Laboratory for Physiology ECG Electrical and Mechanical Events of the Cardiac Cycle

Introduction Last week we talked about the cardiac cycle, the waves induced by depolarization of each of the muscles of the heart and determined that you can record these events using surface electrodes.

Basis for the orderly contraction of the heart

Relationship between mechanical and electrical events Electrical events of the cardiac cycle initiate mechanical events Mechanical events = Contraction and relaxation of the heart muscles which generates pressure to push the blood through the circulatory systems and the opening and closing of the heart valves. Two phases of mechanical events: Diastole: the time during which cardiac muscle is relaxed aka the time during the cardiac cycle BETWEEN contractions Systole: The time during which cardiac muscle is contracting These terms are usually used in referring to contraction of the ventricles but they can also be applied to the atria (i.e. atrial systole etc).

Two phases of mechanical events Systole Diastole

Figure 14-24 - Overview

Two phases of mechanical events Ventricular systole begins about at the peak of the R Wave and ends at the end of the T wave Systole is the time when blood is pumped out of the heart and into the circulatory system Thus electrical depolarization of the ventricles represented by the QRS complex BEGINS BEFORE (about Q)the ventricular contraction starts and is completed long before (about S) the end of ventricular contraction. Depolarization PRECEDES the muscular contraction which makes sense. Ventricular diastole begins at the end of the T Wave and continues until the peak of the next R Wave. Time spent in diastole vs systole at normal resting HR (about 70bpm) you can see that the ventricles spend about 2/3 of the time in diastole (resting) and only 1/3 of the time in systole (Contracting) Even though your heart pumps blood your whole life, the heart actually spends most of its time at rest!

The heart as a PUMP During ventricular systole blood is pushed out of the heart and into the circulatory system creating a pressure wave in the arterial system which is transmitted by the arterial walls This pressure wave is measured when your blood pressure is taken using a blood pressure cuff or sphygmomamometer The smooth muscle walls of the arteries are innervated by the sympathetic division of the autonomic nervous system. Sympathetic innervation serves to set the resistance of the arterial system to blood flow ie when activity in symp is increased, vasoconstriction of the arteries occurs and this results in an increase in resistance to blood flow and a decreased amount of blood flow.

This is particularly true in the arterioles (small arteries) Sympathetic innervation serves to set the resistance of the arterial system to blood flow ie when activity in symp is increased, vasoconstriction of the arteries occurs and this results in an increase in resistance to blood flow and a decreased amount of blood flow. This is particularly true in the arterioles (small arteries) Symp = resistance = Blood Flow BV vasoconstrict

How do we accomplish this increase in sympathetic activity? Place hand in cold water (part I of lab) Cold water causes vasoconstriction….this is why if you cut yourself you run it under cold water to cause the vessels to constrict and decrease blood loss BE SURE to place the hand WITHOUT the transducer in the water. Otherwise we could witness the opposite effect via electric shock!!!

The valves of the heart Purpose of the valves? To keep blood flow unidirectional Four heart valves: the A-V or the mitral and tricuspid valves and the semilunar valves (aortic and pulmonary valves) The closing of these valves causes the lub dub or S1 and S2 sounds that you hear when you listen to the heart using a stethoscope S1=AV valves closing Heard best on lower chest (LUB) S2=Semilunar valves closing Heard best on upper chest (DUB) 2nd part of the lab you will listen and record the heart valve sounds and relate these sounds to the electrical events of the ECG

Figure 14-26 (2 of 5)

Blood Pressure and ECG BP will change as you shift body position from one laying down to standing up This is called orthostatic BP or positional BP The reason behind this phenomena is that when you go from lying down to standing up, gravity pulls the blood down into the lower part of your body and so BP in the upper part falls slightly This fall in BP triggers an autonomic reflex called the baroreceptor reflex which quickly brings BP back to normal. In the last part of the lab you will measure the change in BP when you go from lying down to standing up.

BARORECEPTOR REFLEX Responsible for maintaining homeostatic control of BP It involves the Baroreceptors which are sensory receptors located in the walls of the aorta where they form the aortic body and in the carotid arteries where they form the carotid bodies. Baroreceptors are stretch receptors that monitor the stretch in the walls of the BV and are connected to the cardiovascular control center (CVCC) in the medulla oblongata (Brain stem) CVCC controls sympathetic and parasympathetic activity for the CV system Baroreceptors maintain a tonic level of activity (i.e. they constantly send signals back to the CVCC). For example if the walls of the carotid arteries and aorta stretch as happens when BP increases, they increase their signal rate to the CVCC and the CVCC responds by sending an increase in para and decrease in symp activity. This causes a decrease in HR and dilation of the arterioles and a decrease in BP

BARORECEPTOR REFLEX As BP decreases the stretch in the walls of the aorta and carotid arteries decreases and the BR sense this and decrease signal rates to the CVCC. The CVCC responds by increasing Syp and decreasing Parasymp activity leading to increased HR and vasoconstriction of the arterioles and increased BP. This is a negative feedback loop which acts to maintain BP at a normal levels of about 120/80 mmHg. Stretch of aorta = BP = Baroreceptor = CVCC Parasymp Symp = HR = BP

Baroreceptors detect decrease stretch Baroreceptor Reflex Baroreceptor Reflex CVCC Parasymp Symp Stretch detected by Baroreceptor HR Arterioles constrict Baroreceptors detect decrease stretch Arterioles vasodilate BP

HINTS: Heart sounds: determine where to place the bell of the stethoscope transducer by listening first with your stethoscope to determine where on the chest you can hear best. You may have to move the bell of the stethoscope around to get a good recording Tape the bell of the scope to the chest wall to minimize movement. Orthostatic BP Test: It is very IMPORTANT for the subject to lie flat with their legs hanging over the end of the table and NOT moving for at least 15 minutes. Otherwise you will not get usable recordings and will have to do it again.

HINTS: After the subject stands up after lying flat for 15 mins it is important that they keep the hand with the transducer on it AS STILL AS POSSIBLE. Movement will create noise in the recording and obscure the results. I suggest that before you lie still for 15 mains you practice several times by lying down and getting up WITHOUT moving your hand. Check the BIOPAC and see if you are getting noise in your reading, if so you need to practice some more! The SS30L can be a bit tricky to use. I have had best results using male subjects (larger hearts) and taping the bell of the stethoscope to the chest wall. You can’t hold on to the bell of the stethoscope because your hand causes too much vibration artifact. Also, have them place the ear pieces of the scope on the table top where they will not be moved around by the subjects breathing. You may also have to have the subject breath shallow. Any movement of the stethoscope will create noise artifact in the recording. You might also need to adjust the filters on the biopac a bit. It is possible to get heart sounds with a little effort

NOTE FOR NEXT WEEK!!! ** Next lab will require subjects to exercise fairly rigorously for 9 minutes so chose who will be your subject this week and remember to wear comfortable clothes and shoes for next weeks lab.

Enjoy the lab HAVE FUN DO NOT RUSH!!!!! QUESTIONS? Enjoy the lab HAVE FUN DO NOT RUSH!!!!!