Principles of Defibrillators, Pacemakers, Ventilators and
Pulse Oximetry
The four devices at a glance
All four devices either measure a body signal or support a failing organ: the
defibrillator and pacemaker act on the heart's electrical system, the
ventilator moves air for the lungs, and the pulse oximeter watches how well the
blood is oxygenated.
|
Device |
Organ
system |
What
it does |
Core
physical principle |
|
Defibrillator |
Heart (electrical) |
Delivers one large shock to stop a
chaotic rhythm |
Capacitor stores energy, E = ½CV²,
discharged through the chest |
|
Pacemaker |
Heart (electrical) |
Delivers small, repeated pulses to keep a
minimum heart rate |
Sense the heart's own beat; stimulate
only when it is missing |
|
Ventilator |
Lungs |
Pushes gas into the lungs and lets it out |
Positive pressure overcomes airway
resistance and lung compliance |
|
Pulse oximeter |
Blood |
Measures oxygen saturation (SpO₂) and
pulse rate without a needle |
Beer–Lambert law: oxy- and
deoxyhaemoglobin absorb red and infrared light differently |
Each section below follows the same order: principle → main parts (with a diagram) →
how it works step by step → key numbers → safety points. A revision table
and practice questions close the notes.
1. Defibrillator
A defibrillator stops a chaotic heart
rhythm by sending one brief, high-energy current through the chest so that most
of the heart muscle depolarises at the same instant; the heart's own pacemaker
(the SA node) can then restart a normal beat.
Principle
•
In ventricular fibrillation (VF) different patches of the ventricle
contract at random, so the heart quivers instead of pumping. Blood pressure
drops to zero within seconds.
•
A shock depolarises a critical mass of the myocardium at
once. All those cells enter their refractory period together, the chaos stops,
and the heart is briefly electrically silent.
•
During that silence the SA node, which fires on its own, takes
over again and a normal rhythm can return.
•
A shock does not
"jump-start" a stopped heart. It only resets disorganised activity,
so asystole and PEA are not shockable.
Where the energy comes from
The energy is built up slowly in a
capacitor and released in milliseconds. The stored energy is:
E is energy in joules, C is capacitance in
farads and V is the capacitor voltage in volts.
Worked
example: to store 200 J in a 100 µF capacitor, V =
√(2E / C) = √(400 / 0.0001) = 2000 V.
The current that reaches the heart depends
on the transthoracic impedance,
typically about 50 to 150 Ω in adults. Firm pad contact, gel and a dry chest
keep it low.
Main parts
Defibrillator block diagram · charge path
above, discharge path below
The top row charges the capacitor over a
few seconds; the bottom row delivers the stored energy to the patient in a few
milliseconds when the relay closes.
In
real use
Figure 1b · Defibrillator in real use: pad
placement, AED and the monitor trace before and after a shock
On a real monitor, VF looks like an
irregular wavy line with no clear QRS complexes. The shock appears as one tall
spike, and a successful shock is followed by organised beats with P, QRS and T
waves.
How a shock is delivered, step by step
1.
Attach pads or paddles to the bare
chest: one below the right clavicle, one at the cardiac apex (left mid-axillary
line).
2.
Analyse the ECG. An AED does this
automatically; with a manual device the operator reads the monitor.
3.
Select energy and press CHARGE. The charging circuit raises the
capacitor to kilovolts in a few seconds.
4.
Stand clear: nobody touches the patient
or the bed; oxygen is moved away from the chest.
5.
Press SHOCK. The relay closes and the capacitor discharges through the
wave-shaping circuit and pads, typically over about 5 to 20 ms.
6.
Resume CPR at once and reassess the
rhythm after the next cycle.
