Medical Devices: Monitoring, Diagnostic & Therapeutic

How the devices in a hospital work. Learn patient monitoring (NIBP oscillometry, pulse oximetry, capnography, temperature), diagnostic devices (glucose meters and biosensors, lab analysers), therapeutic devices (pacemakers, defibrillators, infusion pumps, ventilators, dialysis, electrosurgery, lasers), implantables and power, and how sensors transduce physiology — with Python for SpO₂, defibrillator energy and pacemaker battery life.

A modern ICU bed is surrounded by devices measuring and supporting life: monitors, ventilators, infusion pumps, dialysis machines. Each combines sensors, electronics, actuators and software under strict safety requirements.

Understanding the measurement principle behind each device — optical absorption, oscillometry, electrochemistry, impedance — lets engineers design, maintain and troubleshoot them, and spot when a reading may be misleading.

An analogy

A pulse oximeter is like judging a drink's colour by shining two coloured torches through it: oxygen-rich blood and oxygen-poor blood absorb red and infrared light differently, and the ratio reveals the oxygen level.

Patient monitoring

Core vital-sign technologies:

Diagnostic devices and biosensors

Biosensors combine a biological recognition element (enzyme, antibody, DNA) with a transducer (electrochemical, optical, piezoelectric). The glucose meter uses glucose oxidase and measures current proportional to glucose; continuous glucose monitors do this subcutaneously for 10–14 days. Lateral-flow tests (pregnancy, COVID-19 antigen) and lab analysers (spectrophotometry, flow cytometry) complete the diagnostic toolkit.

Therapeutic devices

Pacemakers sense intrinsic beats and pace when needed; ICDs detect ventricular fibrillation and deliver shocks; external defibrillators (AEDs) analyse rhythm and deliver biphasic shocks of 120–360 J. Infusion pumps deliver precise drug flows with occlusion and air-in-line detection. Ventilators control pressure or volume with PEEP. Dialysis machines, electrosurgical units (RF cutting), therapeutic lasers and radiotherapy linacs are other key classes.

Implantables, power and connectivity

Implantables face extreme constraints: hermetic titanium packaging, biocompatible feedthroughs, µA power budgets, MRI conditional safety, wireless telemetry (MICS band, BLE) and cybersecurity. Power comes from primary lithium cells, rechargeable batteries with inductive charging, or energy harvesting. Closed-loop therapies — responsive neurostimulation, artificial pancreas — combine sensing, algorithms and actuation.

Formulas

Worked example: Infusion rate

Given: Dose 5 mg/h, concentration 1 mg/mL

Find: Pump rate.

Answer: 5 mL/h

Worked example: Defibrillator voltage

Given: C = 150 µF, target 200 J

Find: V.

Answer: ≈ 1633 V

Worked example: CGM battery

Given: Sensor 30 mAh, 60 µA average

Find: Life.

Answer: ≈ 21 days

SpO₂, defibrillator energy and pacemaker battery life

# Pulse oximetry: ratio-of-ratios and a typical empirical calibration
red_ac, red_dc = 0.012, 1.00
ir_ac, ir_dc = 0.020, 1.10
R = (red_ac / red_dc) / (ir_ac / ir_dc)
SpO2 = 110 - 25 * R                      # textbook linear calibration
print(f"R = {R:.3f} -> SpO2 ~ {SpO2:.1f}%")
# Defibrillator energy: E = 1/2 C V^2
C, V = 150e-6, 1800.0
print(f"defibrillator capacitor stores {0.5*C*V*V:.0f} J")
# Pacemaker battery life: 1.2 Ah lithium-iodine, 10 uA average drain
print(f"pacemaker battery life ~ {1.2 / 10e-6 / 8766:.1f} years")
# R = 0.660 -> SpO2 ~ 93.5%
# defibrillator capacitor stores 243 J
# pacemaker battery life ~ 13.7 years

Real oximeters use calibration curves derived from human desaturation studies, not the textbook line. Pacemakers achieve decade-long lives by drawing microamps; every design choice (pulse width, sensing duty cycle, telemetry) is weighed against battery life.

Common mistakes

Applications

Key takeaways

Going further