The Heartbeat

Define Sinoatrial node, Atrioventricular node and Purkinje Fibres.
Describe the conduction of electrical signals (action potentials) through the heart.

• Sinoatrial node- The main pacemaker of the heart. It is in the right atrium and has an intrinsic activity of around 90-100 beats per minute. As it is the fastest, it drives the others along (a phenomenon known as "overdrive suppression").
• Atrioventricular node- After an electrical impulse is created through the sinoatrial node, it goes to the other atrium via an interatrial pathway and to the atrioventricular node via the internodal pathway. The impulse becomes delayed at the atrioventricular node due to so many impulses reaching it at once- sorta like traffic slowing down at merge points on the freeway.
• Purkinje Fibres- After passing through the atrioventricular node, the impulses travel through the Bundles of His to the apex of the heart (the bottom of the ventricles). From here, they travel through Purkinje fibres to stimulate the ventricles.

Define automaticity, ectopic beat and chronotropic.

• Automaticity- Able to generate impulses without external stimuli. The conducting fibres of the heart have automaticity, as well as autorhythmicity- they have their own rhythm.
• Ectopic beat- A heartbeat that arises from somewhere other than the usual pacemaker (the sinoatrial node). The place where an ectopic beat arises is called an ectopic pacemaker or ectopic focus.
• Chronotropic- Able to alter heart rate. (Inotropic is another term that refers to the ability to alter the strength of contraction.)

Describe the origin of the heart beat and the actions of autonomic neurotransmitters on heart rate.

As mentioned before, the heart beat arises from the sinoatrial node, travels to the atrioventricular node and then through Bundles of His and Purkinje fibres.

Autonomic neurotransmitters are chronotropic- they alter heart rate. Positive chronotropic agents, such as adrenaline and noradrenaline of the sympathetic nervous system, increase heart rate by acting on β-adrenoceptors. Negative chronotropics agents, such as acetylcholine of the parasympathetic nervous system, decrease the heart rate. They do this by acting on muscarinic receptors. Normally, the heart is mainly under parasympathetic tone, which makes sense: the sinoatrial node's inherent pace is 90-100bpm, but the average resting heart rate is only around 70bpm.

Define EKG/ECG, and list the main waves.
Draw and label a normal EKG.
Explain the cause of the main waves present on a normal EKG.

An EKG/ECG is an electrocardiogram. ("EKG" comes from the German name.) It is a way of measuring the electrical activity of the heart. Note that it doesn't measure mechanical activity (such as whether the heart is actually beating or not)- it only measures the electrical impulses that stimulate the heart to beat.

This is a sketch of a typical ECG:

The first wave is called the P wave. This represents depolarisation of the atria and thus immediately precedes atrial contraction. The next wave is the QRS complex, which represents depolarisation of the ventricles and thus immediately precedes ventricular contraction. Atrial repolarisation actually happens at the same time as the QRS complex, but you don't see it as it's miniscule compared to the size of the QRS complex. Finally, the T wave represents repolarisation of the ventricles.

Predict how simple changes in the electrical activity of the heart will alter the EKG.

Various pathological conditions can alter the ECG. For example, in a 3rd degree heart block, the electrical signal can't travel between the atria and the ventricles via the atrioventricular node. This causes the atria and ventricles to depolarise independently of each other, resulting in a bunch of P waves and a bunch of QRS complexes that don't occur in their usual rhythm.

In ventricular fibrillation, where multiple impulses travel all around the ventricles at once, the ECG pattern is very erratic. This is a medical emergency requiring something like a defibrillator to get the heart back to normal again.

A third condition that affects the ECG is an ectopic beat, as defined earlier in this post. This causes an extra beat to appear on the ECG. Sometimes this beat might be upside down- you see, the ECG measures whether an impulse is travelling away from or towards a certain point (i.e. where the ECG lead is), and so a beat originating from a different spot may appear as negative instead of positive, or vice versa.

Define atrioventricular and semilunar valves.
Describe the function of the heart valves and the origin of the heart sounds.

For this unit, all you really need to know is that atrioventricular valves are between the atria and ventricles, and semilunar valves are between the ventricles and pulmonary artery (for the right ventricle) or aorta (left ventricle). They prevent backflow of blood between ventricles and atria, or between arteries and ventricles. When blood pushes back against the valves, this creates turbulence which can be heard as a heart sound.

For more information on the valves and their sounds, see my previous post on the heart and mediastinum for ANHB2212.

Describe the changes in ventricular pressure and volume that occur during the cardiac cycle.

During diastole, the volume of the ventricles increases. This occurs most rapidly at the beginning of diastole. When the atria contract, the volume increases to its maximum volume (end diastolic volume), usually around 135mL. During this time, the pressure increases slightly, but not very much.

Following this is a period of isovolumetric ventricular contraction. What does this mean? Well, the ventricles are contracting, but the volume stays the same (iso = same, volumetric = volume). This causes pressure to build up until it is higher than that of the aorta or pulmonary artery.

When the pressure is high enough, the ventricles pump out their blood into the arteries. This causes the volume in the ventricles to decrease down to around 65mL (end systolic volume). When ventricular pressure drops below aortic/pulmonary artery pressure, the semilunar valves close. This begins a period of isovolumetric ventricular relaxation, in which the ventricles are relaxing but the volume is not changing.

And thus the cycle starts all over again!

Draw and label a diagram showing the EKG, heart sounds, and ventricular pressure and volume.

Couldn't be bothered drawing a diagram, so I stole one from http://www.cvphysiology.com/Heart%20Disease/HD002.htm.

(Just a heads up: I'm going to ignore sounds S₃ and S₄ for the purposes of this explanation. That's mainly because, according to the website that I stole the graph from, they are usually indicative of pathologies that we don't need to know right now.)

Let's start from the beginning, shall we? First there is a P wave on the ECG, which precedes the last little bit of blood going into the ventricles. Next the QRS complex begins, as does isovolumetric contraction. (Note that waves on the ECG precede the thing that they cause- after all, you need the signal before you get the contraction.) The first heart sound is heard right after the QRS complex: after all, the isovolumetric phases are when everything's closed up and blood can bash itself against the now closed valves.

Anyway, back to the story. Ventricular pressure increases, yada yada yada. Somewhere in there the ventricles repolarise, as indicated by the T wave. A bit after that, isovolumetric ventricular relaxation occurs, and just like the first isovolumetric phase, there's a heart sound. Boom!

And then it all starts over again...