Monophasic vs biphasic waveforms
|
Waveform |
Current
direction |
Typical
adult energy |
Notes |
|
Monophasic damped sine |
One direction only |
360 J |
Older design: an RLC circuit (capacitor +
inductor) shapes the pulse |
|
Biphasic truncated exponential /
rectilinear |
Reverses halfway through |
120 to 200 J |
Modern standard: works at lower energy,
causes less heart-muscle injury, adjusts for patient impedance |
Types of defibrillator
|
Type |
Where
the electrodes are |
Who
uses it |
|
Manual external |
Paddles or pads on the chest |
Doctors and trained staff, in hospital
and ambulances |
|
AED (automated external) |
Adhesive pads on the chest |
Anyone: it analyses the rhythm and gives
voice prompts |
|
ICD (implantable) |
Leads inside the heart |
Implanted under the skin in high-risk
patients; shocks automatically |
|
Internal paddles |
Directly on the heart |
Open-chest cardiac surgery; very low
energy needed |
|
Wearable (vest) |
Electrodes in a garment |
Patients waiting for an ICD |
Synchronised cardioversion
For rhythms that still have organised QRS
complexes (atrial fibrillation, atrial flutter, VT with a pulse), the shock is synchronised to the R wave. A shock
that lands on the T wave, the heart's vulnerable period, can itself trigger VF.
In sync mode the controller waits for the next R wave before firing.
Safety points
•
Shock only VF and pulseless VT; never asystole or PEA.
•
Shout "stand clear"
and check before every shock.
•
Wipe the chest dry; remove
medication patches; keep pads at least 8 cm from an implanted pacemaker.
•
Children: use paediatric pads
and about 4 J/kg.
Clinical example: cardiac arrest at a railway station
Illustrative
teaching case. A 58-year-old man collapses on a
platform. He is unresponsive and not breathing normally. A bystander starts
chest compressions and station staff bring the AED.
|
Time |
What
happens |
What
the device shows or does |
|
0 min |
Collapse; bystander starts CPR |
— |
|
3 min |
AED switched on, pads placed (right upper
chest + left side) |
Voice prompt: "Analysing heart
rhythm, do not touch the patient" |
|
3.5 min |
Rhythm analysed |
VF detected: "Shock advised.
Charging." Capacitor charges in a few seconds |
|
4 min |
Everyone stands clear; SHOCK pressed |
First biphasic shock, about 150 J (device
default) |
|
4–6 min |
CPR resumed immediately for 2 minutes |
AED counts the cycle, then re-analyses |
|
6 min |
Still VF |
Second shock delivered; CPR continues |
|
8 min |
Rhythm check |
"No shock advised": organised
rhythm on the trace |
|
9 min |
Pulse returns (ROSC); ambulance takes
over |
Pads left on; monitor shows sinus rhythm |
Why
it worked: VF is a shockable rhythm, and every
minute without a shock lowers survival sharply. Early CPR kept blood flowing to
the brain; the shock reset the chaotic ventricles so the SA node could take
over.
Second
example: synchronised cardioversion. A 64-year-old
woman in the emergency department has atrial fibrillation at 160 beats/min with
low blood pressure. She is sedated, the defibrillator is switched to SYNC mode (a marker appears on each R
wave), and a synchronised biphasic shock of about 120–150 J is delivered. The
monitor then shows sinus rhythm at 82 beats/min. Without sync, a shock landing
on a T wave could have triggered VF.
2. Pacemaker
A pacemaker keeps the heart from beating
too slowly by delivering small electrical pulses (a few volts, under a
millisecond) through a lead; a modern "demand" pacemaker watches
every beat and fires only when the heart's own beat does not arrive in time.
Principle
•
The heart's natural pacemaker
is the SA node. If it fails, or the
signal is blocked between atria and ventricles (heart block), the heart rate falls and the patient feels dizzy,
faints or becomes breathless.
•
A pacing pulse at the lead tip depolarises a small area of muscle.
That depolarisation then spreads cell to cell through the whole chamber,
producing a full contraction. This is called capture.
•
The pacemaker does two jobs: pacing (sending pulses) and sensing (listening to the heart's own
activity so it does not compete with it).
Main
uses: symptomatic bradycardia, sick sinus syndrome,
complete (third-degree) heart block, and cardiac resynchronisation in heart
failure.
Main parts
Pacemaker block diagram · pacing path and
sensing feedback loop
If a natural beat is sensed before the
timer runs out, the pacemaker stays quiet and restarts its timer; if not, the
output circuit fires a pulse down the lead.
In
real use
Figure 2b · Pacemaker in real use: implant
position, lead route and a paced ECG
On a chest X-ray the generator sits below
the left collarbone with the lead curving into the heart. On the ECG, a
ventricular-paced beat shows a sharp vertical spike followed by a broad QRS,
because the impulse spreads cell to cell instead of through the normal
conduction system.
