Measurement methods in anaesthesia

SI Units and Pressure Measurement

SI Units in Anaesthesia

The International System of Units (SI) provides a standardised framework for measurement. Key SI units relevant to anaesthesia include:

Pressure is defined as force per unit area:

$$P = \frac{F}{A}$$

The SI unit of pressure is the pascal: $1\ \text{Pa} = 1\ \text{N·m}^{-2}$

Because 1 Pa is a very small pressure, the kilopascal (kPa) is more clinically practical. Multiple pressure units are used in medicine, requiring fluency in conversion:

Blood pressure is conventionally expressed in mmHg; airway pressures in cmH₂O; gas cylinder pressures in kPa or bar.

Measurement of Volumes, Flows, and Pressures

Pressure Gauges and Transducers

Pressure transducers convert a mechanical pressure signal into an electrical signal. In invasive arterial blood pressure monitoring, a fluid-filled column transmits pressure to a strain-gauge transducer. Key components and sources of error are discussed under invasive blood pressure measurement.

Flowmeters

Variable-orifice (Rotameter) flowmeters use a bobbin suspended in a tapered tube. Flow is read at the top of the bobbin. These are gas-specific due to dependence on gas viscosity (at low flows, laminar, viscosity-dependent) and density (at high flows, turbulent, density-dependent).

Pneumotachograph

The pneumotachograph is a fixed-orifice, variable-pressure flowmeter. It measures the pressure drop across a known resistance (a mesh or bundle of parallel tubes):

$$\dot{V} = \frac{\Delta P}{R}$$

• Two sensitive pressure transducers are positioned on either side of a resistance
• Provides bidirectional flow measurement
• Used in ventilators and respiratory function testing
• Accurate across a wide flow range; Pitot-tube variants minimise added resistance

Wright Respirometer

• Measures expired tidal volume and minute volume
• A vane rotates in response to gas flow
• Best positioned on the expiratory limb
• Under-reads at low flows (vane inertia) and over-reads at high flow rates
• Not bidirectional

Measurement of Blood Pressure

Non-Invasive Blood Pressure (NIBP)

Auscultatory method (sphygmomanometry):

• Cuff inflated above systolic; Korotkoff sounds auscultated during deflation
• Too small a cuff → falsely elevated reading
• Requires slow cuff deflation for accuracy

Oscillotonometry (DINAMAP):

• Device for indirect non-invasive automatic mean arterial pressure measurement
• Detects oscillations in cuff pressure during deflation
• MAP detected at the point of maximum oscillation
• Systolic and diastolic pressures are algorithmically derived

Mean arterial pressure (MAP):

$$\text{MAP} = \text{DBP} + \frac{1}{3}(\text{SBP} - \text{DBP})$$

Or equivalently: MAP = diastolic pressure + one-third of pulse pressure.

Finapres system: Continuous non-invasive measurement using a finger cuff with infrared plethysmography; maintains constant finger volume via servo-controlled cuff pressure.

Invasive Arterial Blood Pressure (IABP)

Common sites: Radial (most common), brachial, femoral arteries. Allen's test performed before radial cannulation.

System components: Arterial cannula → fluid-filled tubing → pressure transducer → amplifier/display

Zeroing: Transducer levelled at the phlebostatic axis (mid-axillary line, 4th intercostal space); opened to atmosphere to zero.

Sources of error:

Fourier analysis: The arterial waveform is a complex periodic wave; accurate reproduction requires the system's natural frequency to be well above the fundamental frequency of the pulse (~1-3 Hz), with adequate damping coefficient (~0.64 optimal).

Additional waveform information: Stroke volume estimation, fluid responsiveness assessment, systolic pressure variation.

Complications: Haematoma, arterial thrombosis, distal ischaemia, infection, accidental intra-arterial drug injection.

Measurement of Cardiac Output

Methods include:

• Pulmonary artery catheter (thermodilution): Cold injectate bolus; Stewart-Hamilton equation used; requires PAC placement with associated risks
• Doppler (oesophageal/transthoracic): Measures blood flow velocity in aorta; requires cross-sectional area estimation
• Bio-impedance: Changes in thoracic electrical impedance with each heartbeat
• Arterial waveform analysis (PiCCO, LiDCO): Pulse contour analysis; requires calibration
• Fick principle: $\dot{Q} = \frac{\dot{V}O_2}{CaO_2 - C\bar{v}O_2}$

Temperature Measurement

Core temperature sites (in order of accuracy): Pulmonary artery > oesophagus > nasopharynx > rectum > tympanic membrane > bladder.

Electrocardiography (ECG)

• Measures surface electrical potentials generated by myocardial depolarisation/repolarisation
• Electrode potentials range: 0.5-2 mV
• Standard display: 25 mm/s sweep speed, 1 mV/cm sensitivity
• Silver/silver chloride electrodes are used, minimise electrode-skin impedance
• Monitoring mode: Narrower frequency response (0.5-40 Hz), reduces artefact, suitable for rhythm monitoring
• Diagnostic mode: Wider frequency response (0.05-150 Hz), preserves ST-segment morphology

Sources of interference:

• Electrostatic induction (reduced by copper screening of leads)
• Electromagnetic interference (diathermy, electrical equipment)
• Patient movement artefact
• Electrode placement errors

Clinical utility: Rhythm monitoring, ST-segment analysis (ischaemia), electrolyte disturbance detection, pacemaker function.

Oximetry (Pulse Oximetry)

Principles

Pulse oximetry exploits the Beer-Lambert law: light absorption is proportional to the concentration of absorbing species and path length.

