302 Physiology of tachycardia Flashcards
Why is tachycardia worrying?
It can be a reflection of a normal response or an indication that something more serious is going on
What is the equation for cardiac output?
CO = SV x HR
What are the red flags signs when someone is tachycardic?
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
Why are perioperative arrhythmias serious?
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
What does perioperative mean?
Occurring or performed at or around the time of an operation
What are the 4 main areas of electrical conducting in the heart?
SA Node
AV node
Bundle of Hiss
Left and Right bundles
How is heart rate controlled within the heart?
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)
Which factors affect the intrinsic firing rate of the heart?
-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
What does depolarisation mean?
A shift in ions (Na+, K+, Ca2+) across the cell membrane to generate an electrical impulse
Causes contraction of heart muscle
What does repolarisation mean?
The return of the ions to their resting state
Cause relaxation of heart muscle
What is the effect of hyperkalaemia on the cardiac action potential?
Lowers cell action potential
Prevents repolarisation
This can result in cardiac muscle paralysis – conduction blocks
What does the P wave indicate?
Atrial depolarisation (therefore Atrial contraction)
What does a QRS complex indicate?
Ventricular depolarisation (therefore Ventricular contraction)
What does a T wave indicate?
Ventricular repolarisation (therefore Ventricular relaxation)
What is PEA?
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
What is systole?
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
What is diastole?
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%)
How long does a cardiac cycle take?
Cardiac cycle lasts 0.85s
Systole lasts 0.3s
Diastole lasts 0.55s
What is the effect of tachycardia on the cardiac cycle?
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
At which stage of the cardiac cycle do the coronary arteries fill?
During diastole
Where are the baroreceptors in the heart?
Within the carotid sinuses and the aortic arch
Baroreceptors in the aortic are only responsive to increases in arterial pressure
How do baroreceptors respond to decreased arterial pressure?
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
What are the causes of tachycardia?
-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
What is the sepsis 6?
Take: Lactate, UO, Blood cultures
Give: Fluids, Antibiotics, Oxygen
What are some signs of sepsis?
Tachycardia
Hypotension
Acidosis
High or low temp (can be normothermic)
Rising lactate
Low urine output
How does awareness of tachycardia make it worse?
These are all also the signs seen in anxious or stressed patients awake due to increased circulating catecholamines
How long does tachycardia caused by intubation take to settle?
10-15mins
What are the ECG changes associated with hyperkalaemia?
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
What is Wolff-Parkinson-White Syndrome?
A condition that makes the heart suddenly beat abnormally fast, in an abnormal heart rhythm called supraventricular tachycardia (SVT)
What is supraventricular tachycardia (SVT)?
When the heart suddenly beats much faster than normal
What is the treatment for supraventricular tachycardia (SVT)?
Vagal manoeuvres
Specific cardiac medications
Rarely require cardioversion
What are vagal manoeuvres?
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
How do vagal manoeuvres work?
It decreases the speed and frequency of electrical impulses in the heart