•
Pulse generator: battery, timing
circuit, output circuit and sense amplifier sealed in a titanium can.
•
Lead(s): insulated wires from the
generator through a vein into the right atrium and/or right ventricle. The tip
electrode touches the heart wall.
•
Programmer: an external unit that talks
to the generator wirelessly to read data and change settings.
Key electrical ideas
•
Stimulation threshold: the smallest
pulse that reliably captures the heart, often under 1 V at 0.4 ms. The output
is set at about 2 to 3 times the
threshold as a safety margin.
•
Strength–duration curve: a shorter pulse
needs a higher voltage. Rheobase is
the lowest voltage that captures with a very long pulse; chronaxie is the pulse width at twice the rheobase, where energy
use is close to its minimum.
•
Sensing: the ventricular R wave inside
the heart is roughly 5 to 20 mV and the atrial P wave about 1.5 to 5 mV. The sensitivity setting (in mV) is the
smallest signal the device will count as a beat.
•
Rate and intervals: a lower rate of 60
beats/min means a pacing interval of 60 000 / 60 = 1000 ms.
\text{Pacing interval (ms)} =
\frac{60\,000}{\text{rate (beats/min)}}
The pacing code (NBG code)
Pacing modes are written with up to five
letters. The first three are the ones students must know.
|
Position |
Meaning |
Letters
used |
|
I |
Chamber paced |
O = none, A = atrium, V = ventricle, D =
dual (A + V) |
|
II |
Chamber sensed |
O, A, V, D |
|
III |
Response to a sensed beat |
O = none, I = inhibit, T = trigger, D =
dual (I + T) |
|
IV |
Rate modulation |
O = none, R = rate-responsive (speeds up
with activity) |
|
V |
Multisite pacing |
O, A, V, D |
Examples:
•
VVI: paces the ventricle, senses the
ventricle, inhibits when a beat is sensed. The commonest single-chamber mode.
•
AAI: the same in the atrium; used when
AV conduction is normal.
•
DDD: paces and senses both chambers,
keeps the atria and ventricles in step. Common for heart block.
•
VOO / DOO: fixed-rate (asynchronous)
pacing with no sensing. A magnet placed over many devices switches them into
this mode.
•
DDDR: DDD plus a sensor that raises the
rate during exercise.
Types of pacemaker
|
Type |
How
it is applied |
When
it is used |
|
Transcutaneous (external) |
Large pads on the chest |
Emergency, short term; painful at high
outputs |
|
Temporary transvenous |
Lead through a vein to the RV, external
box |
Hours to days, until a permanent device
is fitted |
|
Permanent single / dual chamber |
Generator under the skin, 1 or 2 leads |
Long-term bradycardia and heart block |
|
Biventricular (CRT) |
Third lead to the left ventricle |
Heart failure with poorly coordinated
ventricles |
|
Leadless |
Capsule implanted inside the RV |
Single-chamber pacing without a lead or
pocket |
Safety points
•
Check for capture on the ECG: every pacing spike should be followed by a wide
QRS complex.
•
Failure to sense (pacing over the
patient's own beats) and failure to
capture (spike with no QRS) are the two classic faults.
•
Electrocautery, MRI and strong
magnets can interfere; only MRI-conditional systems may be scanned, under
protocol.
•
The battery's elective replacement indicator warns
months before the battery runs out.
Clinical example: complete heart block in an elderly
patient
Illustrative
teaching case. A 76-year-old woman comes to the
emergency department after fainting twice at home. She is pale and drowsy.
Pulse 34 beats/min, blood pressure 80/50 mmHg.
|
Stage |
Finding
or action |
Pacing
detail |
|
ECG |
P waves at 80/min march through with no
link to QRS complexes; ventricular rate 34/min |
Diagnosis: complete (third-degree) heart block |
|
Emergency |
Atropine has little effect |
Transcutaneous
pacing: pads on chest and back, rate set to
70/min, current raised step by step until every spike is followed by a wide
QRS (capture) and a pulse is felt |
|
Same day |
Transcutaneous pacing is painful and not
reliable long term |
Temporary
transvenous lead passed through a vein into the
right ventricle; external box set to VVI 70/min |
|
Next 1–2 days |
Heart block persists |
Permanent
dual-chamber pacemaker implanted under the left
collarbone, programmed DDD at a
lower rate of 60/min |
|
Follow-up clinic |
Feels well, no more faints |
Programmer checks threshold (e.g. 0.75 V
at 0.4 ms), output set to 2.5 V for a safety margin, battery status good |
Why
DDD? The atria are still beating normally. DDD
senses each P wave and paces the ventricle after a set delay, so the atria and
ventricles stay in step and the heart rate rises naturally with exercise.