Two wavelengths of light are used:

• Red light: 660 nm (absorbed preferentially by deoxyhaemoglobin)
• Infrared light: 940 nm (absorbed preferentially by oxyhaemoglobin)

The ratio of absorbances at these two wavelengths (R/IR ratio) is used to calculate $SpO_2$. The pulsatile component of absorbance is isolated to distinguish arterial blood from venous blood and tissue.

Accuracy

Clinically accurate within the range 70-100% $SpO_2$.

Sources of Error

Key limitation: $SpO_2$ does not reflect $PaCO_2$ or ventilation adequacy.

Terminology:

• $SpO_2$: Pulse oximeter-measured saturation
• $SaO_2$: Arterial oxygen saturation measured by co-oximetry
• $FO_2Hb$: Fractional oxyhaemoglobin (co-oximetry, includes all Hb species)

Infrared Gas Analysis and Capnography

Principles of Infrared Absorption

Molecules with two or more different atoms absorb infrared radiation at characteristic wavelengths (due to molecular vibration/rotation). Homodiatomic gases (O₂, N₂) cannot be measured by infrared analysis.

Key absorption wavelengths:

The Beer-Lambert law applies: absorbance is proportional to gas concentration and path length. Glass absorbs infrared radiation and cannot be used for windows in analysers (sapphire or calcium fluoride used instead). Nitrous oxide can interfere with CO₂ analysis due to overlapping absorption spectra in some designs.

Capnography

Mainstream analysers: Sensor placed directly in the airway circuit; no sampling delay, but adds dead space and weight to circuit; cannot simultaneously measure other gases easily.

Sidestream analysers: Gas sampled from circuit via fine tubing at 50-200 mL/min; delay of up to 1-2 seconds acceptable (less than 38 seconds); lighter, can measure multiple gases simultaneously; prone to sampling tube occlusion/leaks and water vapour interference.

Normal Capnograph Waveform

• Phase I: Anatomical dead space gas (no CO₂)
• Phase II: Mixed dead space/alveolar gas (rapid CO₂ rise)
• Phase III: Alveolar plateau (gradual CO₂ rise)
• End-tidal CO₂ (EtCO₂): Approximates alveolar PCO₂

In healthy lungs: $EtCO_2 \approx PaCO_2$ (EtCO₂ typically 2-5 mmHg less than PaCO₂). In COPD, the alveolar plateau slopes upward due to uneven emptying.

Clinical uses of capnography:

• Confirmation of tracheal intubation (most reliable)
• Ventilator disconnection alarm
• Assessment of ventilation adequacy
• Detection of air/pulmonary embolism (sudden decrease in EtCO₂)
• Diagnosis of malignant hyperthermia (rising EtCO₂)
• CPR effectiveness monitoring

Photoacoustic spectroscopy: An alternative infrared detection method using a microphone to detect pressure waves generated by absorbed infrared energy; more stable than conventional thermal detection.

Paramagnetic and Fuel Cell Oxygen Analysis

Paramagnetic Oxygen Analyser

Oxygen is weakly paramagnetic (attracted into a magnetic field) due to two unpaired electrons. Most other anaesthetic gases are diamagnetic (weakly repelled).

Differential pressure paramagnetic analyser (modern standard):

1. Two chambers separated by a sensitive pressure transducer
2. Sample gas delivered to measuring chamber; reference gas (room air, 21% O₂) to reference chamber
3. Electromagnet switched on/off at ~100-110 Hz, creating an alternating magnetic field
4. Oxygen molecules attracted into the magnetic field create a pressure difference proportional to oxygen concentration
5. Differential pressure measured and displayed as % O₂

• Response time: Fast (10-20 ms), capable of breath-to-breath measurement
• Accuracy: ~0.1%
• Nitrous oxide is diamagnetic; nitrogen is weakly diamagnetic, neither interferes significantly

Fuel Cell (Galvanic Cell)

Components: Lead anode, gold cathode, potassium hydroxide electrolyte, oxygen-permeable membrane (Teflon).

Reactions:

• Cathode: $O_2 + 4e^- \rightarrow 4OH^-$
• Anode: $Pb + 2OH^- \rightarrow PbO + H_2O + 2e^-$

Current generated is proportional to oxygen partial pressure. Calibrated using 100% O₂ and room air (21% O₂).

Basic Pulmonary Function Tests

Spirometry

Spirometry measures lung volumes and flow rates using a pneumotachograph or rolling-seal spirometer.

Obstructive pattern: FEV₁/FVC < 0.7, FVC relatively preserved (COPD, asthma) Restrictive pattern: FVC reduced, FEV₁/FVC ≥ 0.7, TLC reduced

Flow-Volume Loops

Graphical display of flow vs. volume during forced inspiration and expiration. Characteristic patterns for obstruction (scooping of expiratory limb), restriction (narrow but normal shaped), and fixed upper airway obstruction (plateau in both phases).

Wright Respirometer

Used clinically to measure tidal volume and minute ventilation at the bedside or intraoperatively. Best positioned on the expiratory limb. Under-reads at low flows; over-reads at high flows.

Clinical Relevance

Pressure transducer zeroing errors are a common and important source of error in invasive monitoring: for every 10 cm of incorrect height, approximately 7.4 mmHg error is introduced. All invasive pressure measurements must be zeroed at the phlebostatic axis before clinical decision-making.

Sources

• ANZCA clinical guidelines
• ANZCA Professional Documents (PS series)