4.4 Monitors: Pulse Oximeters, Blood Pressure Devices, and ECG Equipment
Key Takeaways
- Domain I of the PFT content outline lists ECG monitors as item 11 and monitors (pulse oximeters and blood pressure devices) as item 19, both of which must be set up, maintained, calibrated, and troubleshot by the technologist.
- Pulse oximeters are calibrated by the manufacturer against human volunteer desaturation studies and cannot be field-calibrated; FDA clearance requires accuracy (ARMS) within 3% over 70-100% saturation.
- A plethysmographic waveform with poor perfusion, motion artifact, or nail polish invalidates an SpO2 reading long before the displayed number looks abnormal.
- Automated oscillometric blood pressure devices measure mean arterial pressure directly and derive systolic and diastolic values by algorithm, so a cuff that is too small falsely elevates and a cuff that is too large falsely lowers readings.
- Exercise-quality ECG requires skin preparation to below 5 kilohms of impedance, Mason-Likar limb-lead repositioning onto the torso, and a 0.05 to 150 Hz diagnostic bandwidth.
4.4 Monitors: Pulse Oximeters, Blood Pressure Devices, and ECG Equipment
Three monitoring devices appear by name in Domain I of the PFT Detailed Content Outline: ECG monitors (item 11) and monitors — (a) pulse oximeters and (b) blood pressure (item 19). Each is listed under both Set Up, Maintain, Calibrate and Troubleshoot, which means the exam expects you to know how the hardware works, what the technologist can legitimately adjust, and how each device fails in a way that produces a believable but wrong number. That last point matters most: a monitor that fails obviously gets replaced, while a monitor that fails subtly gets charted.
Pulse Oximeters
Operating Principle
A transmission pulse oximeter shines two wavelengths of light — 660 nm (red) and 940 nm (infrared) — through a vascular bed such as a fingertip or earlobe, and a photodiode on the far side measures transmitted intensity. Oxyhemoglobin and reduced (deoxygenated) hemoglobin absorb these two wavelengths very differently: reduced hemoglobin absorbs more red light, oxyhemoglobin absorbs more infrared. The device computes the ratio of ratios:
The pulsatile (AC) component is arterial; the non-pulsatile (DC) component is venous blood, capillary blood, tissue, and bone. Dividing AC by DC at each wavelength isolates the arterial signal from everything static in the light path — which is precisely why a pulse oximeter cannot read without a pulse.
What You Cannot Calibrate
There is no field calibration for a pulse oximeter. The relationship between $R$ and saturation is empirical, derived by the manufacturer from controlled desaturation studies on healthy volunteers whose arterial blood was simultaneously analyzed by a laboratory CO-oximeter. That lookup table lives in firmware. FDA clearance requires an accuracy root-mean-square (A$_{RMS}$) of within 3% over the 70–100% range; below 70% the calibration curve is extrapolated, not measured, because volunteers cannot ethically be desaturated further. Practical consequence: an SpO$_2$ of 84% may be several points off in either direction, and a low reading during a six-minute walk should be confirmed against the trend and the waveform, not treated as a laboratory value.
Technologist responsibilities are therefore verification, not calibration: inspect the probe for a cracked emitter window, confirm the displayed pulse rate matches a manually palpated rate, confirm a clean plethysmographic waveform or perfusion index, and compare against a functioning reference device or a co-oximetry SaO$_2$ when a reading is clinically implausible.
Artifacts That Produce Believable Wrong Numbers
| Artifact | Mechanism | Typical Effect |
|---|---|---|
| Low perfusion (cold, vasoconstriction, hypotension) | AC signal shrinks toward the noise floor | Erratic or absent reading; drift toward 85% |
| Motion (walking, tremor, shivering) | Venous blood becomes pulsatile and is misread as arterial | Falsely low SpO$_2$ |
| Nail polish / artificial nails | Fixed absorbance in the light path; dark blue, green, and black are worst | Falsely low; rotate the probe 90° to a lateral position |
| Ambient light (procedure lamp, sunlight) | Photodiode saturation | Erroneous high or unstable values |
| Venous congestion / probe too tight | Venous pulsation added to AC | Falsely low |
| Carboxyhemoglobin | COHb absorbs at 660 nm almost identically to O$_2$Hb | Falsely high — the classic smoke-inhalation trap |
| Methemoglobin | Absorbs strongly at both wavelengths, driving $R$ toward 1.0 | Reading pulled toward a fixed ~85% regardless of true saturation |
The last two are the reason CO-oximetry exists. A two-wavelength pulse oximeter measures functional saturation and is blind to dyshemoglobins by design.
Non-Invasive Blood Pressure Devices
Manual Auscultatory Measurement
An aneroid or mercury sphygmomanometer with a stethoscope remains the reference method. Systolic pressure is the first Korotkoff sound (phase I) and diastolic pressure is the disappearance of sound (phase V). Aneroid manometers drift and must be verified against a calibrated reference at least annually and whenever the needle does not rest at zero with the cuff deflated.
