Chapter 1: Perform vital-signs monitors maintenance

Biomedical engineers in Kenya play a critical role in ensuring the reliability and accuracy of vital-sign monitoring devices used in healthcare facilities. Among these, blood pressure machines are fundamental for patient diagnosis and management in hospitals, clinics, and county health centers. Proper maintenance of these devices guarantees consistent performance, minimizing downtime and errors that could affect patient care. This chapter focuses on understanding blood pressure monitors, their types, methods of measurement, and essential maintenance practices within the Kenyan biomedical engineering context.

1.1 Meaning and function of a Blood Pressure Machine (BP)

A blood pressure machine measures the pressure that circulating blood exerts on the walls of the arteries. The result is written as two numbers in millimetres of mercury (mmHg): the systolic pressure, reached when the heart contracts, and the diastolic pressure, the lowest value between beats, for example 120/80 mmHg. The reading helps clinical staff to detect high or low blood pressure and to follow the effect of treatment, and it is one of the vital signs recorded at almost every patient contact. A biomedical engineering technician who maintains these machines must understand how each method works, because a leak, a drifting gauge or a worn sensor gives readings that look believable but are wrong.

1.1.1 Methods of blood pressure measurements

Blood pressure can be measured by several methods, each with its own use, accuracy and maintenance needs. Non-invasive methods use a cuff around the upper arm. The invasive method measures pressure directly inside an artery.

Manual auscultatory method

• Procedure: a cuff is inflated to stop the flow in the brachial artery, then deflated slowly while the clinician listens with a stethoscope for Korotkoff sounds. The first sound gives the systolic pressure and the disappearance of the sounds gives the diastolic pressure.
• Significance: it is the reference method against which automatic devices are compared, when performed correctly.
• Limitations: it needs trained staff, and results are affected by hearing, noise in the room and the speed of deflation.

Oscillometric method

• Procedure: an automatic device senses the small pressure oscillations that the arterial wall produces in the cuff during deflation, and its processor calculates the systolic and diastolic values from them.
• Significance: it needs no stethoscope and little training, so it is used in health facilities and for home monitoring.
• Limitations: accuracy may fall with irregular heart rhythm, very low blood pressure or arm movement during the measurement.

Palpatory method

• Procedure: the clinician feels the radial pulse, inflates the cuff until the pulse disappears, then deflates slowly and notes the pressure at which the pulse returns, which is the systolic pressure.
• Significance: it is useful when Korotkoff sounds cannot be heard, for example in a noisy room.
• Limitations: it gives only the systolic pressure and is less precise.

Invasive intra-arterial measurement

• Procedure: a catheter is placed in an artery and connected through a fluid-filled line to a pressure transducer, which sends a continuous signal to a monitor.
• Significance: it gives a beat-by-beat pressure and is used in intensive care and during major surgery.
• Limitations: it is invasive, carries a risk of infection and bleeding, and needs a transducer that is zeroed and levelled correctly.

Ambulatory blood pressure monitoring

• Procedure: the patient wears a small automatic cuff device that measures at set intervals over 24 hours.
• Significance: it shows how blood pressure changes through the day and night, and helps to identify raised pressure that appears only during a visit to a health facility.
• Limitations: the patient must wear the cuff correctly and keep the arm still during each reading, and the battery must be fresh.

1.1.2 Types and parts of blood pressure machines

Blood pressure machines differ in design but share the same basic parts: a cuff, a way of inflating it, a way of measuring the cuff pressure and a way of releasing the air. Knowing the parts makes it easier to trace a fault.

Types of blood pressure machines

• Mercury sphygmomanometers: use a column of mercury to show the pressure. They are accurate but are being phased out because of the toxicity of mercury and the risk of spills.
• Aneroid sphygmomanometers: use a mechanical dial gauge with a metal capsule. They are portable and contain no mercury, but their calibration drifts and they must be checked.
• Digital automatic monitors: use an electronic pressure sensor and a microprocessor to calculate and display the readings.
• Hybrid models: combine an electronic sensor with a manual cuff or a display that imitates a column.

