C33 Lab Marking Schemes.doc

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Biological Sciences
Stephen Reid

BGYC33: ECG I and ECG II Laboratory ECG I Data and Calculations (2 marks) Questions 1. What do your data (i.e., different intervals; heart rate, etc.) say about the heart of the subject(s)? Is the activity normal? Do the data suggest any abnormalities? (2 marks) This is a relatively open question but the students should compare heart rate and the various distances (see below and the lab manual) with the standard values listed in the lab manual. They should note whether the P wave, QRS complex and T wave are in the normal orientation and state if the heart seems normal or if something is wrong. Give 0.5 marks for comparing heart rate to normal. Give 0.5 marks for discussing the distances Give 0.5 marks for talking about the shapes/heights/up or down nature of the waves Give 0.5 marks for diagnosis of healthy or abnormal ECG. P-R distance (time of conduction through the atria and AV nodes) Q-T distance (ventricular systole) T-Q distance (ventricular diastole) DURATION PHASE (second) P wave 0.06 – 0.11 P-R interval 0.12-0.20 P-R segment 0.08 QRS complex (R) < 0.12 S-T segment 0.12 Q-T interval 0.36-0.44 T wave 0.16 2. Explain the changes in heart rate between conditions (a. inspiration versus expiration; b. regular breathing versus exercise; c. lying down versus sitting up; d. exercise versus recovery from exercise). Describe the physiological mechanisms causing these changes. (4 marks) a. inspiration versus expiration (1 mark) Heart rate should be greater during inspiration than during expiration. During inspiration, the thoracic pressure decreases causing a drop in central venous pressure and a decrease in venous return. Cardiac output and blood pressure drop accordingly which activates the baroreflex (carotid sinus and aortic arch baroreceptors). Baroreceptor activity decreases leading to a reflex increase in heart rate which would help to raise cardiac output and blood pressure. During expiration, the diaphragm and intercostal muscles relax. Thoracic volume decreases which causes an increase in central venous pressure, venous return, EDV, SV, CO and BP. This increases baroreceptor firing leading to a decrease in HR. b. regular breathing versus exercise (1 mark) Exercise leads to an increase in general sympathetic output and a decrease in parasympathetic output. At the level of the SA node, these effects will cause an increase in heart rate. c. lying down versus sitting up (1 mark) Immediately upon standing, the arterial pressure in the head and upper part of the body will fall. The baroreflex will therefore lead to an increase in heart rate via increased sympathetic and decreased parasympathetic activity. d. exercise versus recovery from exercise (1 mark) During recovery from exercise, the exercise-induced increase in sympathetic tone begins to subside and parasympathetic tone begins to increase (from the exercise-induced decrease). As such, HR should begin to decrease as one recovers from exercise. In this lab, the one minute difference between the 2 recording times (start and end) may not have been enough to see a difference. 3. After a heart attack involving areas of myocardial tissue death, the survivor’s ECG is altered in characteristic ways. One alteration often seen is in the T-wave, which may go down instead of up. Name another ECG region that would show an alteration from normal in the ECG just described. You need not say exactly what the abnormality is, just where does it occur? Explain your answer. (2 marks) The T-wave represents ventricular repolarisation. If repolarisation of the ventricle is abnormal then it is likely that depolarisation of the ventricle is also abnormal. Therefore the QRS complex would be the other ECG region that would likely be abnormal. ECG II Data and Calculations Graph 1 (1 mark) Graph 2 (1 mark) Graph 3 (1 mark) Questions 1. Explain the physiological reason(s) for any differences in the mean electrical axis under the following conditions. a. lying down versus sitting up. (1 mark) If the heart is angulated to the left, the mean electrical axis will shift to the left. This occurs when a person lies down due to the abdominal contents pressing upwards against the diaphragm causing the angle of the heart to shift leftwards. If the heart is angulated towards the right, the mean electrical axis will shift to the right. This happens when a person stands up. b. inspiration versus expiration. (1 mark) Breathing out; heart moves to the left; axis moves to the left Breathing in; heart moves to the right; axis moves to the right. 