Automated Oscillometric Devices
An oscillometric monitor does not hear Korotkoff sounds. It inflates the cuff above systolic pressure, then bleeds pressure while sampling the amplitude of arterial wall oscillations transmitted into the cuff. The point of maximum oscillation amplitude is the mean arterial pressure (MAP) — the only value the device actually measures. Systolic and diastolic values are then derived by a proprietary algorithm from the oscillation envelope. This explains two exam-relevant behaviors: oscillometric devices are least reliable in atrial fibrillation and other irregular rhythms, where beat-to-beat amplitude varies chaotically, and they can disagree with auscultation at the extremes of pressure while still reporting a correct MAP.
Cuff Selection — the Dominant Error Source
The bladder width should be about 40% of arm circumference and the bladder length should encircle 80–100% of the arm.
- Cuff too small (or wrapped too loosely): more pressure is needed to compress the artery → falsely high readings. This is by far the most common blood pressure error in adults with large arms.
- Cuff too large: falsely low readings.
- Arm below heart level: falsely high (hydrostatic column); arm above heart level: falsely low. Support the arm at mid-sternal level.
During cardiopulmonary exercise testing, the cuff must be placed on the arm and the tubing routed so the patient's swing does not pull it, and readings should be taken during the last 30–45 seconds of each workload stage.
ECG Monitors
Signal Acquisition and Skin Preparation
The ECG amplifier measures microvolt-level potentials, so the dominant technical variable is skin–electrode impedance. Preparation for an exercise-quality tracing means shaving hair at the electrode site, abrading the stratum corneum with a prep pad or fine abrasive, and cleaning with alcohol and allowing it to dry fully. Target impedance is below 5 kΩ; poorly prepped skin runs an order of magnitude higher and produces baseline wander that mimics ST-segment shift.
Silver/silver-chloride (Ag/AgCl) wet-gel electrodes are standard. Dried-out electrodes are a leading cause of intermittent artifact, so check the expiration date and the seal on the pouch.
Lead Placement for Exercise: the Mason-Likar Modification
A resting 12-lead ECG places the limb electrodes on the wrists and ankles, which is unusable during treadmill or cycle exercise. The Mason-Likar modification moves the arm electrodes to the infraclavicular fossae and the leg electrodes to the lower torso above the iliac crests. This reduces motion artifact dramatically but shifts the QRS axis rightward and can increase inferior-lead voltage — a Mason-Likar tracing is not interchangeable with a resting diagnostic 12-lead, and the report should note which configuration was used.
Precordial leads are unchanged: V1 fourth intercostal space right sternal border, V2 fourth intercostal space left sternal border, V4 fifth intercostal space midclavicular line, V3 midway between V2 and V4, V5 anterior axillary line level with V4, V6 midaxillary line level with V4.
Calibration and Filter Settings
- Standard calibration: a 1 mV signal produces exactly 10 mm of vertical deflection, and paper speed is 25 mm/s (so one small box = 0.04 s, one large box = 0.20 s). The calibration pulse must be verified on every recording; a monitor running at half standard (5 mm/mV) will make real ST depression look trivial.
- Diagnostic bandwidth: 0.05 to 150 Hz. The 0.05 Hz low-frequency corner is what preserves the ST segment. Switching to the "monitor" filter setting (roughly 0.5–40 Hz) suppresses baseline wander but distorts ST segments, which is unacceptable for exercise testing.
- Notch filter: a 50/60 Hz notch removes power-line interference but should not be used to paper over bad skin prep.
Troubleshooting Matrix
| Finding | Likely Cause | Action |
|---|---|---|
| Wandering baseline | High impedance, respiration, loose electrode | Re-prep skin, replace electrode, secure lead wires |
| Fine 60 Hz interference on all leads | Power-line coupling, ungrounded equipment | Unplug nearby devices, enable notch filter, check ground |
| Artifact isolated to one lead | That electrode or its wire | Replace the single electrode and lead wire |
| Flat line, one lead only | Disconnected wire | Reconnect; do not assume asystole without a second lead |
| Sudden amplitude halving | Gain switched to 5 mm/mV | Restore standard 10 mm/mV and re-record |
A patient rescued from a house fire has an SpO2 of 99% on room air but appears confused and dyspneic. Why can the pulse oximeter reading not be trusted, and what measurement is required?
An automated oscillometric monitor is used on a large-armed patient with a standard adult cuff that barely encircles the upper arm. What error should the technologist anticipate, and what does the device actually measure?
Before a cardiopulmonary exercise test, a technologist sets the ECG monitor to its 0.5 to 40 Hz filter to eliminate baseline wander. What is the consequence?