Visual Diagnosis

A Hypoxic Teen Inpatient

Eva Delgado, MDMorning Report

Overview

Case Presentation Work-up based on Differential Significance of the Diagnosis Understanding the Presentation Treating the Pathology Take Home Points

Case Presentation

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

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

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,

New pulse ox trialed, well-placed, with same reading on different digits

Work-up to Diagnosis

Chart Review: History- tumor burden/growth,

medications, hypercoagulability, similar past episodes

Labs/Studies: CXR ABG/VBG CBC

CBC: Hbg 8.6, HCT 25 VBG: 7.36/44/53/25 -0.1 MetHb: 40% CO-Hb: 0.9% Diagnosis: Methemoglobinemia

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A Review of Hemoglobin

Hemoglobin = 4 globin chains, 4 heme molecules, each bound to Fe++

Oxygen bindsIron (Fe++)

Hemoglobin Methemoglobin

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

MetHb is 2% of normal blood Endogenous enzymes reduce to Hb Methemoglobinemia:

Oxidation >2% MetHb Oxyhemoglobin curve Danger: tissue hypoxia

*MetHb level 70% fatal

Etiologies of Methemoglobinemia

Congenital: Deficiency of enzymes that reduce

MetHb back to Hb G6PD Deficiency Hb M Disease

Diet: Nitrites from nitrate-rich food or from

well water oxidation of Hb

Etiologies of Methemoglobinemia

Meds = most common cause 42% of 138 cases due to Dapsone Local Anesthetics:

Prilocaine, Lidocaine, Tetracaine, and… Topical benzocaine most severe ↑MetHb

Ifosfamide Bactrim Methylene Blue*

Tobias et al, J of Intensive Care Medicine, 2009.

Presentation: Clinical

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

Chocolate-colored blood

ABG with normal or high pO2 AND low O2sat on pulse oximetry

“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

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

MetHb absorbs at both wavelengths

MetHb < 20% detected by deoxyHb sensor

MetHb > 20% detected by oxyHb sensor

Newer pulse ox coming with more wavelengths

SpO2 = BLUESaO2 = RED

So and Farrington, J Ped Healthcare 2008

Detecting Methemoglobinemia

Co-oximetry: Measurement of oxygen concentration

in blood using numerous UV light wavelengths Detects oxyHb, deoxyHb, MetHb,

carboxyHb

Reducing MetHb

Physiologic response to MetHb: NADH-dependent MetHb reductase

shifts MetHb back to Hb Rate of conversion = 15%/hour Infants have small supply of enzyme

Infants and elderly = 50% of Methemoglobinemia cases

So and Farrington, J Ped Healthcare 2008

Treating Methemoglobinemia

Poison Control! Methylene Blue:

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

Patient treated with Methylene Blue Developed chills, rigors, SpO2 70% No intervention, improved – 96% Added Benzocaine and Lidocaine to

allergy list (along with Ifosfamide) Risk of recurrence with re-exposure

Guay, Anesthesia and Analgesia, 2009.

Take Home Points

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

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.