Topic Overview: Physiological Measurements to Monitor Health
Welcome to this revision guide on Physiological Measurements to Monitor Health! In healthcare and medical physics, monitoring a patient's vital signs allows doctors to detect diseases early, evaluate how well organs are working, and make life-saving decisions. Don't worry if medical physics seems intimidating at first — we will break down each physiological measurement step-by-step, exploring how the instruments work, the underlying physics and biology, and what the readings actually mean.
In this chapter, we will cover four vital monitoring techniques:
• Heart Rate and Pulse Measurement
• Electrocardiography (ECG)
• Blood Pressure Measurement (Sphygmomanometry)
• Pulse Oximetry
1. Heart Rate and Manual Pulse Measurement
Every time your heart beats, it pumps blood through your circulatory system, creating a pressure wave that travels along your arteries. We can detect this pressure wave at peripheral sites on the body where an artery lies close to the surface and over a bone.
Common Pulse Points
• Radial pulse: Located on the inside of the wrist, just below the base of the thumb.
• Carotid pulse: Located in the neck, alongside the windpipe (trachea).
Correct Measurement Procedure
1. Gently place the tips of your index and middle fingers over the artery (such as the radial artery at the wrist).
2. Important Examiner Tip: Never use your thumb to take a pulse! The thumb contains its own strong arterial pulse, which can easily be mistaken for the patient's pulse.
3. Use a calibrated timer or stopwatch to count the beats over a set time period:
• For a regular rhythm, count for \(30\text{ seconds}\) and multiply by \(2\).
• For an irregular rhythm (or for greater accuracy), count for a full \(60\text{ seconds}\).
Baseline Values and Variations
• Normal Resting Heart Rate: Typically between \(60\text{–}100\text{ bpm}\) (beats per minute) in a healthy adult.
• Bradycardia (\(< 60\text{ bpm}\)): A slow resting heart rate. While it can sometimes indicate heart issues, it is often a normal, non-pathological finding in well-trained endurance athletes because their hearts have a higher stroke volume (each beat pumps more blood, so fewer beats are needed per minute).
• Tachycardia (\(> 100\text{ bpm}\)): A fast resting heart rate. This can be caused non-pathologically by recent exercise, acute emotional stress, anxiety, or consuming caffeine.
Key Takeaway: Always use the index and middle fingers (never the thumb) to palpate a pulse at peripheral sites like the radial or carotid artery. A normal adult resting pulse is \(60\text{–}100\text{ bpm}\).
2. Electrocardiography (ECG)
The heart does not beat mechanically on its own; its contraction is triggered by waves of electrical activity. An Electrocardiogram (ECG) detects and records these electrical signals over time.
Underlying Physics and Mechanism
During each cardiac cycle, action potentials (waves of depolarisation and repolarisation) sweep across the heart muscle (the myocardium). These moving ionic charges generate tiny electrical currents and electrical dipoles that spread outward through the conductive body fluids to the skin. Highly sensitive electrodes placed on the skin surface detect these minute voltage fluctuations and display them as a characteristic waveform.
ECG vs. EEG: Don't Confuse Them!
• ECG (Electrocardiogram): Measures the electrical activity of the heart/myocardium.
• EEG (Electroencephalogram): Measures the electrical activity of cerebral cortical neurons in the brain.
The ECG Trace Anatomy
A standard single cardiac cycle on an ECG consists of three key components:
1. P Wave:
• Represents atrial depolarisation.
• This is the electrical wave spreading through the atria, which triggers atrial contraction (systole).
• Common Pitfall: The P wave is the electrical depolarisation, not the mechanical contraction itself!
2. QRS Complex:
• Represents rapid ventricular depolarisation.
• This strong electrical event triggers ventricular contraction (systole).
• Did you know? Atrial repolarisation happens at the same time, but it is masked on the trace by the much larger QRS complex.
3. T Wave:
• Represents ventricular repolarisation (the recovery/recharging phase of the ventricles before the next beat).
Diagnostic Applications
Doctors analyze ECG traces to identify various cardiac conditions:
• Arrhythmias: Irregular or abnormal rhythms.
• Fibrillation: Rapid, uncoordinated twitching of the heart muscle (atrial or ventricular fibrillation).
• Myocardial Infarction (Heart Attack): Often recognized by specific changes such as ST-segment elevation or depression.
• Long QT Syndrome: A condition where there is delayed repolarisation of the ventricles (prolonged interval between the start of the Q wave and the end of the T wave). This can lead to sudden, potentially fatal episodic ventricular tachyarrhythmias.
Limitations of Standard Resting ECGs
A standard resting ECG only records the heart's electrical activity over a very short time window (typically a few seconds to a minute). Intermittent or episodic abnormalities — such as arrhythmias or Long QT episodes that are triggered only during exercise, sudden stress, or adrenaline release — might not appear during a routine resting trace. A completely normal resting ECG does not rule out intermittent cardiac disorders!
Key Takeaway: An ECG detects electrical dipoles from myocardial depolarisation and repolarisation: P wave (atrial depolarisation), QRS complex (ventricular depolarisation), and T wave (ventricular repolarisation).
