Student Learning Outcomes

• Define blood pressure, pulse, heart sounds, systolic and diastolic pressures.
• Summarize the pressure/volume relationship of the aorta, atria and ventricles during one cardiac cycle.
• Measure blood pressure, radial pulse, apical pulse, using sphygmomanometer, stethoscope.
• Differentiate partial heart block from complete heart block.
• Explain the effects of physical and chemical modifiers of the heart.

Introduction
The cardiac cycle covers one heart beat; one contraction (systole) and one relaxation (diastole) of both atria, first, and ventricles, second. It is this contraction and relaxation that is responsible for getting blood into and from the heart to all over the body. Remember the intrinsic conduction system begins with the SA node initiating and transmitting electrical impulses. These impulses will get the atria to contract and after, once the signal passes to the AV node, the AV bundles, the bundles branches and Purkinje fibers, the ventricles will contract.

Cardiac Cycle – Effect of Pressure
When the atria contract, blood will be ejected into the ventricles. Subsequently, the ventricles will contract ejecting blood into the pulmonary trunk and the aorta. This contraction and relaxation involved in getting blood into and out of the chambers is what one cardiac cycle entails. During contraction and relaxation of the chambers there is a change from high pressure (contraction) to lower pressure (relaxation). The presence of high pressure during contraction is responsible for pushing blood into the chambers.

Let’s begin the cycle with the atria. At the beginning of the cycle they are relaxed and blood from the IVC, SVC and pulmonary veins enters the atria. The AV valves are opened so blood passively enters the ventricles. This phase is responsible for about 70-80% of blood filling the ventricles. Eventually, the atria contract ejecting the remaining blood into the ventricles. By the time the electrical impulse gets from the AV node to the Purkinje fibers, the ventricles begin to contract, increasing in pressure and causing the AV valves to close. In closing, blood rushes back onto the AV valves forming the first heart sound (S1). Now all four valves are closed and pressure within the ventricles continues to increase without change in the volume of blood, (Isovolumetric contraction). When the pressure in the ventricle exceeds the pressure in the aorta and the pulmonary trunk the aortic valve open and blood is ejected into the aorta and pulmonary trunk. Eventually the ventricles begin to relax and the pressure drops. When pressure in the aorta exceeds the pressure in the ventricles the semilunar valves close prevent the back-flow of blood. Blood in the aorta rushes back onto the closed semilunar valve, causing to the second heart sound (S2). Closing of the semilunar valves leads to a small increase in aortic pressure referred to as the dicrotic notch. The ventricle continues to relax (isovolumetric relaxation). When the pressure in the atria exceeds the pressure in the ventricle the AV valves open allowing blood to once again flow passively into the ventricles. The process will continue with every heartbeat.

Cardiac Cycle – Change in Volume
With contraction and pumping of blood from atria to ventricles and to aorta and pulmonary trunk, the volume of blood also changes. Let’s discuss the change in volume of blood in the ventricles during one cardiac cycle.
During passive filling of the ventricles, the pressure in the ventricle is relatively low but higher in the atria so blood flows into the ventricles increasing ventricular volume. Eventually the atria contract increasing in pressure and ejecting more blood into the ventricles leading to a slight increase in ventricular volume. The volume in the ventricles, about 140 ml, is now known as the end diastolic volume (EDV). The ventricles begin to contract, increasing ventricular pressure forcing the AV valves to close. The ventricle will eventually contract increasing the pressure in the ventricles until it exceeds that of the aorta forcing the semilunar valves open and ejecting blood into the aorta. This amount is known as the stroke volume which is about 70ml. All the blood in the ventricle never gets pumped out. About 70 ml remains and is known as the end systolic volume. Following contraction, the ventricles will begin to relax reducing the pressure in the ventricles.

Phases of cardiac cycle

1. Ventricular filling – Passive flow of blood from atria
2. Atrial contraction – ejection of blood from atria to ventricles
3. Isovolumetric contraction – all valves are closed ventricular pressure increases without increase in volume as ventricles continue to contract
4. Ventricular contraction – ventricles are contracting, pressure continue to increase.
5. Ventricular ejection – pressure in ventricles exceed pressure in aorta and PV, semilunar valves open blood is ejected from ventricles.
6. Isovolumetric relaxation – ventricles are relaxing valves close, pressure decrease without change in volume.

Phases of the cardiac cycle. Atria receives blood from IVS, SPV and pulmonary veins. Blood enters the ventricles passively though AV valves (ventricular filling phases). Atria contract ejecting more blood into ventricles (atria contraction). Atria begins to relax and ventricles begin contracting. All valves are closed and ventricles are continuing to contract (isovolumetric contraction). Pressure in ventricles are higher than pressure in aorta and pulmonary trunk (PT), semilunar valves open. Ventricles eject blood into the PT and aorta (ventricular ejection). Ventricles are relaxing pressure in PT and aorta higher than in ventricles, semilunar valves close. Ventricles continue to relax (isovolumetric relaxation). Pressure in ventricles are lower than in atria, AV valves open. Cycle repeats.

