March 15 Lectures

Partridge
cell to cell propagation in cardiac cells uses gap junctions.  speed of propagation depends on slope of phase 0 of cell action potential which can be altered by electrolyte imbalance (high K for example) and autonomic drugs.  Agents that alter speed of conduction are said to have a "dromotropic" effect.

p. 130 Costanzo is wrong is stating that phase 0 of SA node action potential is due to T-type Ca channels.  It is L-type.  Here is a nice figure on the SA node action potential:

From http://www.cvphysiology.com/Arrhythmias/A004.htm
Phase 4 starts with funny channels. If.  As the membrane potential reaches about -50 mV, another type of channel opens. This channel is called transient or T-type Ca⁺⁺ channel. As Ca⁺⁺ enters the cell through these channels down its electrochemical gradient, the inward directed Ca⁺⁺ currents further depolarize the cell. As the membrane continues to depolarize to about -40 mV, a second Ca⁺⁺ channel opens. These are the so-called long-lasting, or L-type Ca⁺⁺ channels.
heart muscle can't cramp.  twitches can't summate.

total period of systole is about 300 msec (total conduction time through heart).  diastole is about 500 msec. at resting heart rate (ca. 70 bpm).  at higher heart rates, period of systole shortens only slightly; diastole period is decreased so filling time decreases and can lead to decreased stroke volume.

Intrinsic firing rate of the SA node (without autonomic tone) is about 100 bpm.  The normal autonomic tone is predominantly vagal which lowers the innervated SA node resting rate to about 70 bpm.



http://www.cvphysiology.com/Arrhythmias/A005.htm


Resta

P. 82. Flow path of blood through heart and order of valves:  tricuspid then biscuspid (mitral);
Memory tool:  "try it before you buy it"
 
p. 85. LeCesne

Leads I, II, and III

memory trick:  count the L's

RA - LA  = lead I (1 L)

RA - LL = lead II (2 Ls)

LA - LL = lead III (3 Ls)

Fast and Easy way to calculate heart rate (and more accurate with irregular rhythms) = count R waves in rhythm strip and multiply by 6.  Standard 8.5 x 11 inch EKG rhythm strip gives 10 seconds of recording.  beats in 10 seconds x 6 = BPM.

For box method, you can also count the big boxes between R waves.  Divide 300 by the number of big boxes.  There are 5 big boxes per second (25 mm) so 300 big boxes per minute.  1 big box between R waves = 300/1 or 300 bpm.  2 big boxes = 300/2 or 150 bpm, etc.


most books give the normal electrical axis as -30 to + 90 degrees instead of 0 to + 90 degrees.  When I and aVF are both positive, axis is in range of 0 to + 90 degrees.  When leads I and II are positive, then the axis is within the larger normal range of -30 to + 90 degrees.

Biphasic leads (equally positive and negative) are a great tool because the electrical axis is perpendicular to that lead.

In this ECG, lead aV_L is biphasic.  The positive perpendicular axis to aV_L is +60°. Therefore, the mean electrical axis is +60°, which is normal.  It could also be -60° but if that were true, aVF would be negative, not positive.

Timm

Mneumonic for valve defects (left ventricle)

aortic regurgitation diastolic (H)ARD

aortic stenosis systolic ASS

mitral stenosis diastolic MSD

mitral regurgitation systolic MRS

HARD ASS MSD (missed) MRS

If chambers are connected via a septal defect, the direction of shunted blood flow is from higher to lower pressure OR if pressures are equal, from stiffer chamber to more compliant chamber.

Sounds

website for heart sounds: http://depts.washington.edu/physdx/heart/tech2.html