• Debrief on last Friday's porcine heart lab
• Autonomic Nervous System
• Pacing
• Determination of Heart Rate
• Intro to Rhythm
• Practice

Your lab write--ups looked great! Hope you had fun. One correction for several groups: When I asked you to draw the porcine heart in cross section, I sure got some interesting renderings! Let's look at the porcine heart in cross section...

<-- Note the left ventricle looks like a donut! Thick concentric muscular walls. The right ventricle loks more like a horseshoe, has thinner walls, and 'wraps around' the left ventricle. If I ask you to draw this on the midterm exam, will you remember? Good :-)

SNS: stimulation of cardiac beta adrenergic receptors (ß1) increase:

Rate of SA node pacing

Rate of conduction

Force of contraction

Irritability of foci

PSNS: Stimulation of cardiac cholinergic receptors results in decreased:

Rate of SA node pacing

Rate of conduction

Force of contraction (weak effect?)

Irritability of foci

Fun with the PSNS!

Intrinsic vs. ectopic pacemaker

SA node: intrinsic pacer. Rate 60-100.

Normal sinus rhythm (NSR): 60-100 BPM. ‘P’ for every QRS; All ‘P’s look the same. R-R intervals regular.

Sinus bradycardia
Sinus tachycardia (see image below)

Ectopic pacemaker (ectopic focus): Any focus pacing a beat that is NOT the SA node.

Supraventricular

Junctional

Ventricular

 

It’s EASY!…as long as R-R intervals are regular!

Are they?

If YES:

Memorize this sequence

“300-150-100-75-60-50”.

Chant the sequence as you count the # of large boxes between 2 R waves.

Here's another way to determine rate:

Divide number of large boxes between 2 R waves into 300

60 sec/min ÷ 0.20 sec/big box = 300 big boxes in 1 minute

Divide number of little boxes between 2 R waves into 1500

60 sec/min ÷ 0.04 sec/little box = 1500 little boxes in 1 minute

 

If R-R intervals are irregular, then what?:

Be sure you have a ‘rhythm strip’ with at least 6 seconds of rhythm.

Count # R-R intervals in six seconds (30 big boxes). Add a zero.

Sinus rhythm: variations include…

NSR

Sinus bradycardia

Sinus tachycardia

Sinus arrhythmia (HR variability associated w/ respiration)

What’s this one?… Yep! It's sinus arrhythmia!

Irregular rhythm:

Example: Atrial fibrillation! Key features: no p waves; course baseline between R waves; R-R intervals are ‘irregularly irregular”

Escape rhythm:

An ectopic focus escapes the overdrive suppression of the SA node to pace at

its inherent rate… Junctional rhythm is shown below!

Escape beat:

An ectopic focus transiently escapes overdrive suppression for 1 beat!

We looked at an example of a ventricular escape beat from you text (see page 121 and 337). Note that escape beats and rhythms are late, rather than early! They follow a pause in conduction! This becomes important when we begin distinguishing between premature and escape beats. One is early and one is late!

 

At the end of class, I distributed a handout and we reviewed three tracings for rate and rhythm. Keep your handout to review when midterm time comes around! Also, please check out the EKG websites linked to the course home page! Keep practicing and have fun!

Lab on Friday! Resting 12-lead EKG. Please meet in the EMS Exercise Physiology Lab. Student Rec Center. Don't forget you ID! If you're female and interested in begin a subject, please bring a swimsuit top. See you there bright and early!