Cardiovascular Physiology Practical

• 4BBL1061 Fundamentals of Physiology L9 introduces physiology practicals, instructed by Dr Greg Knock
• The course includes practicals on cardiovascular and respiratory physiology, and fuel homeostasis and diagnosing diabetes

Practical 1: Cardiovascular Physiology (Nov 23)

• Involves studying the effects of posture, exercise, and facial immersion in cold water on Mean Arterial Pressure (MAP) and heart rate

Effects of Posture on MAP

• Moving from a supine (lying down) to a standing position involves several physiological adjustments
• Gravity opposes the upward flow of venous blood causing reduced venous return and stroke volume (Starling, L7)
• Cardiac Output (CO) and MAP is reduced
• Reduced baroreceptor firing causes increased sympathetic and decreased parasympathetic output
• Increased Total Peripheral Resistance (TPR) and Heart Rate (HR) improves venous return
• MAP returns to normal or above normal
• Increase in HR cannot fully compensate for reduced Stroke Volume, therefore CO is still reduced
• TPR increase brings MAP up to or slightly above normal
• Transient initial MAP drop is normally corrected within seconds

Orthostatic Hypotension

• Dizziness or fainting results without the baroreceptor reflex due to orthostatic hypotension

Maintaining MAP during exercise

• During moderate exercise there is a 2-fold increase in cardiac output and a 6-fold increase in muscle and skin blood flow
• There is also a big decrease in TPR

= MAP During Exercise

• A big decrease in MAP during exercise is not seen, since vasodilation occurs in muscle and skin, dropping TPR^
• A Potential Drop in MAP results in the baroreceptor reflex and activation of brain 'exercise centres'
• Reduced parasympathetic output and increased sympathetic output leads to an increased heart rate and stroke volume
• Increased cardiac output occurs since CO = HR x SV
• There is vasoconstriction in other parts of the body (kidney and GI tract), causing partial restoration of TPR
• Overall, MAP does not change much and is typically only slightly increased

(3) The Mammalian Diving Reflex

• Facial immersion in cold water (holding breath) initiates the diving reflex
• Reduced heart rate, reduced cardiac output and selective vasoconstriction in muscle, skin, kidneys, and gut, but NOT in brain and heart occurs
• TPR increases greatly leading to a net increase in MAP
• Total blood flow reduces, directing a greater percentage to the brain and heart, conserving oxygen

Techniques

• Calculating MAP involves measuring systolic and diastolic blood pressure using a sphygmomanometer
• Recording and interpreting the ECG is another technique

Calculating Mean Arterial Blood Pressure (MAP)

• Systolic pressure (top number) and diastolic pressure (bottom number) change in aorta and large arteries during each heartbeat
• Pulse pressure is the difference between systolic and diastolic pressure
• MAP = diastolic BP + (pulse pressure/3)

Measuring MAP with a Sphygmomanometer

• Typical resting values, measured from the brachial artery using a sphygmomanometer, are diastolic = 80 mmHg and systolic = 120 mmHg
• Using these values, MAP = 80 + (120-80)/3 = ~93 mmHg

Using a 3-Lead Electrocardiogram (ECG)

• The 3-lead ECG setup involves placing electrodes on the right collar-bone (black), left collar-bone (green), and lower left rib (red)

Reading the ECG and Measuring Intervals

• Waves: P wave represents atrial depolarisation, QRS complex represents ventricular depolarisation, and T wave represents ventricular repolarisation
• P-R (or P-Q) interval: From the start of the P wave to the first deflection of the QRS complex
• Q-T interval: From the first deflection of the QRS complex to the end of the T wave
• T-P interval: From the end of the T wave to the start of the next P wave

Calculating Heart Rate from the ECG

• The heart rate in beats per second is the reciprocal of the duration of the R-R interval in seconds (1/R-R)
• HR (bpm) = 60/R-R interval (in secs)

ECG Intervals and Heart Rate Changes

• The duration of the P-R interval (atrial depolarisation) does not significantly change with heart rate
• The Q-T interval (ventricular action potential) tends to shorten during exercise
• T-P interval (period of diastole) depends on the rate of diastolic depolarisation of the SA node

