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Tintinalli chaper 207
記得用 藍色藥水治療藍血人 Chocolate - brown color
高鐵血紅蛋白血症 (methemoglobinemia) : 藍血人
亞甲基藍 (methylene blue) : 藍色藥水
Saturation gap :
- ABG : 測量血中 PaO2, PaCO2, pH, 用 PaO2 推估血氧飽和度
- Pulse oximeter : 測量末梢微血管 660nm 與 940nm 光波推估血氧飽和度
- Co-oximeter : 直接測量血中 oxyhemoglobin, deoxyhemoglobin, methemoglobinemia, carboxyhemoglobin
是 methemoglobinemia 病患血氧飽和度的標準
用這 3 個工具測量 methemoglobinemia 病患的血氧飽和度, 結果如何 ?
- ABG : 如果血中PaO2正常, 就會得到正常的血氧飽和度
- Pulse oximeter 和 co-oximeter 結果如圖
pulse oximeter 血氧飽和度讀數為圓點(SpO2), 會高估病患實際血氧飽和度
當MetHb超過30~35%, SpO2會維持在85%左右
Co-oximeter 血氧飽和度讀數為方點 (SaO2) 是methemoglobinemia 病患血氧飽和度的標準
Pulse oximetry will read 80-85% regardless of the oxygen level,
potentially overestimating the true oxygen saturation.
回到一開始的問題, saturation gap 到底是哪 2 個讀數間的差距呢 ？
Pulse oximeter, ABG, co-oximeter 各自saturation之間都有 gap
** 臨床實務上真正有幫助的是看到 Cyanosis 病患
Pulse oximeter 的 SpO2 竟然「高達」85 - 90%
抽 ABG一看, 血氧飽和度竟然「高達」近 100%
心中出現「今天見鬼了 !」意念時, 就要想到 methemoglobinemia
而與 co-oximeter 間的 gap, 因為看到 co-oximeter 報告時已確定是 methemoglobinemia
有沒有 gap 就沒那麼重要了
In patients with methemoglobinemia, the pulse oximeter will report a falsely elevated value
for arterial oxygen saturation percentage.
The specific values vary by oximeter, but typically report approximately 85%.
Arterial blood gas results may be initially deceptive 騙人的 because the partial pressure of oxygen,
a measure of dissolved, not bound, oxygen, is normal.
氧分壓是正常的, 問題是血紅素無法攜帶氧氣
所以 ABG 用氧分壓推算 SpO2 不準確
Thus, calculation of oxygen saturation from measured partial pressure
by the blood gas analyzer will produce a falsely elevated result.
一般大概 < 80%
SpO2 會開始 Cyanosis
blue gray discoloration
ferric form is unable to bind oxygen for transport and is termed methemoglobin.
Normally, <1% - 2% of circulating hemoglobin exists as methemoglobin;
Higher concentrations define the condition of methemoglobinemia.
是由於氧化過度或還原酶HbM reductase缺乏
使 Fe2+被氧化成 Fe3+ 後一直保持氧化狀態而不可逆 (ferrous to ferric)
HbM 累積過多而產生此病
HbM 會影響攜氧能力, 臨床表現以紫紺及缺氧症狀為主
高鐵血紅素(methemoglobin)不具有可逆氧合作用的能力
是正常血紅素結構的二價鐵 (Fe2+)被氧化成三價鐵 (Fe3+)
這種被變性後的血紅素無法成功攜帶氧氣供身體利用
- Cytochrome b5 reductase : reducing nearly 95% of methemoglobin produced under typical circumstances.
- NADPH-methemoglobin reductase : minimal importance and is responsible for <5%,
crucial for the antidotal effect of methylene blue
G6PD deficiency with a resultant deficit of NADPH
are not at increased risk of developing methemoglobinem
如果顯著高鐵血紅素(methemoglobin)在血液中積聚, 會導致 cyanosis
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基因 : 可以通過一個顯性遺傳血紅素蛋白異常引起或由隱性遺傳的高鐵血紅蛋白還原酶引起
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May be acquired 接觸氧化劑使二價鐵被氧化成三價鐵Classical drug causes include :Antibiotics (trimethoprim, sulfonamides, and dapsoneLocal anesthetics (articaine, benzocaine, and prilocaine, lidocaine)Aniline dyes 苯胺染劑, metoclopramide, rasburicase, chlorates, and bromates.Ingestion of compounds containing nitrates 硝酸鹽 (such as the patina chemical bismuth nitrate)or 亞硝酸鹽 (Nitrite)NEJM : Methemoglobinemia can occur after exposure to a number of medications,including topical anesthetic agents such as benzocain
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Drugs in conventional doses rarely produce clinically significant methemoglobinemiaBenzocaine is the local anesthetic most commonly associated with methemoglobinemia.
Cyanosis : methemoglobin levels between 10-15% in a nonanemic individual.
Anemic patients may not exhibit cyanosis until the methemoglobin level rises well above 10%
because cyanosis detection is dependent on the concentration of methemoglobin, not the percentage.
At methemoglobin levels between 20% - 30%, anxiety, headache, weakness, and lightheadedness develop,
and patients may exhibit tachypnea and sinus tachycardia.
50% - 60% impair oxygen delivery to vital tissues, resulting in myocardial ischemia, dysrhythmias,
depressed mental status (including coma), seizures, and lactate-associated metabolic acidosis.
Levels above 70% are largely incompatible with life.
Consider methemoglobinemia in patients with cyanosis,
particularly if cyanosis does not improve with supplemental oxygen
Patients with methemoglobin-associated cyanosis generally
are less symptomatic than equivalently appearing patients with hypoxemia-induced cyanosis.
一般 20-30% 若無症狀, 支持治療即可
會自己代謝掉
Gastric decontamination is of limited value
A single dose of activated charcoal is likely sufficient
Antidotal therapy with methylene blue is reserved for symptomatic patients or for those asymptomatic patients
with methemoglobin levels > 25%.
甲基藍作為電子接受者
G6PD 病人使用甲基藍可能會產生溶血貧血
Dapsone-exposed patients may require repetitive dosing of methylene blue
The initial dose of methylene blue is 1 mg/kg
(0.1 mL/kg of the 1% solution or approximately 7 mL in an adult) IV slowly over 5 minutes
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