Troubleshooting
example. Two weeks later the same patient feels
dizzy again. Her ECG shows pacing spikes with no QRS after some of them. This is failure to capture. A chest X-ray shows the ventricular lead has
moved (lead displacement). The lead is repositioned and capture returns.
3. Mechanical ventilator
A mechanical ventilator breathes for a
patient by raising the pressure at the airway above the pressure in the
alveoli, so gas flows into the lungs; when the pressure is released, the
elastic lungs and chest wall recoil and push the gas back out.
Principle
•
Normal breathing uses negative pressure: the diaphragm pulls
down, pressure in the chest falls and air is sucked in.
•
A modern ventilator uses positive pressure: it pushes gas in.
Old "iron lung" machines used negative pressure around the body.
•
Expiration is passive. The ventilator
simply opens the expiratory valve and the lungs empty by elastic recoil.
•
How much pressure is needed
depends on two properties of the respiratory system:
o
Compliance (C): how easily the lungs
stretch. C = change in volume / change in pressure. Normal is roughly 50 to 100
mL/cmH₂O; stiff lungs (ARDS, fibrosis) have low compliance.
o
Resistance (R): how hard it is for gas
to flow through the tubes and airways. It rises with a narrow ET tube,
secretions or bronchospasm.
These are combined in the equation of motion:
P_{aw} = \frac{V_T}{C} + R \times
\dot{V} + PEEP
P_aw is airway pressure, V_T is tidal
volume and V̇ is gas flow.
Main parts
Mechanical ventilator circuit · inspiratory
and expiratory limbs
Gas travels out along the top row and comes
back along the bottom row; the controller opens one valve at a time so the
patient breathes in and then out.
In
real use
Figure 3b · Ventilator in real use: patient
circuit and a typical volume-control screen
A real ventilator screen shows pressure and
flow against time. In volume control the inspiratory flow is a flat-topped
square block; the expiratory flow is negative and decays back to zero as the
lungs empty. The numbers on the side are the measured values the nurse records
each hour.
The four phases of every breath
1.
Trigger: what starts the breath. Time (the machine's set rate) or the patient (a small drop in pressure or
flow when the patient tries to breathe).
2.
Target / limit: what is controlled
during inspiration. Either a set volume
or a set pressure.
3.
Cycle: what ends inspiration. Set time, delivered volume, or flow falling
to a set fraction.
4.
Expiration and baseline: the expiratory
valve opens and pressure falls to PEEP,
which keeps the alveoli open at the end of each breath.
Reading the pressure waveform
Airway pressure during a volume-controlled
breath (illustrative shape)
The gap between peak and plateau reflects
airway resistance; the gap between plateau and PEEP (the driving pressure)
reflects lung stiffness. A rising peak with a steady plateau points to a
blocked tube or bronchospasm; a rising plateau points to stiffer lungs or a
pneumothorax.
