postop AF is associated with:

2.3 fold increase risk of CVA
Higher incidence of MI, CCF, Ventricular dysrrhythmias, Renal failure
Tachycardia isn’t good for coronary artery perfusion
Increases the risk of intraoperative MI

Question 5

What does perioperative mean?

Answer 5

Occurring or performed at or around the time of an operation

Question 6

What are the 4 main areas of electrical conducting in the heart?

Answer 6

SA Node
AV node
Bundle of Hiss
Left and Right bundles

Question 7

How is heart rate controlled within the heart?

Answer 7

Normally determined by the pacemaker activity of the sinoatrial (SA) node – this is sinus rhythm
Located in the posterior wall of the right atrium

The SA node exhibits automaticity that is determined by spontaneous changes in Ca2+, Na+ and K+

If unmodified by neurohumoral factors, it will exhibit a spontanenous firing rate of 100-115bpm (NB this intrinsic firing rate decreases with age)

Question 8

Which factors affect the intrinsic firing rate of the heart?

Answer 8

-Neuro factors
Autonomic NS
Sympathetic stimulation leading to an increase in heart rate via adrenergic receptors
Usually vagal tone (parasympathetic) predominates therefore resting heart rate is 60-80/min

-Humoral factors
Circulating catecholamines acting at the β1 adrenoceptors on SA nodal cells
Thyroxine
Temperature

Question 9

What does depolarisation mean?

Answer 9

A shift in ions (Na+, K+, Ca2+) across the cell membrane to generate an electrical impulse

Causes contraction of heart muscle

Question 10

What does repolarisation mean?

Answer 10

The return of the ions to their resting state

Cause relaxation of heart muscle

Question 11

What is the effect of hyperkalaemia on the cardiac action potential?

Answer 11

Lowers cell action potential
Prevents repolarisation

This can result in cardiac muscle paralysis – conduction blocks

Question 12

What does the P wave indicate?

Answer 12

Atrial depolarisation (therefore Atrial contraction)

Question 13

What does a QRS complex indicate?

Answer 13

Ventricular depolarisation (therefore Ventricular contraction)

Question 14

What does a T wave indicate?

Answer 14

Ventricular repolarisation (therefore Ventricular relaxation)

Question 15

What is PEA?

Answer 15

A cardiac arrest when the ECG shows a heart rhythm that should produce a pulse, but does not

A signal without the heart actually pumping

Question 16

What is systole?

Answer 16

Period of ventricular contraction#

AV valves close to prevent back flow
Isovolumetric contraction (rapid increase in pressure without increase in volume)

Aortic and Pulmonary valves open
Ejection of blood occurs (60-80% of vol)
= Stroke Volume ~70mls

Question 17

What is diastole?

Answer 17

Period of ventricular relaxation

Isovolumetric relaxation
All valves closed
As atria fill with blood returning to heart, pressure increases

AV valves open and passive filling of ventricle (high pressure gradient)
Contraction of atria completes this filling (20-30%)

Question 18

How long does a cardiac cycle take?

Answer 18

Cardiac cycle lasts 0.85s

Systole lasts 0.3s
Diastole lasts 0.55s

Question 19

What is the effect of tachycardia on the cardiac cycle?

Answer 19

HR 180/min
Cardiac cycle lasts 0.3s instead of 0.85s

Systole and diastole both 0.15s
Diastole reduces more than systole with increased HR
Systole cannot reduce any further without compromise of SV

Question 20

At which stage of the cardiac cycle do the coronary arteries fill?

Answer 20

During diastole

Question 21

Where are the baroreceptors in the heart?

Answer 21

Within the carotid sinuses and the aortic arch

Baroreceptors in the aortic are only responsive to increases in arterial pressure

Question 22

How do baroreceptors respond to decreased arterial pressure?

Answer 22

Increases the heart rate to compensate

Respond very quickly to maintain a stable BP but this response diminishes with time
Hence the patient may initially look like they are fine and then suddenly decompensate

Question 23

What are the causes of tachycardia?

Answer 23

-Negative fluid status
Eg. fluid loss from vomiting, lack of oral intake, or blood loss
-Sepsis
-Thyroid supplements causing increased T3
-Adrenal storm
Eg. Phaeochromocytoma
-Awareness
-Pain
-Response to intubation
-Drug interactions
Eg. Anaesthetic causing vasodilation, anaphylaxis
-Underlying cardiac condition
-PE

Question 24

What is the sepsis 6?

Answer 24

Take: Lactate, UO, Blood cultures
Give: Fluids, Antibiotics, Oxygen

Question 25

What are some signs of sepsis?

Answer 25

Tachycardia
Hypotension
Acidosis
High or low temp (can be normothermic)
Rising lactate
Low urine output

Question 26

How does awareness of tachycardia make it worse?

Answer 26

These are all also the signs seen in anxious or stressed patients awake due to increased circulating catecholamines

Question 27

How long does tachycardia caused by intubation take to settle?

Answer 27

10-15mins

Question 28

What are the ECG changes associated with hyperkalaemia?

Answer 28

Mild: peaked T wave, prolonged PR segment

Moderate: Loss of P wave, prolonged QRS complex, ST segment elevation, Ectopic beats and escape rhythms

Severe: progressive widening of QRS complex, sine wave, ventricular fibrillation, asystole, axis deviations, bundle branch blocks, fascicular blocks

Question 29

What is Wolff-Parkinson-White Syndrome?

Answer 29

A condition that makes the heart suddenly beat abnormally fast, in an abnormal heart rhythm called supraventricular tachycardia (SVT)

Question 30

What is supraventricular tachycardia (SVT)?

Answer 30

When the heart suddenly beats much faster than normal

Question 31

What is the treatment for supraventricular tachycardia (SVT)?

Answer 31

Vagal manoeuvres
Specific cardiac medications
Rarely require cardioversion

Question 32

What are vagal manoeuvres?

Answer 32

1.Sit down or lie down.
2.Take a deep breath and hold it by closing your throat.
3.Bear down hard, as if you’re trying to go to the bathroom.
4.Strain hard for about 10 to 15 seconds.
5.Release the air when you’re done.
6.Wait at least a minute before you try again

Question 33

How do vagal manoeuvres work?

Answer 33

It decreases the speed and frequency of electrical impulses in the heart