4.3 Pressure Devices, Valves & Gas Delivery Systems

Pressure, Valves, and Gas Paths Share One Theme

Many PFT measurements are only as valid as the plumbing. Domain I groups several related competencies that the exam often tests together:

• Pressure-measuring devices (manometers, electronic transducers) used for MIP/MEP, mouth pressures, and box-related pressures (I.A.13 on the PFT DCO)
• Valves that direct inspired and expired gas, demand-deliver test mixtures, or enforce one-way flow (I.A.6)
• Gas delivery systems—blenders, flowmeters, regulators, and cylinders/wall outlets—for oxygen assessment/titration and for delivering challenge or specialty mixtures (I.A.12)

Troubleshooting and QC items for these subsystems ask whether you can spot a zero-offset manometer, a one-way valve stuck open, a blender with inadequate inlet pressure, or a flowmeter bobbin that does not read true flow.

Manometers and Pressure Transducers

What they measure in the PFT lab

Analog aneroid manometers and electronic pressure transducers both appear in labs. Electronic systems need warm-up, zeroing, and digital calibration factors; aneroid gauges need visual zero checks and periodic comparison to a reference.

Setup for MIP/MEP equipment

1. Select a flanged mouthpiece or manufacturer interface that the patient can seal; nose clip on.
2. Confirm the occlusion valve or shutter closes fully for the pressure effort (a leaky occlusion under-reads true muscle pressure).
3. Zero the transducer/manometer at atmosphere before efforts.
4. Verify the device range covers expected adult pressures (MIP often tens of cm H2O negative; MEP higher positive values—know that out-of-range clipping is an equipment limit, not physiology).
5. Use a small leak orifice if required by standards/lab protocol to prevent glottic closure artifact—ensure the orifice is the specified size and not obstructed or omitted inconsistently.
6. Infection-control barriers per policy without blocking the pressure port.

Calibration and QC of pressure devices

Apply a known pressure with a reference manometer, calibrated syringe-pump, or electronic calibrator:

• Check zero at open atmosphere.
• Check span at one or more positive and negative points in the clinical range.
• Document tolerance (manufacturer/lab). Typical exam thinking: if the reference is −60 cm H2O and the device reads −45 cm H2O after proper zeroing, the system fails—do not blame the biological control subject first.

Shared transducers used for both box pressures and muscle pressures must meet the range and accuracy needed for each application; a sensor optimized only for tiny box ΔP may be inappropriate for high MEP unless the system is designed for both.

Directional, Demand, and One-Way Valves

Valves enforce the gas path the protocol assumes.

One-way (check) valves

• Allow flow in one direction only in breathing circuits, non-rebreathing assemblies, and some demand systems.
• Stuck open → rebreathing, CO2 accumulation, diluted inspired test gas, or loss of separation between inspired and expired limbs.
• Stuck closed → patient cannot inspire or expire through that limb; high pressure, panic, or aborted test.
• Reassembled backward after cleaning → classic post-maintenance failure; always function-check direction with a simple flow test after reassembly.

Directional valve assemblies / sliding valves

Used to switch between room air, test gas bags, occlusion, or analyzer paths. Setup requires:

• Full travel to detents
• No cross-leak between ports
• Correct labeling so “test gas” actually selects the test-gas limb

Demand valves

Demand systems deliver gas when the patient generates a negative pressure trigger (common in some O2 delivery and specialized gas systems). Setup/QC concerns:

• Adequate supply pressure upstream
• Trigger sensitivity within spec (too hard → patient fights the valve; too easy → free flow/waste)
• Clean patient connection and functioning exhalation pathway
• No sticky diaphragm from moisture or contamination

Blenders, Flowmeters, and Gas Delivery

Oxygen titration and assessment

Oxygen assessment during walk or exercise may use nasal cannula or other interfaces with a flowmeter fed by cylinder or wall oxygen. Setup truths:

• Confirm gas identity (O2) and secure cylinder restraint; crack-purge regulators as trained.
• Set flowmeter upright; read the float per manufacturer (center vs top of bobbin).
• Know that indicated L/min is device- and gas-specific; a flowmeter designed and labeled for O2 should not be assumed accurate for arbitrary gases.
• Tubing length, humidifier addition, and leaks at connectors reduce delivered flow below the set value—inspect the entire path.

Air–oxygen blenders

Blenders mix air and oxygen to a set FiO2 when both inlet pressures are adequate and balanced within manufacturer windows.

High-yield failure modes:

• Low inlet pressure on air or O2 → inaccurate FiO2 or alarm
• Disconnected analyzer port if FiO2 is being verified
• Downstream leaks so the patient does not receive the blender setting
• Using blender output without verifying with an O2 analyzer when protocol/lab policy requires verification for critical tests

Specialty and challenge gases

Bronchoprovocation agents, DLCO test gas, hypoxic mixtures, or other specialty cylinders demand:

• Correct cylinder label and lot, not “the tank that was nearby”
• Compatible regulator (some gases need specific materials)
• Leak-free connections from cylinder → demand valve/bag → patient
• Scavenging or room ventilation policies for certain agents
• Expiration dating and storage conditions per SDS and lab SOP

An RPFT who can run spirometry but cannot secure a challenge-gas delivery path is incomplete on Domain I.

Flowmeter verification (QC concept)

Reference methods may include calibrated flow standards or comparison devices. For blended O2, many labs verify delivered FiO2 with a calibrated oxygen analyzer at the patient connection. Document set vs measured values; remove devices that drift beyond limits.

Integrated Troubleshooting Logic

When results look wrong, classify the subsystem:

1. Pressure sensing — zero, span, occluded tubing, wet transducer, wrong units (kPa vs cm H2O display confusion in mixed device fleets).
2. Valve mechanics — direction, sticking, incomplete occlusion for MIP/MEP or TGV.
3. Gas supply — empty cylinder, closed wall valve, inadequate blender inlet pressures, wrong gas.
4. Interface leak — mouthpiece, nose clip, cannula prongs, mask seal.

Scenario A — Low MIP

A myopathy clinic sends a patient for MIP/MEP. Values are far below prior studies. The technologist notices the aneroid needle rests at +5 cm H2O with the port open to air, and the flanged mouthpiece is cracked. Instrument-first actions: replace mouthpiece, zero/replace manometer, verify with a pressure standard, then repeat efforts. Reporting “severe inspiratory weakness” from a +5 cm H2O zero error is a Domain I miss.

Scenario B — O2 titration walk

During a walk test, SpO2 falls despite “3 L/min O2.” Inspection shows the flowmeter set to 3 but the tubing disconnected at the tank wrench joint and the cylinder near empty. Fix the delivery system, replace/repair, confirm continuous flow at the cannula with the correct gas, then restart per protocol. Desaturation on a disconnected cannula is not an indication to increase the written prescription without restoring delivery integrity.

Scenario C — Challenge circuit

After cleaning, a one-way valve is reinserted backward on a bronchial challenge circuit. The patient cannot inspire test aerosol properly and coughs against a closed path. Function-check valves after every reassembly—a required QC habit, not optional perfectionism.

Documentation and Out-of-Service Rules

• Log pressure-device calibration and failures.
• Tag defective blenders/flowmeters; do not leave an inaccurate O2 device in the walk-test kit.
• After valve maintenance, record function checks.
• Link out-of-service decisions to patient safety: wrong FiO2 and false MIP both change clinical pathways.

Study Hook

/practice/rpftPractice questions with detailed explanations