Common modes
|
Mode |
What
is set |
How
it behaves |
Typical
use |
|
Volume control (AC-VC) |
Tidal volume, rate |
Fixed volume every breath; pressure
varies with the lungs |
Sedated, paralysed patients |
|
Pressure control (AC-PC) |
Inspiratory pressure, time, rate |
Fixed pressure; volume varies with the
lungs |
Stiff lungs, limiting pressure |
|
SIMV |
Volume or pressure, rate |
Set mandatory breaths, patient can
breathe between them |
Weaning (older approach) |
|
Pressure support (PSV) |
Support pressure, PEEP |
Every breath started by the patient,
machine assists |
Weaning, awake patients |
|
CPAP |
PEEP only |
Constant pressure, patient does all the
breathing |
Spontaneous breathing trial, sleep apnoea |
|
BiPAP / NIV |
Two pressures (IPAP, EPAP) |
Delivered through a tight face mask, no
tube |
COPD flare-ups, avoiding intubation |
Key settings and starting values (adult)
|
Setting |
Meaning |
Typical
starting value |
|
Tidal volume (V_T) |
Volume of each breath |
6 to 8 mL/kg of predicted body weight |
|
Respiratory rate |
Breaths per minute |
12 to 20 /min |
|
FiO₂ |
Fraction of oxygen in the gas |
0.21 to 1.0, lowered as SpO₂ allows |
|
PEEP |
Pressure left in the lungs at end of
expiration |
5 cmH₂O, higher in ARDS |
|
I:E ratio |
Inspiration time : expiration time |
1:2 |
|
Plateau pressure |
Pressure in alveoli during a pause |
Keep below 30 cmH₂O |
|
Trigger sensitivity |
Effort needed to start a breath |
about −1 to −2 cmH₂O or 1 to 3 L/min |
Minute
ventilation = tidal volume × rate. Example: 500 mL
× 14 /min = 7 L/min.
Alarms and safety
•
High pressure: blocked or kinked tube,
secretions, patient biting or coughing, bronchospasm, pneumothorax.
•
Low pressure / low volume: disconnection
or leak, the most dangerous alarm to ignore.
•
Apnoea, low minute volume, FiO₂ and gas-supply / power failure alarms are also standard.
•
If the patient deteriorates
suddenly, think DOPE: Displaced
tube, Obstructed tube, Pneumothorax, Equipment failure. Disconnect and
hand-ventilate with a bag if in doubt.
•
Complications to know:
barotrauma and volutrauma (lung injury from pressure or over-stretch), low
blood pressure (less venous return), ventilator-associated pneumonia, and
oxygen toxicity.
Clinical example: severe pneumonia leading to ARDS
Illustrative
teaching case. A 45-year-old man with severe
pneumonia is breathing 38 times a minute. His SpO₂ is 82% even on a high-flow
oxygen mask, and he is exhausted. He is intubated and put on a ventilator in
the ICU. His predicted body weight is 60 kg.
|
Setting |
Value
chosen |
Reason |
|
Mode |
Volume control (AC-VC) |
Sedated patient; guaranteed volume each
breath |
|
Tidal volume |
360 mL (6 mL/kg × 60 kg) |
Low volume protects stiff, injured lungs |
|
Rate |
20 /min |
Keeps minute ventilation about 7.2 L/min
to clear CO₂ |
|
FiO₂ |
0.8 at first, lowered later |
Correct severe hypoxia quickly |
|
PEEP |
10 cmH₂O |
Keeps collapsed alveoli open, improves
oxygenation |
Checking
the screen: plateau pressure 28 cmH₂O (below the 30
limit), so the settings are lung-protective. SpO₂ rises to 94% and FiO₂ is
reduced to 0.5 over the next hours.
Alarm
on day 2 — reading the waveform:
|
What
the screen shows |
Likely
cause |
Action |
|
Peak pressure jumps from 32 to 45 cmH₂O, plateau unchanged at 28 |
Airway resistance has risen: secretions
blocking the tube |
Suction the tube; peak falls back to 32 |
|
Peak and
plateau both rise suddenly, SpO₂ falls, breath sounds absent on one side |
Lung compliance has dropped: tension pneumothorax |
Emergency chest decompression, then chest
drain |
|
Low-pressure and low-volume alarms, no
chest movement |
Disconnection of the circuit |
Reconnect at once; hand-ventilate with a
bag if needed |
Weaning: as the pneumonia improves he is switched to pressure support, then
passes a spontaneous breathing trial on CPAP with low support, and is extubated
on day 6.
4. Pulse oximetry
A pulse oximeter measures the percentage of
haemoglobin carrying oxygen (SpO₂) by shining red and infrared light through a
fingertip and comparing how much of each is absorbed by the pulsing arterial
blood; normal SpO₂ is about 95 to 100%.
Principle
Pulse oximetry combines two ideas.
1.
Spectrophotometry (the Beer–Lambert law). The
amount of light a substance absorbs depends on how much of it is in the path
and how far the light travels:
A = \varepsilon \, c \, l
\qquad\qquad I = I_0 \, e^{-\varepsilon c l}
A is absorbance, ε is the absorption
coefficient of the substance, c its concentration, l the path length, I₀ the
light going in and I the light coming out.