Key components

• Cuff: an inflatable bladder in a cloth cover that squeezes the artery. It comes in different sizes, and a cuff that is too small or too large gives wrong readings.
• Pressure gauge or display: shows the pressure as a mercury column, a dial or a digital screen.
• Inflation bulb and release valve: manual devices use a hand bulb to inflate the cuff, and a valve that lets the air out at a controlled rate.
• Pump and valve in automatic devices: a small electric pump inflates the cuff and an electric valve deflates it.
• Pressure sensor and microprocessor: in digital monitors they detect the cuff pressure and its oscillations and calculate the result.
• Tubing and connectors: carry air between the cuff and the gauge or the electronic unit, and must be airtight.
• Power source: batteries or a mains adapter for digital devices.

1.1.3 Blood pressure monitors

Blood pressure monitors are the devices and systems that measure, display, record and sometimes transmit blood pressure and other vital signs. Many of them are combined in bedside or portable vital-signs monitors that also measure pulse and breathing. The classes of monitors and the features that influence selection come first, then three sensing devices: the stethoscope, used with manual blood pressure measurement, and pulse and respiratory sensors, used by monitors.

Classes of monitors

• Manual monitors: the user inflates the cuff and interprets the sounds or the dial, so they work without power and are simple to repair.
• Semi-automatic monitors: the user inflates by hand and the device deflates and displays the result, which reduces observer error.
• Fully automatic monitors: inflate and deflate automatically and show systolic, diastolic and pulse rate, and some store readings or send them to another system.

Features that influence selection

• Accuracy: the model should be validated against an accepted protocol such as ISO 81060-2.
• Cuff sizes: a range of sizes allows correct measurement on children and adults.
• Portability: a light, battery-powered unit is easier to use in outreach and home visits.
• Display and controls: clear numbers and simple buttons reduce user mistakes.
• Connectivity: a USB or wireless connection lets readings be exported to patient records.

1.1.3.1 Stethoscope

A stethoscope carries body sounds from the patient to the ears of the listener. In blood pressure measurement it is used to hear the Korotkoff sounds under the cuff, and it is also used to listen to the heart and lungs. An acoustic stethoscope has no electronics, so most of its faults are mechanical.

• Chest piece: has a diaphragm, which picks up higher-pitched sounds, and sometimes a bell, which picks up lower-pitched sounds.
• Tubing: carries the sound to the ears and should be flexible, without cracks, and not too long.
• Binaural tubes and ear tips: the metal tubes that lead to the ears, with soft ear tips that must seal the ear canal.
• Electronic stethoscopes: amplify the sound and may filter noise, display a waveform or record it, and need a battery and sometimes software.
• Maintenance: clean the chest piece and ear tips with an alcohol wipe between patients. Replace cracked tubing and hardened ear tips. Check the diaphragm for cracks or loose rim, and check the battery and controls of electronic models.

1.1.3.2 Pulse sensors

Pulse sensors detect each heartbeat and are used to display heart rate and sometimes a pulse waveform. A photoplethysmographic sensor is widely used in monitors: a clip or probe on the finger or earlobe with two light-emitting diodes and a photodiode. The photodiode detects changes in the light passing through the tissue as the volume of blood changes with each beat. The same probe is used to measure oxygen saturation, which is covered in the topic on pulse oximetry.

• Other pulse sensing: the pulse can also be taken from an electrocardiogram signal, or from a pressure sensor placed over an artery.
• Common faults: the signal is lost or irregular when the patient moves, when the fingers are cold or poorly perfused, when nail polish or dirt is on the finger, or when strong room light reaches the sensor.
• Cable and probe faults: broken wires near the connector, a failed light-emitting diode, or a dirty window produce a weak or missing trace.
• Maintenance: clean the probe with the wipe approved by the manufacturer, check the clip spring and the cable, and test the probe on a healthy volunteer or with a pulse simulator.
• Safety: probes are moved to another finger at the interval stated in the manual, because prolonged pressure or heat can damage the skin.

