Chapter 4: Rate Flashcards
First thing to determine on a ECG
Rate
Units measurement for rate
Cycles /min
Heart’s normal pacemaker
SA node
- the dominant center of automaticity
- generates sinus rhythm
Normal rate range
60-100 beats per minute
Location of SA node
within the upper-posterior wall of the right atrium
Sinus bradycardia
When the SA node paces the heart at a rate slower than 60 beater/min
Causes of sinus bradycardia
-parasympathetic excess (ex: conditioned athletes at rest)
Potential consequences of sinus bradycardia
-may slow heart rate to point that blood flow to the brain is reduced causing syncope
What happens when the SA node pacemaking fails
One of the other potential pacemakers (automaticity foci) will assume pacemaking activity at their inherent rate
Atrial automaticity focus
The automaticity foci within the atrial conduction system
Junctional automaticity foci
AV junction -the middle and distal regions of the AV node, has an automaticity focus
-proximal end of the AV node has no automaticity foci
Ventricular automaticity focus
-automaticity foci in purkinje fibers including he Hiis Bundle and the bundle branches and their subdivisions
Rate of pacing of the different automaticity foci
1) Atrial automaticity focus - 60-80 /min
2) Junctional automaticity focus - 40-60/min
3) Ventricular automaticity focus - 20-40/min
* 8Range for these regions - but any specific focuus has a distinct inherent rate of pacing
Overdrive suppresion
Rapid automaticity suppresses slower automaticity
- characteristic of ALL automaticity centers
- once an automaticity focus actively begins pacing it wil overdrive- suppress all lower (slower) foci, including foci at the same level
When does a backup pacemaker take over
When a higher pacemaker fails - backup pacemaker is no longer over drive suppressed an will emerge to pace at its inherent rate
Automaticity focus when SA node fails
Atrial automaticity focus
Automaticity focus when atrial focus fails
AV junctional automaticity focus
Idio-junctional rhythm
Rhythm produced when junctional automaticity focus is dominant focus
Conditions for junctional automaticity focus to become the active pacemaker
Complete conduction block in the AV node above the AV junction -so AV node receive node pacing stimuli from above
-Will pace the ventricles at inherent rate
When does ventricular automaticity focus become the pacemaker
1) If all pacemaking centers above it have failed
2) If there is a complete block of conduction below the AV node (including he AV junction) that prevents pacing stimulus above it from reaching the ventricles
Idio-ventricular rhythm
Rhythm produced when ventricular automaticity focus is pacemaker
How to calculate the rate
1) Find a R wave that peaks on a heavy black line (the start line)
2) Count off 300, 150, 100. 75. 60, 50 for each black lines that follows the start lines
Basis of rate estimation - explanation of the method
Doing x/300 where x is the number of time units between two heavy black lines (number of time units between two consecutive black lines = 1)
Because the unit of time between two heavy black lines is 0.2 sec or 1/300th of a minute
Estimation of very slow rates (bradycardia)
1) Look at marking on the top margin of the ECG strip - two consecutive marks = 3 second interval
2) Taking two of the three second intervals have a 6 second strip
3) Count the number of complete (R wave to R wave) cycles in 3 seconds - with marked bradycardia there will be few cycles
4) Ten of the 6 second strips = 1 min, number of cycles per minute is the rate therefore the number of cycles per 6 second strip x10 = rate