•
Deoxyhaemoglobin (Hb) absorbs much more red light (660 nm). Deoxygenated blood
looks dark.
•
Oxyhaemoglobin (HbO₂) absorbs more infrared light (940 nm). Oxygenated
blood looks bright red because it lets red light through.
•
At about 805 nm (the isosbestic point) both absorb equally.
2.
Plethysmography. With each heartbeat a small extra
volume of arterial blood enters the finger, so the absorbed light rises and
falls. The oximeter uses only this pulsing (AC) part, which belongs to arterial blood. The steady (DC) part, from skin, bone, venous blood
and non-pulsing arterial blood, is cancelled out.
A: absorption of light by Hb and HbO2
(schematic) · B: AC and DC parts of the absorbed light
Panel A is why two wavelengths are used;
panel B is why the device needs a pulse. Only the shaded bumps in B change with
each beat, and they come from arterial blood alone.
Ratio of ratios
The processor divides the pulsing part by
the steady part at each wavelength, then compares the two:
R = \frac{AC_{660} /
DC_{660}}{AC_{940} / DC_{940}}
•
Low R (around 0.5) means
well-oxygenated blood: about 100% SpO₂.
•
R = 1 means about 85% SpO₂.
•
A common classroom
approximation is SpO₂ ≈ 110 − 25 × R.
The real conversion uses a calibration curve stored in the device,
made by testing healthy volunteers breathing low-oxygen mixtures. Below about
70 to 80% the curve is less reliable, because volunteers cannot safely be
desaturated further.
Main parts
↗ Pulse oximeter block diagram · light path
and signal processing
Light travels across the top row through
the finger; the electronics in the bottom row turn the received light into the
number on the screen.
In
real use
↗ Figure 4b · Pulse oximeter in real use:
finger clip probe and the monitor display
The red glow you see through a fingertip
under the clip is the red LED; the infrared LED is invisible to the eye. Trust
the number only when the pleth wave is regular and the displayed pulse rate
matches the patient's heart rate.
•
Probe types: transmission (LEDs and photodiode on opposite sides of a finger,
toe or ear lobe) and reflectance
(both on the same side, e.g. forehead).
•
Display: SpO₂ (%), pulse rate, and the plethysmograph (pleth) waveform. A
clean, regular pleth wave is the best sign that the reading can be trusted.
SpO₂, SaO₂ and PaO₂
|
Term |
Meaning |
Measured
by |
|
SpO₂ |
Oxygen saturation estimated from pulse
oximetry |
Pulse oximeter (non-invasive) |
|
SaO₂ |
True arterial oxygen saturation |
Arterial blood gas with co-oximetry |
|
PaO₂ |
Partial pressure of oxygen dissolved in
arterial blood (mmHg or kPa) |
Arterial blood gas |
Because of the S-shaped oxygen–haemoglobin
dissociation curve, an SpO₂ of about 90%
corresponds to a PaO₂ of about 60 mmHg (8 kPa). Below this point saturation
falls steeply.
Sources of error
|
Problem |
Effect
on reading |
Why |
|
Poor perfusion (cold hands, shock,
vasoconstriction) |
Unreliable or no reading |
The AC signal is too small |
|
Movement, shivering |
False readings |
Motion creates fake pulses |
|
Carbon monoxide poisoning |
Falsely
high |
Carboxyhaemoglobin absorbs red light like
HbO₂ |
|
Methaemoglobinaemia |
Reads close to 85% |
Absorbs red and IR about equally, so R ≈
1 |
|
Dark nail polish, dyes (e.g. methylene
blue) |
Falsely low |
Extra absorption of light |
|
Darker skin pigmentation |
May overestimate at low saturations |
Calibration largely done on lighter skin |
|
Bright ambient light, loose probe |
Erratic readings |
Stray light reaches the photodiode |
|
Severe anaemia |
Less reliable |
Very little haemoglobin to measure |
|
Venous pulsation (e.g. tricuspid
regurgitation) |
Falsely low |
Venous blood is counted as arterial |
Safety points
•
SpO₂ shows oxygenation, not ventilation. A patient on oxygen can have a normal
SpO₂ while CO₂ rises dangerously.