1.1.3.3 Respiratory sensors

Respiratory sensors detect breathing and are used to display the respiratory rate and to raise an alarm if breathing stops. The type that a monitor uses depends on the model and on the way it is set up for the patient.

• Impedance sensing: electrodes on the chest, normally the ECG electrodes, detect the small change in electrical impedance as the chest expands and contracts.
• Airflow sensors: a thermistor or a temperature sensor at the nose or mouth detects the change in temperature between the inhaled and the exhaled air.
• Respiratory belts: a belt round the chest or abdomen holds a sensor that detects the stretch caused by breathing.
• Gas sensing: a capnograph measures carbon dioxide in the breath and gives a respiratory rate as well, as described in the topic on the capnograph.
• Common faults: poor electrode contact, a loose belt, a blocked sampling line, movement artefacts and a broken lead all give false rates or false alarms.
• Maintenance: use fresh, correctly placed electrodes, inspect leads for breaks, clean reusable sensors with an approved wipe and verify alarm limits with a simulator or an observed breathing check.

1.1.4 Common faults and maintenance of blood pressure machines and pressure monitors

Blood pressure machines are used many times a day, so they wear out in predictable ways: leaks, drifting calibration, battery faults and sensor wear. Regular checks find these faults before they affect patient care.

Common faults

• Leaks in the cuff or tubing: the pressure falls during measurement, giving unreliable readings. They come from punctured bladders, perished rubber or loose connectors.
• Calibration drift: the gauge or sensor reads high or low by an amount that changes over time, so readings look steady but are wrong.
• Battery or power faults: weak batteries or a faulty adapter cause erratic readings, a blank screen or an error message.
• Sensor faults: in oscillometric devices a worn or contaminated pressure sensor produces inconsistent results.
• Valve faults: a sticking release valve gives a deflation rate that is too fast or too slow.
• Mechanical damage: dropping the unit or storing it with a tightly wrapped cuff can damage the gauge, the pump and the tubing.

Maintenance practices

• Calibration check: compare the device with a reference manometer or a pressure simulator at the interval in the facility schedule or the manufacturer's manual.
• Cuff inspection: check the bladder, the cover and the hook-and-loop closure and replace worn cuffs.
• Battery management: use the correct type of battery, replace all together and remove them when the device is stored for a long period.
• Cleaning and disinfection: wipe the cuff and the casing with the disinfectant recommended in the manual, and do not let liquid enter the tubing or the case.
• Storage: keep devices in a dry, dust-free place away from direct sunlight and heat.

Troubleshooting steps

1. Note the symptom, for example inconsistent readings, error message or no inflation.
2. Inspect the cuff, tubing and connectors for visible damage.
3. Test for leaks by inflating the cuff and tubing to a set pressure and watching how fast it falls.
4. Check the calibration against the reference device.
5. Check the batteries or adapter, then test the pump and valve.
6. Replace the faulty part or send the device to the supplier for repair.
7. Record the fault, the repair and the test results in the equipment log.

Chapter Summary

This chapter explored the maintenance of vital-signs monitors, beginning with blood pressure machines by detailing their measurement methods, types, parts, and common faults alongside maintenance practices. It then examined electromagnetic blood flowmeters and the radiographic method for estimating blood flow, emphasizing their operational principles. Pulse oximetry was discussed, focusing on its working mechanism and upkeep requirements. The chapter further explained the function and types of capnographs, outlining their components, measurement techniques, typical faults, and maintenance routines. Blood gas analyzers were covered next, including their working principles, parts, control devices, and maintenance procedures. The principles and components of EEG and ECG machines were described, alongside their specific maintenance, safety measures, and calibration needs. The chapter concluded with an overview of glucometers and bilirubinometers, highlighting their basic components, maintenance, safety protocols, and calibration processes, as well as the components of a diagnostic set essential for comprehensive patient assessment.