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Pulse Oximetry Physics and Pitfalls

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Monitoring I · 9 min read

Two wavelengths, one number. Knowing when SpO₂ lies (CO, MetHb, low perfusion) prevents the worst missed hypoxia.

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Explain the Beer-Lambert two-wavelength principle.
Recall the SpO₂ behavior in MetHb and COHb.
Identify low-perfusion + motion + nail-polish artifacts.
Decide when cooximetry is mandatory.

Beer-Lambert law at 660/940 nm

KeyTwo wavelengths chosen because oxyHb and deoxyHb have very different absorbances at each.

660 nm (red): deoxyHb absorbs strongly, oxyHb weakly.

940 nm (infrared): opposite — oxyHb absorbs strongly, deoxyHb weakly.

Ratio of pulsatile (arterial) absorbances at the two wavelengths empirically calibrated against arterial co-oximetry SaO₂.

Provides accuracy of ±2% in 80-100% range; less accurate below 80%.

Pulse oximetry SpO2 vs SaO2: pulse oximetry estimates arterial saturation noninvasively using two wavelengths (660 nm red, 940 nm IR); accuracy +/- 2% at 70-100%, less reliable in low perfusion, motion, dyshemoglobinemias.

Pulsatile signal extraction

Total absorbance = pulsatile AC component (arterial blood) + non-pulsatile DC component (tissue, venous blood, bone)

Algorithm isolates AC/DC ratio at each wavelength.

Pulsatile signal depends on perfusion — low cardiac output, vasoconstriction, hypothermia all attenuate signal.

Modern signal-extraction algorithms (Masimo SET) tolerate more motion than older devices.

Useful with cautery + shivering with newer technology.

Pulse oximetry in low perfusion states: dampened waveform, slow capillary refill, vasoconstriction, hypothermia cause unreliable readings; ear/forehead sensors and signal-extraction tech may help.

Methemoglobin 85% plateau

KeyMetHb absorbs equally at both 660 and 940 nm — drives ratio toward 1.0, which the calibration curve maps to SpO₂ ~85%.

Result: SpO₂ stuck at ~85% regardless of true SaO₂ (which could be much higher or lower).

Causes

topical benzocaine
prilocaine in EMLA
dapsone
nitrites
antimalarials

Treatment: methylene blue 1-2 mg/kg IV (contraindicated in G6PD — use ascorbic acid 1.5 g IV q4h).

Pulse oximetry: CO-Hb and metHb spectra explain inaccuracies; standard 2-wavelength oximetry confuses COHb with O2Hb (falsely high SpO2) and pulls MetHb toward ~85%; co-oximetry uses 4+ wavelengths.

Carbon monoxide false-normal

KeyCarboxyhemoglobin absorbs similarly to oxyHb at 660 nm.

Standard 2-wavelength pulse ox reads COHb as oxyHb falsely normal SpO₂ despite severe hypoxia.

Smoke inhalation, CO poisoning, dehydrated CO₂ absorbent + volatile generation.

Diagnose with co-oximetry (multi-wavelength) on arterial blood gas.

Treatment: 100% O₂ + hyperbaric for severe (LOC, seizure, neurologic deficit, MI, pregnancy).

Co-oximetry advantages over pulse oximetry: measures actual oxyhemoglobin, carboxyhemoglobin, methemoglobin, and deoxyhemoglobin using 4+ wavelengths; gold standard for dyshemoglobinemia.

Low perfusion + motion artifact

KeyMost common cause of intra-op SpO₂ failure (>50% in surveys).

Vasoconstriction (cold, hypotension, vasopressors, PVD), motion (shivering, jostling), cautery noise, nail polish (rare in modern tech).

Mitigations

warm the digit
switch site (ear, forehead reflectance)
try multiple fingers
Masimo SET tolerates more motion

Forehead reflectance probe particularly useful in centralized vasoconstriction.

Causes of artifact (false low or unreliable readings) in pulse oximetry: motion, nail polish, ambient light, low perfusion, dyshemoglobinemia, IV dyes (methylene blue); recognize before treating perceived hypoxia.