3. Blood Pressure Measurement
Blood pressure (BP) is defined as the lateral force exerted per unit area by circulating blood upon the systemic arterial walls. It is traditionally measured in millimeters of mercury (\(\text{mmHg}\)).
Systolic vs. Diastolic Pressure
• Systolic Pressure: The peak arterial pressure reached during ventricular contraction (systole).
• Diastolic Pressure: The minimum baseline arterial pressure maintained during ventricular relaxation (diastole).
• Standard Healthy Reference: Around \(120/80\text{ mmHg}\) (stated as "120 over 80").
• Hypertension (High Blood Pressure): Typically diagnosed at values \(\ge 140/90\text{ mmHg}\).
How a Sphygmomanometer Works (Step-by-Step)
A sphygmomanometer consists of an inflatable cuff, a pressure gauge, and a bulb or pump. It is used alongside a stethoscope (or digital pressure sensor):
1. Cuff Placement: The cuff is wrapped securely around the upper arm at heart level.
2. Occlusion: The cuff is inflated to a pressure higher than the patient's expected systolic pressure. This completely compresses and closes the brachial artery, stopping blood flow.
3. Controlled Deflation & Systolic Detection: Air is slowly released from the cuff. When the cuff pressure falls just below systolic pressure, blood begins to spurt through the partially compressed artery. This creates turbulent blood flow, which produces distinct tapping sounds known as Korotkoff sounds heard through the stethoscope. The pressure reading at the first sound is the systolic pressure.
4. Diastolic Detection: As the cuff deflates further, the artery fully opens and blood flow transitions from turbulent flow back to smooth, silent laminar flow. The pressure reading at the moment the Korotkoff sounds completely disappear represents the diastolic pressure.
Exam Warning: Remember that Korotkoff sounds are caused by turbulent flow in a partially constricted vessel. When blood flows smoothly (laminar flow), it is silent!
Key Takeaway: Sphygmomanometers measure systolic and diastolic pressure in \(\text{mmHg}\). Systolic pressure is recorded when turbulent Korotkoff sounds start; diastolic is recorded when sounds disappear as laminar flow returns.
4. Pulse Oximetry
A pulse oximeter is a non-invasive medical device that clips onto a thin, translucent part of a patient's body (most commonly a finger, toe, or earlobe) to measure peripheral capillary oxygen saturation (\(\text{SpO}_2\)).
The Optical Principle (Spectrophotometry & Beer-Lambert Law)
Pulse oximetry relies on differential spectrophotometry, which is based on the Beer-Lambert law (the principle that the amount of light absorbed by a substance depends on its concentration and the path length of the light). Haemoglobin changes colour and light absorption properties depending on whether or not it is carrying oxygen:
• Deoxygenated Haemoglobin (\(\text{Hb}\)): Absorbs more Red light (wavelength \(\approx 660\text{ nm}\)).
• Oxygenated Haemoglobin (\(\text{HbO}_2\)): Absorbs more Infrared (IR) light (wavelength \(\approx 940\text{ nm}\)).
Memory Aid for Wavelengths
• Red (\(660\text{ nm}\)) = Loved by Deoxy-Hb (think: "Red light reads raw/deoxygenated blood").
• Infrared (\(940\text{ nm}\)) = Loved by Oxy-Hb (\(\text{HbO}_2\)).
How the Sensor Works
1. Two light-emitting diodes (LEDs) shine alternating beams of Red light (\(\approx 660\text{ nm}\)) and Infrared light (\(\approx 940\text{ nm}\)) through the tissue.
2. A photodiode detector on the opposite side measures the intensity of light transmitted through the finger.
3. Separating Arterial Blood from Background Tissue:
• DC Component (Constant): Constant absorption caused by skin, bone, venous blood, and resting tissue.
• AC Component (Pulsatile): Variable absorption caused exclusively by the rhythmic pulses of arterial blood entering with each heartbeat.
4. The microprocessor calculates the ratio of red to infrared light absorbed during the pulsatile (AC) phase to determine \(\text{SpO}_2\).
Clinical Reference Values
• Normal \(\text{SpO}_2\): \(\ge 95\%\text{–}100\%\) breathing room air.
• Hypoxaemia: Oxygen saturation falling below \(90\%\text{–}92\%\), signalling insufficient oxygen delivery to tissues.
Key Takeaway: Pulse oximeters shine \(660\text{ nm}\) (Red) and \(940\text{ nm}\) (Infrared) light through a pulsatile vascular bed. \(\text{Hb}\) absorbs more red light, while \(\text{HbO}_2\) absorbs more infrared light.
Quick Revision: Common Exam Traps to Avoid
• Don't mix up electrical triggers with mechanical actions: The P wave is atrial depolarisation, not atrial contraction.
• Don't mix up ECG and EEG: ECG = Heart; EEG = Brain.
• Don't use the thumb to take a pulse: Always use the index and middle fingers.
• Understand Korotkoff sounds: They occur during turbulent flow, not laminar flow.
• Don't swap the oximeter wavelengths: \(\text{Hb}\) absorbs at \(660\text{ nm}\) (Red); \(\text{HbO}_2\) absorbs at \(940\text{ nm}\) (Infrared).
• Remember the limitation of resting ECGs: Episodic arrhythmias (e.g., in Long QT syndrome) may not show up on a standard resting trace.