Heart Sounds
During the cardiac cycle there are two prominent heart sounds heard, S1 which is as a result of closing of the AV valves and, S2 closing of the semilunar valves. A third and a fourth sound, S3 and S4, can be heard but rarely in healthy individuals. In regular medical physicals exams, heart sounds or auscultation is performed to evaluate the heart or lungs. In terms of the heart, the sounds of the four valves are evaluated. The figure below shows the location the stethoscope is placed in order to hear the valves. An abnormal sound associated with the heart is called a heart murmur. Heart murmurs are as a result of turbulent flow of blood through the heart. Usually heart murmurs are caused by problem with one or more heart valves caused by regurgitation or obstruction (stenosis).

Heart Rate

Heart rate is the number of complete heart beats per minute. The heart rate can increase or decrease depending on the health condition of an individual or the activities involved. Pulse is the repetitive throbbing felt as your heart contracts and relaxes. There are several locations on the body a typical pulse can be felt. These areas are where arteries can be found superficially. The most common areas used are the radial and the brachial pulse. Pulse measurement represents heart beats and can be used to measure heart rate, the number of beats per minute. Pulse rate is different from pulse pressure which is the difference between the systolic pressure and the diastolic pressure. Pulse rate indicates how fast or slow the heart beats while pulse pressure indicates the force the heart generates at each beat. Both can signal pathological conditions. Normal pulse should be 60-100 while normal pulse pressure should be 40-60 mm Hg.

Cardiac output, the amount of blood pumped by the ventricles in one minute, is important to maintain blood circulation and tissue and cell perfusion. If cardiac output is too low, then the brain and other vital organs will not get sufficient amount of blood to function. So to abnormally high cardiac output, high-ouput heart failure, can also be as dangerous. Therefore, it is important to maintain normal cardiac output which is in the range of 5 ml per minute in young adult male. Factors affecting cardiac out are heart rate (HR) and stroke volume (SV). Stroke volume is the amount of blood pumped out of the ventricles every heart beat or cycle. It is the amount of blood in the ventricles before it contracts, end diastolic volume or EDV, minus the amount of blood remaining in the heart after ejecting blood, end systolic volume or ESV. If you increase or decrease HR and or SV cardiac output will be affected. For example, if the HR is 70 bpm and SV is 80 ml/beat then CO is 70 x 80 = 5600 ml/minute. If the HR increases to 80 bpm and SV remains the same, then CO will now be 6400 ml/minute. Each of these is affected by several factors and conditions. HR is affected by autonomic nervous system, hormones like epinephrine and norepinephrine, exercise, stress, and age. Stroke volume is affected by fitness levels, contractility, heart size and several other factors.

Exercise and heart rate
During exercise, the heart tries to compensate for the added increase in activity for trying to receive and deliver blood to vital organs, except the brain which tend not to be affected greatly. As more blood is needed by organs and tissues, there is also an increase in blood returning to the heart. Therefore, the heart has to increase its activity, the pumping action, to compensate. Frank-Starling principle states that increasing the amount of blood in the ventricles will lead to increase contractility or force of contraction. In so doing the amount of blood ejected from the heart will increase. Chronotropic drugs are drugs will affect heart rate. Drugs that increase HR are positive chronotropic while those that decrease are negative chronotropic. Some drugs are also labeled as inotropic drugs; they affect contractility of the heart.

Other factors affecting HR

Blood Pressure
Heart and health conditions can be assess with blood pressure measurement, which is the pressure of the blood in systemic arteries. It is recorded as two numbers; systolic pressure and diastolic pressure. Systolic pressure is the pressure from contraction of the ventricles (left ventricle) while diastolic pressure is pressure during relaxation of the ventricles. The values are recorded as two numbers with the systolic, the higher number, over the diastolic, the lower number. Normal blood pressure is in the range of 120/80 mm Hg. When blood pressure is consistently high, above 130/80, medication may be prescribed. Another important measurement taken into consideration is the mean arterial pressure (MAP). This represents the average pressure of blood within your arteries. It indicates the flow of blood to organs and tissues. It is calculated by adding diastolic blood pressure to a third of the difference between systolic and diastolic blood pressure. Normal MAP range is 70-110 mm Hg.

 

MAP = diastolic BP + ((sytolic-distolic BP))/3

Activity 1 – Pressure and Volume

Activity 2 – Pulse Measurement

Activity 3 –  Blood Pressure Measurement

Activity 4 – Effect of Exercise on BP