The Heart as a Pump and ECG

Explain the mechanical events occurring in the heart during one cardiac cycle (Pump)

• Cardiac Inotropy

Explain the electrical events occurring in the heart during one cardiac cycle (Initiation of heartbeat)

• Cardiac Ion Channels and the Heart Beat

One thing not discussed in the above post is that the I_h current involved in pacemaker cells is also called the "funny current." The "funny channels" involved in the funny current are leaky to both sodium and potassium, but long story short, sodium wins, which is why sodium gets to come into the cell and slowly bring pacemaker cells to the threshold potential. As the "funny channels" close (remember, sodium channels become deactivated after being open for long enough), T-type (transient) calcium channels take over, bringing the cell to threshold for the upstroke (which, as the post linked above describes, is carried out by L-type calcium channels in cardiac muscle).

Discuss the Wiggers diagram

Here is the Wiggers diagram, courtesy of Wikimedia Commons:



Essentially, the Wiggers diagram sums up various heart measurements into one handy diagram.

Aortic, Atrial, and Ventricular Pressures

At the top, you have aortic, atrial, and ventricular pressure. The pressures given are for the left side of the heart (you can tell because the pressure is high and because the left ventricle pumps blood into the aorta). At first, both atrial and ventricular pressures are quite low. After the mitral valve closes, isovolumetric contraction (i.e. contraction without changing the volume of blood) occurs, causing the pressure in the ventricle to increase as the blood has nowhere else to go. (Atrial pressure remains low as the atria are still "open" to the veins that come before them.)

Aortic pressure normally remains between 80mmHg and 120mmHg (i.e. your diastolic and systolic pressures). Ejection of blood occurs when the ventricular pressure exceeds the aortic pressure for the first time (i.e. at the first dotted line), causing the aortic valve to open and allow blood to enter the aorta. Once the ventricular pressure falls below the diastolic pressure, the aortic valves snap shut and the ventricles relax, causing the pressure to fall. When the pressure in the ventricles falls below the atrial pressure, the mitral valve opens, allowing filling to occur again.

You might have also noticed the letters "a," "c," and "v" on the atrial pressure curve. I don't think we need to know these, but I'll describe them for completeness. The a wave shows the increase in pressure during atrial contraction. The c wave shows the increase in atrial pressure as contraction of the ventricles causes the mitral valve to bulge back into the atrium. Finally, the v wave shows an increase in pressure as blood from the veins (v wave = venous wave) fills the atrium.

Ventricular Volume

The next curve down represents ventricular volume. You'll notice that ventricular volume increases during diastole (when the heart is filling) and decreases during systole (when the heart is pumping out blood). You might also notice that diastole occurs in two phases: ventricular volume increases and plateaus before increasing and reaching a plateau again. The first phase of increasing and reaching a plateau is due to passive ventricular filling. The second phase is due to atrial contraction providing a last little "push" to get some extra blood into the ventricles.

Electrocardiogram and Heart Sounds

• The Heartbeat