Visual Diagnosis
A Hypoxic Teen Inpatient
Eva Delgado, MD
Morning Report
Overview
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Case Presentation
Work-up based on Differential
Significance of the Diagnosis
Understanding the Presentation
Treating the Pathology
Take Home Points
Case Presentation
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16 y/o F with synovial sarcoma in L
thigh/hip, metastatic to lungs at
diagnosis.
Radiation  wound complication,
transferred to ortho  debridement,
revision of hip arthroplasty
Case Presentation
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Patient with long course and poor
nutrition, so pediatric hospitalist service
consulted.
Day peds MD ordered NG tube with local
anesthetic and CXR to confirm placement.
RN calls Night peds MD due to
desaturation shortly afterward.
Physical Exam
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VS: RR 12, O2sat 85-90% on 10L NC
Gen: No ↑WOB, alert
HEENT: No cyanosis
Chest: CTA b/l, good aeration
CV: RRR, no murmur
Ext: wwp, cap refill < 2 sec,
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New pulse ox trialed, well-placed, with
same reading on different digits
Work-up to Diagnosis
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Chart Review:
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History- tumor burden/growth,
medications, hypercoagulability, similar
past episodes
Labs/Studies:
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CXR
ABG/VBG
CBC
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CBC: Hbg 8.6, HCT 25
VBG: 7.36/44/53/25 -0.1
MetHb: 40%
Diagnosis: Methemoglobinemia
CO-Hb: 0.9%
A Review of Hemoglobin
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Hemoglobin = 4 globin chains, 4
heme molecules, each bound to
Fe++
Oxygen binds
Iron (Fe++)
Hemoglobin  Methemoglobin
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Oxidation = loss of 4 electrons from
Fe atoms
Methemoglobin = 4 globin chains,
4 heme molecules, each bound to
Fe+++
**Can’t bind O2 in this form**
Significance to Practice
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MetHb is 2% of normal blood
Endogenous enzymes reduce to Hb
Methemoglobinemia:
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Oxidation  >2% MetHb
Oxyhemoglobin curve
Danger: tissue hypoxia
*MetHb level 70% fatal
Etiologies of Methemoglobinemia
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Congenital:
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Deficiency of enzymes that reduce
MetHb back to Hb
G6PD Deficiency
Hb M Disease
Diet:
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Nitrites from nitrate-rich food or from
well water  oxidation of Hb
Etiologies of Methemoglobinemia
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Meds = most common cause
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42% of 138 cases due to Dapsone
Local Anesthetics:
Prilocaine, Lidocaine, Tetracaine, and…
 Topical benzocaine  most severe ↑MetHb
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Ifosfamide
Bactrim
Methylene Blue*
Tobias et al, J of Intensive Care Medicine, 2009.
Presentation: Clinical
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No resp distress if MetHb < 20%;
+dyspnea > 45%
Cyanosis if MetHb > 15-20%
Hypoxemia manifested as low oxygen
saturation on pulse oximetry
SpO2 not responsive to ↑FiO2
Presentation: Serology
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Chocolate-colored blood
ABG with normal or high pO2
AND low O2sat on pulse oximetry
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“The Saturation Gap”
VBG with elevated MetHb level
Chung et al.J of Emergency Medicine, 2009.
Tobias, J of Intensive Care Medicine, 2009.
Pulse Oximetry
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Measures O2 saturation of Hb
Sensor contains infrared (940nm) and
red (660nm) wavelengths of light
Oxygenated Hb absorbs light at 660nm
vs. deoxygenated Hb at 940nm
Ratio of wavelengths absorbed gives
O2sat
Pulse Oximetry and Methemoglobin
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SpO2 = BLUE
SaO2 = RED
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MetHb absorbs at both
wavelengths
MetHb < 20% detected
by deoxyHb sensor
MetHb > 20% detected
by oxyHb sensor
Newer pulse ox coming
with more wavelengths
So and Farrington, J Ped Healthcare 2008
Detecting Methemoglobinemia
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Co-oximetry:
Measurement of oxygen concentration
in blood using numerous UV light
wavelengths
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Detects oxyHb, deoxyHb, MetHb,
carboxyHb
Reducing MetHb
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Physiologic response to MetHb:
NADH-dependent MetHb reductase
shifts MetHb back to Hb
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Rate of conversion = 15%/hour
Infants have small supply of enzyme
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Infants and elderly = 50% of
Methemoglobinemia cases
So and Farrington, J Ped Healthcare 2008
Treating Methemoglobinemia
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Poison Control!
Methylene Blue:
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Give if symptomatic and MetHb < 20%
or if level > 30%
Facilitates reduction via NADPH MetHb
Reductase
NADPH enzyme produced by G6PD*
N-acetylcysteine in G6PD Deficiency
Back to the Case
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Patient treated with Methylene Blue
Developed chills, rigors, SpO2 70%
No intervention, improved – 96%
Added Benzocaine and Lidocaine to
allergy list (along with Ifosfamide)
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Risk of recurrence with re-exposure
Guay, Anesthesia and Analgesia, 2009.
Take Home Points
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Low SpO2 and lack of response to
↑FiO2 implies abnormal Hb
Don’t forget ABG or VBG, and the
saturation gap with normal pO2
Beware local/topical anesthetics
Review history and meds if unsure
Watch for new pulse oximeters!!
Works Cited
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Tobias and Ramachandra, “Intraoperative Diagnosis of
Unsuspected Methemoglobinemia Due to Low Pulse Oximetry
Values,” Journal of Intensive Care Medicine, July 2009.
So and Farrington, “Topical Benzocaine-induced
Methemoglobinemia in the Pediatric Population,” Journal of
Pediatric Health Care, 2008.
Chung et al., “Severe Methemoglobinemia Linked to Gel-Type
Topical Benzocaine Use: A Case Report,” Journal of
Emergency Medicine, 2009.
Guay, “Methemoglobinemia Related to Local Anesthetics: A
summary of 242 Episodes,” Anesthesia and Analgesia, 2009.
Bong, Hilliard and Seefelder, “Severe Methemoglobinemia
from Topical Benzocaine 7.5% (baby orajel) Use for Teething
Pain in a Toddler,” Clinical Pediatrics, 2009.
Mack, “Focus on Diagnosis: Co-oximetry,” Pediatrics in
Review, 2007.