Chapter 4: Rate

Question 1

First thing to determine on a ECG

Answer 1

Rate

Question 2

Units measurement for rate

Answer 2

Cycles /min

Question 3

Heart’s normal pacemaker

Answer 3

SA node

• the dominant center of automaticity
• generates sinus rhythm

Question 4

Normal rate range

Answer 4

60-100 beats per minute

Question 5

Location of SA node

Answer 5

within the upper-posterior wall of the right atrium

Question 6

Sinus bradycardia

Answer 6

When the SA node paces the heart at a rate slower than 60 beater/min

Question 7

Causes of sinus bradycardia

Answer 7

-parasympathetic excess (ex: conditioned athletes at rest)

Question 8

Potential consequences of sinus bradycardia

Answer 8

-may slow heart rate to point that blood flow to the brain is reduced causing syncope

Question 9

What happens when the SA node pacemaking fails

Answer 9

One of the other potential pacemakers (automaticity foci) will assume pacemaking activity at their inherent rate

Question 10

Atrial automaticity focus

Answer 10

The automaticity foci within the atrial conduction system

Question 11

Junctional automaticity foci

Answer 11

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

Question 12

Ventricular automaticity focus

Answer 12

-automaticity foci in purkinje fibers including he Hiis Bundle and the bundle branches and their subdivisions

Question 13

Rate of pacing of the different automaticity foci

Answer 13

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

Question 14

Overdrive suppresion

Answer 14

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

Question 15

When does a backup pacemaker take over

Answer 15

When a higher pacemaker fails - backup pacemaker is no longer over drive suppressed an will emerge to pace at its inherent rate

Question 16

Automaticity focus when SA node fails

Answer 16

Atrial automaticity focus

Question 17

Automaticity focus when atrial focus fails

Answer 17

AV junctional automaticity focus

Question 18

Idio-junctional rhythm

Answer 18

Rhythm produced when junctional automaticity focus is dominant focus

Question 19

Conditions for junctional automaticity focus to become the active pacemaker

Answer 19

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

Question 20

When does ventricular automaticity focus become the pacemaker

Answer 20

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

Question 21

Idio-ventricular rhythm

Answer 21

Rhythm produced when ventricular automaticity focus is pacemaker

Question 22

How to calculate the rate

Answer 22

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

Question 23

Basis of rate estimation - explanation of the method

Answer 23

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

Question 24

Estimation of very slow rates (bradycardia)

Answer 24

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