Site selection

FINGERstandard, easiest access; rotate sites in long cases to prevent pressure injury.

EAR: less motion artifact, faster response to central O2 changes (closer to brain perfusion).

FOREHEAD reflectance probe: best when peripheral perfusion poor (vasoconstriction, low CO, vasopressor infusion); responds faster to central changes.

TOE: pediatric option, lower priority.

NASAL ALARuseful in burn patients with no available digit sites.

RIGHT HAND PRE-DUCTAL in neonates with possible PDA shunt — measures pre-shunt arterial sat.

PULSE CO-OXIMETRY (Masimo Rainbow): multi-wavelength measures SpO2 + SpCO + SpMet + total Hb non-invasively — useful in burn, CO exposure, methemoglobin, anemia screening.

Near-infrared spectroscopy (NIRS) regional oxygen saturation: light source/detector pair, measures venous-weighted tissue O2 saturation, common in cardiac and pediatric anesthesia; affected by anemia, edema, melanin.

Right hand vs left hand in special situations

KeyRIGHT hand = pre-ductal (proximal to the ductus arteriosus insertion into the descending aorta).

LEFT hand or feet = post-ductal

NEONATES with possible PDA + persistent fetal circulation: pulse ox on right hand AND a foot — divergence (right higher than foot) confirms right-to-left shunt across PDA.

CONGENITAL HEART surgery: site choice depends on lesion anatomy.

AORTIC DISSECTIONright vs left arm BP discrepancy can be the first clue (Stanford A dissection commonly involves left subclavian).

Tetralogy of Fallot anatomy: pulmonary stenosis, RVH, overriding aorta, and VSD produce right-to-left shunt with cyanosis. Magnitude depends on RVOT obstruction and SVR; tet spells improve with knee-to-chest position and phenylephrine.

Common alarm scenarios + practical management

KeyFALSE LOW alarms common; do not ignore but verify.

CHECKLIST when SpO2 alarms: (1) verify probe placement + perfusion at site (warm digit, change site if cold/cyanotic).

Check pleth waveform — flat/absent indicates poor signal, not true hypoxia.

Confirm with second site or finger blood gas if uncertain.

Examine patient: cyanosis, breath sounds, ETCO2, ventilator settings, tube position.

ABG with co-oximetry if rapid clinical change with SpO2 NORMAL but suspected hypoxia (CO, MetHb).

NEVER ignore — alarm fatigue causes deaths.

Set audible variable-pitch tone always on — pitch change is the fastest way to detect early desaturation.

Anesthesia machine pre-use checklist (6-point): power and alarms, oxygen supply and backup, leak test, ventilator check, gas delivery, suction and scavenging. Performed at start of day and between cases.

⚠ Common pitfalls

Trusting SpO₂ in fire-victim / smoke-inhalation patients — COHb reads falsely normal; need cooximetry.
Using nail polish black/blue/green — interferes with 660 nm absorbance; remove or use ear/cheek probe.
Calling poor waveform 'machine error' — it's often low perfusion (vasopressor candidate).
Reading SpO₂ as PaO₂ — the relationship is sigmoidal; 90% SpO₂ ≈ 60 mmHg PaO₂.

💎 Clinical pearls

MetHb produces a 'flatline' at SpO₂ ~85% regardless of true sat — cooximetry confirms; treat with methylene blue 1-2 mg/kg.
Methylene blue itself causes a transient SpO₂ drop (artifact, not desat) — anticipate it.
Sickle-cell patients in crisis: cooximetry preferred over pulse-ox for management decisions.
Cyanosis is visible when ≥5 g/dL deoxy Hb — anemic patients desat without obvious cyanosis.

Recap

MetHb produces a 'flatline' at SpO₂ ~85% regardless of true sat — cooximetry confirms; treat with methylene blue 1-2 mg/kg.
Methylene blue itself causes a transient SpO₂ drop (artifact, not desat) — anticipate it.
Sickle-cell patients in crisis: cooximetry preferred over pulse-ox for management decisions.
Cyanosis is visible when ≥5 g/dL deoxy Hb — anemic patients desat without obvious cyanosis.

Mark each section done to complete the module.