Practical Two: Respiratory Physiology

• Expansion of the lungs and movement of the ribs makes the lungs expand

Respiratory Physiology Practicals

• Control of breathing and hyperventilation effects on breath-holding
• Measuring lung volumes with the pneumotachograph spirometer
• evaluation of lung function with the vitalograph

Control of Breathing

• The main determinant of breathing rate is arterial CO2
• Groups of specialised neurons in the medulla (central chemoreceptors) detect changes in arterial PCO2
• If PCO2 increases, ventilation increases and if PCO2 decreases, ventilation decreases

Effects of Hyperventilation

• This is where over-ventilation occurs in proportion to metabolism, commonly caused by anxiety
• CO2 is expired faster than it is produced in the body, leading to arterial PCO2 dropping below normal (5.3kPa) causing a ventilatory drive inhibition

Lung Volumes and Capacities

• Tidal Volume (VT): The volume of air inspired and expired during normal regular breathing (~500ml in males, ~400ml in females)
• Inspiratory Reserve Volume (IRV): The maximum amount of lung volume that can be increased above VT (~3000ml in males, ~2100ml in females)
• Expiratory Reserve Volume (ERV): The maximum extra volume that can be expired after expiring VT (~1200ml in males, ~900ml in females)
• Residual Volume (RV): The volume remaining after maximal expiratory effort (~1200ml in males, ~1000ml in females); the lungs cannot be completely emptied

Lung Capacities

• These are combinations of two or more lung volumes
• Inspiratory Capacity (IC): IRV + VT (~3500ml in males, ~2500ml in females)
• Vital Capacity (VC): IRV + VT + ERV (~4700ml in males, ~3400ml in females)
• Functional Residual Capacity (FRC): ERV + RV (~2400ml in males, ~1900ml in females)
• Total Lung Capacity (TLC): IRV + VT + ERV + RV (~5900ml in males, ~4400ml in females)
• Lung volumes also vary between individuals by age, height, build, and lifestyle

Measuring Lung Volumes

• Some lung volumes and capacities can be measured directly using a pneumotachograph spirometer

Using the Pneumotachograph

• The pneumotachograph involves using a filter, nose-clip, and mouthpiece

Reading the Spirometer Trace

• The spirometer trace measures tidal volume, inspiratory reserve volume, expiratory reserve volume, and vital capacity
• ATPS refers to Ambient Temperature and Pressure, Saturated
• To convert to Body Temperature and Pressure, Saturated (BTPS), multiply the volume measured by 1.09

Calculating Ventilation Rate from Spirometer Recordings

• Ventilation is the flow of air in and out of the alveoli
• Minute ventilation is the volume of air breathed every minute
• Minute ventilation equals breathing rate (breaths per min) multiplied by tidal volume (vol of each breath), ml/min or L/min

Vitalograph Evaluation of Lung Function

• It records changes in the volume of air expired over time during a single forced expiration
• It can be used to measure forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC), and to calculate the Forced Expiratory Ratio (FER)

Interpreting Vitalograph Traces

• ATPS to BTPS volume corrections are not required from vitalograph traces
• The FEV1 is the Forced Expiratory Volume in the first second of expiration
• The FVC is the Forced Vital Capacity

Using Vitalograph to identify Healthy Lungs Function

• In healthy lungs FER is around 75%
• Obstructive Lung Disease: Airways easily collapse, making it difficult to breathe out (e.g., emphysema or asthma) will show Low FER
• Restrictive Lung Disease: Breathing out is easy but vital capacity is reduced (e.g., lung fibrosis) will show High FER
• In restrictive lung disease may force out more than 75% in 1 sec but is unable to expand lung to full, so FVC reduced

Obstuctive Lung Diseases

• It is characterized by FER less than 50%
• There is inability to force out 75% volume in 1 second

Attending Practicals 1 & 2

• Read the practical worksheet in the 'Practicals' section on the module KEATS page
• Those wishing to be a volunteer subject in the practical, should read the information in the appendix
• Attend the assigned session where a printed copy of the practical worksheet will be provided

Practical 3: Fuel Homeostasis

• This involves measuring glucose and ketone concentration in samples of 'blood'
• Identifying samples taken before or after a ‘meal' and identifying healthy and diabetic subjects samples occurs in this practical

Fuel Homeostasis

• No real blood is used
• No needles are involved
• No eating is involved