2. What factors affect the orientation of the mean electrical axis of the heart? (3 marks) 0.5 marks for each of the below to a maximum of 3 marks. a. Posture b. Breathing c. Ventricular hypertrophy. d. Branch bundle block. e. Muscular destruction such as heart attack. f. Chronic obstructive lung disease g. pulmonary embolism h. congenital heart defects i. severe pulmonary hypertension j. hypertension k. aortic stenosis l. ischaemic heart disease 3. How would the electrical axis of the heart change during branch bundle (left and right) block? (2 marks) Normally, the lateral walls of the two ventricles depolarise at the same time because both branch bundles transmit the electrical impulse to each side at the same time. As a result, the currents blocked, depolarisation of the two ventricles does not occur simultaneously and the depolarisations currents do not neutralise each other. (give 1 mark for explaining this) If the left bundle is blocked then the cardiac depolarisation spreads through the right ventricle faster than the left ventricle. As a result, much of the left ventricle remains at rest when the right ventricle has become depolarised. This means that the direction of depolarisation has been strongly biased from right to left. There is an intense left axis deviation. (give 0.5 marks) If the right bundle is blocked, the opposite thing happens. The left ventricle depolarises first and so the wave of depolarisation heads from left to right. There is a strong right axis deviation. (give 0.5 marks) BGYC33 Blood Pressure Laboratory Data and Calculations (3 marks) A. Systolic Measurements (Table 16.2) B. Diastolic Measurements (Table 16.3) C. BPM Measurements (Table 16.4) D. Summary of Mean Blood Pressure Data (Table 16.5) E. Timing of Korotkoff Sounds (Table 16.6) F. Calculation of Pulse Speed Questions 1. Using a flow chart, describe the baroreflex? (1 mark) Below is the baroreflex. All of the changes will be opposite if the MAP increases instead of decreases. This should be replicated almost identically since it is right out of the lecture notes. ↓ M AP B aroreceptors ↓ Parasympathetic ↑ Sympathetic (vagal) activity activity ↑ H R ↑ SV ↑ TPR ↑MAP 2. What effect would cutting the nerves to the carotid sinus have on changes in blood pressure associated with a change in posture (i.e., standing versus lying down)? (1 mark) If you cut the nerves to the carotid sinus then the carotid sinus baroreceptors will not be able to report changes in MAP to the brain. However, the aortic arch baroreceptors will still be able to signal changes in MAP. Full marks should be awarded if the students mention one of these two different than if the carotid sinus nerves hadn’t been cut because the aortic arch is still functional. 2) the change in blood pressure would be greater with the nerves cut since the baroreflex doesn’t respond to its normal extent due to the absence of input from the carotid sinus. 3a. Does your systolic and/or diastolic arterial pressure change as your heart rate changes? (1 mark) BP = CO X TPR = HR X SV X TPR. Therefore, if HR increases then BP (MAP) would increase. This increase could however, lead to compensatory changes via the baroreflex leading to a decrease in SV and/or TPR which could prevent any observed increase in BP. (1 mark: 0.5 marks for stating increase; 0.5 marks for mentioning the baroreflex) 3b. How does this change affect pulse pressure? (1 mark) Pulse Pressure (PP) = Systolic Pressure (SP) – Diastolic Pressure (DP) Pulse pressure is directly related to SV and inversely related to HR. When SV increases, SP increases more than DP. If HR increases, filling time decreases, EDV decreases, SV decreases and PP decreases. If HR decreases, filling time increases, EDV increases, SV increases and PP decreases. (1 mark: 0.5 marks for the change in PP; 0.5 marks for mentioning filling time and EDV) 4. Why is the baroreflex unimportant in the long term regulation of blood pressure? (1 mark) The baroreceptors become desensitised and adapt to any new level of blood pressure. For example, at a normal MAP (i.e., 80 mmHg) the baroreceptors are firing at a certain rate. If BP increases to 120 mmHg then the baroreceptors will increase their rate of firing. However, if BP remains elevated at 120 mmHg then over the course of about 3 days, the baroreceptor firing rate will decrease to what it used to be at 80 mmHg. In other words, the baroreceptors now see 120 mmHg as being the new normal and don’t try to regulate it downward. 5. What is the CNS isc
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