ECG and heart sounds practical (Dr. Houston)

Question 1

What is an ECG ?

Answer 1

Electrocardiogram = the pattern of electrical activity recorded at the body surface

Question 2

Where does blood enter the heart ?

Answer 2

Blood enters the atrial chambers of the heart at a low pressure and leaves the ventricles at a higher pressure.

Question 3

How does the blood circulate w/ respect do the distribution of pressure in the different chambers in the heart ?

Answer 3

The heart pumps blood from areas of high pressure to areas of low pressure.

Question 4

Where does blood returning from the body go to ?

What happens to the blood ?

Answer 4

Blood returning from the body arrives at the right side of the heart through the right atrioventricular (mitral) valve and is pumped through the lungs. Oxygen is picked up and carbon dioxide is released.

Question 5

Where does oxygenated blood enter the lungs ?

Answer 5

Oxygenated blood arrives at the left side of the heart through the left atrioventricular (tricuspid) valve, from where it is pumped back to the body

Question 6

What is respiratory sinus arrythmia (RSA) ?

Answer 6

RSA is heart rate variability in synchrony with respiration, by which the R-R interval on an ECG is shortened during inspiration and prolonged during expiration.

Question 7

Is cardiac contraction dependent upon nerve supply ?

Do nerves have any effect at all ?

Answer 7

NO !
This is regulated by pacemaker cells in the sinoatrial node (SA node).
Innervation by the PS (vagus) and S nerves does modify the basic cardiac rhythm.

Question 8

How do pacemaker cells work ?

Answer 8

They rhythmically produce APs that spread through the muscle fibers of the atria –> contraction that pushes blood into the ventricles.
APs spread slowly through the AV node (only electrical connection between atria and ventricule) –> allowing atrial contraction to contribute to ventricular filling
APs spread rapidly through the AV bundle + Purkinje fibers to excite both ventricles.

Question 9

What is the purpose of recording cardiac activity by placing recording electrodes on the skin ?

Answer 9

cardiac cycle = sequential contraction of atria and ventricles –> combined electrical activity of the different myocardial cells produces electrical currents that spread through the body fluids –> currents are large enough to be detected by recording electrodes placed on the skin

Question 10

What are the 3 phases of the APs recorded from atrial + ventricular fibers ?

Answer 10

• a rapid depolarization,
• a plateau depolarization (which is very obvious in ventricular fibers)
• a repolarization back to resting membrane potential

Question 11

What 3 distinct features can be observed in on cardiac cycle ?
What do these correspond to ?

Answer 11

• P-wave produced by atrial depolarization
• QRS complex produced by ventricular depolarization; atrial repolarization also occurs during this time, but its contribution is insignificant.
• T-wave produced by ventricular repolarization

Question 12

How does the heart ensure that blood does not flow backwards ?

Answer 12

By detecting pressure differences between 2 chambers:
the valves close automatically whenever there is a pressure difference across the valve that would cause backflow of blood.

Question 13

What is the characteristtic sound produced by the heart ?

What is this sound due to ?

Answer 13

The closure of the valves gives rise to audible variations = heart sounds = ‘lub-dup’ sound.

• lower-pitched ‘lub’ = early phase of ventricular contraction = closing of the atrioventricular (mitral and tricuspid) valves.
• higher-pitched “dup” = closing of aortic valves (aortic and pulmonary)

Question 14

How can malfunctions of valves in the heart be detected ?

Answer 14

W/ a stethoscope.

Question 15

What are the 4 sequence of events that occur w/in each cardiac cycle ?

Answer 15

• Ventricular diastole: blood returning to the heart
• Atrial systole: filling of ventricles is complete (accounts for ~20% of atrial filling)
• Isovolumic ventricular contraction: as ventricles begin to contract the pressure in them rises and exceeds that in the atria –> closes the AV valves. Until the pressure in the left ventricle exceeds that in the aorta (and in the right ventricle exceeds that in the pulmonary artery), the volume of the ventricles can not change
• Ventricular systole: aortic and pulmonary valves open and blood is ejected into the aorta and pulmonary arteries

Question 16

What is the consequence of the so-called isovolumic ventricular contraction ?

Answer 16

Ventricular systole causes pressure to in the left ventricle to shoot to ~120 (mm Hg)

Question 17

How does the amplitude of the P-wave, QRS-complex and T-wave vary between rest and exercice ?
What about the intervals between cardiac cycles ?

Answer 17

The amplitudes of the P-wave and QRS-complex are increased during rest, but that of the T-wave is diminished
Equally at rest, the interval between 2 cardiac cycles increases.

Question 18

How are the R-R, P-R, QRS, Q-T and T-Q intervals affected by exercise ?
What about the heart rate ?

Answer 18

During exercise, The R-R and T-Q intervals are diminished and the heart rate increased.

Question 19

“Lub” = systole = ventricular contraction
“Dup” = diastole = vetricular relaxation
When do these sounds occur in the cardiac cycle

Answer 19

The “lub” sound (closing of atrioventricular valves) occurs right after the QRS complex.
The “dup” sound (closing of semilunar valves) occurs after the T-wave.