•
A finger probe reacts later than an ear probe because the
blood takes longer to reach it.
•
Move the probe every few hours
on long-term monitoring to avoid pressure injury or burns.
•
Always look at the patient and
the pleth wave, not just the number.
Clinical examples: when the number is right, and when it
lies
Illustrative
teaching cases.
|
Case |
Oximeter
reading |
What
was really happening |
Lesson |
|
Pneumonia
in the ward. A 70-year-old with fever and cough;
breathing 28 /min |
SpO₂ 88%, clean pleth wave |
Genuine low oxygen. Oxygen by nasal
cannula raised SpO₂ to 94% |
A good pleth wave means the reading can
be trusted; act on it |
|
Charcoal
stove in a closed room. Family found drowsy with
headache in the morning |
SpO₂ 99% |
Carbon monoxide poisoning. Blood-gas
co-oximetry showed carboxyhaemoglobin 25%; true oxygen content low |
CO fools the oximeter (falsely high).
Give 100% oxygen; never be reassured by SpO₂ alone |
|
After
surgery, cold hands. Patient shivering in
recovery |
SpO₂ 84%, flat noisy pleth wave |
Poor finger perfusion, not true hypoxia.
Probe moved to the ear lobe and hands warmed: SpO₂ 97% |
Check the pleth wave before acting on a
low number |
|
After
a methylene blue injection during surgery |
SpO₂ drops suddenly to 85% for a few
minutes |
Blue dye absorbs red light; the patient's
real oxygen was normal |
Dyes give falsely low readings; check the
patient, not just the monitor |
|
Opioid
painkiller on the ward, patient on 4 L/min oxygen |
SpO₂ steady at 97% |
Breathing slow and shallow; blood CO₂
rising dangerously |
SpO₂ does not show ventilation. Count the
breathing rate; capnography detects rising CO₂ |
|
Home
monitoring of a viral lung infection |
SpO₂ falls from 96% to 91% over two days,
patient feels fine |
Early silent hypoxia |
A steady downward trend means "seek
medical review", even without symptoms |
Quick revision and practice questions
|
|
Defibrillator |
Pacemaker |
Ventilator |
Pulse
oximeter |
|
Key principle |
Simultaneous depolarisation resets
chaotic rhythm |
Small pulse captures muscle; sense before
pacing |
Positive pressure vs compliance and
resistance |
Beer–Lambert law + pulsatile (AC) signal |
|
Key formula |
E = ½CV² |
Interval (ms) = 60 000 / rate |
P = V/C + R × flow + PEEP |
R = (AC/DC)red / (AC/DC)IR |
|
Key numbers |
Biphasic 120–200 J; monophasic 360 J;
child 4 J/kg |
Output 2.5–5 V, 0.4 ms; lower rate ~60
/min |
V_T 6–8 mL/kg; PEEP 5 cmH₂O; Pplat <
30 cmH₂O |
660 nm and 940 nm; normal 95–100% |
|
Main hazard |
Shock to bystanders; burns |
Failure to sense or capture |
Barotrauma, disconnection |
False reading (CO, motion, low perfusion) |
Practice questions
1.
Calculate the voltage needed to
store 360 J in a 200 µF capacitor. (Answer:
V = √(2 × 360 / 0.0002) = 1897 V, about 1.9 kV.)
2.
Why is a shock synchronised to
the R wave during cardioversion, but not during defibrillation of VF?
3.
Explain why asystole is not
treated with a shock.
4.
Decode the pacing mode DDD and describe what the pacemaker
does when it senses an atrial beat.
5.
A VVI pacemaker is set at 70
beats/min. What is its pacing interval? (Answer:
60 000 / 70 ≈ 857 ms.)
6.
On a volume-controlled
ventilator the peak pressure rises sharply but the plateau pressure is
unchanged. What is the likely cause?
7.
A patient has V_T 450 mL and
rate 16 /min. Find the minute ventilation. (Answer:
7.2 L/min.)
8.
Why does a pulse oximeter need
two wavelengths of light, and why does it use only the pulsing part of the
signal?
9.
A fire victim has SpO₂ 99% but
is confused and breathless. Explain why the reading may be wrong.
10.
Distinguish SpO₂, SaO₂ and
PaO₂, and state the PaO₂ that matches an SpO₂ of 90%.
