在呼吸生理學中，「loop gain 環路增益」是用於衡量換氣的回饋調控機制的穩定性，它反映了身體對血中氣體濃度（主要是二氧化碳）的擾動產生反應的震盪程度。
它的定義是「controller gain（化學感受器敏感性—主要是腦幹的後斜方核）」與「plant gain（肺部呼吸換氣的效率）」的乘積。
Loop gain in the respiratory system is a measure of the stability of the feedback loop that controls ventilation, reflecting how heavily the ventilatory response to disturbances in blood gas levels (mainly PaCO2).
It is defined as the product of controller gain (sensitivity to blood gases, primarily driven by central chemoreceptors in the brainstem–retrotrapezoid nucleus, RTN) and plant gain (how effectively the lungs change blood gases).
Loop gain > 1 表示容易過度換氣（過度補償），導致不穩定、振盪的呼吸。
A loop gain >1 indicates that the respiratory system tends to overcompensate, leading to unstable, oscillating breathing. (Breathing tends to go off-balance and takes a long time for the body to regain balance.)
Loop gain < 1 表示呼吸系統能維持穩定幅度的自我修正。
A loop gain <1 indicates a stable, self-correcting system.
High loop gain是導致潮式呼吸及中樞型睡眠呼吸中止症的主要致病機轉，也是睡眠呼吸中止症的亞型之一。
High loop gain is a major pathogenic mechanism leading to Cheyne-Stokes respiration and central sleep apnea, and is also one of the endotypes of obstructive sleep apnea.
導致高環路增益的原因，首先從其定義去想，就是controller gain升高或是plant gain升高。一般來說，睡著時的controller gain比清醒時低，尤其是快速動眼期(REM)，但在阻塞性睡眠呼吸中止症(OSA)患者中，非快速動眼期(NREM)期間的controller gain常常升高，這可能是因為反覆呼吸暫停造成「間歇性缺氧」會使化學感受器敏感度升高的關係。
We can trace back from its definition to understand factors that lead to high loop gain—increased controller gain or increased plant gain. Generally, controller gain is lower in REM sleep compared to wakefulness, but it can be high during NREM sleep in patients with OSA, because repeated apnea causes intermittent hypoxia, which increases chemoreceptor sensitivity.
任何原因造成全身血液循環時間延長，都會延遲大腦收到訊號變化而提高loop gain，常見原因是心輸量減少。通常可以對應中醫的心氣虛、心陽虛或心腎陽虛伴隨水飲氾濫，若加上交感神經過度亢進，則涉及陽亢，短期大多以氣滯或氣鬱表現，病程拖長都是走向陰陽兩虛，夾一堆病理產物痰濕血瘀。
Lengthened circulation time, often found in decreased cardiac output, also causes a delay in signalling blood-gas changes to the brain, which boosts overall loop gain. This pathological condition may be classified as heart qi deficiency, heart yang deficiency or heart-kidney yang deficiency pattern with water retention in traditional Chinese medicine. Sympathetic hyperactivation is common in such conditions, similar to yang hyperactivity. In the short term, it is mostly manifested as qi stagnation. But as sickness progresses, it always leads to deficiency of both yin and yang, with loads of pathological products such as phlegm, dampness and blood stasis.
酒精、鎮靜劑和鴉片類藥物因為會抑制呼吸中樞，也會增加loop gain。
Alcohol, sedatives, and opioids can inhibit respiratory drive and increase loop gain.
另一方面，造成plant gain升高的因素，則有肥胖和仰臥姿勢（造成肺容積下降，功能殘氣容積降低），以及身體對血氣變化的緩衝作用減弱。
有幾種狀況會使身體血氣變化的緩衝作用減弱：慢性阻塞性肺病、氣喘、肺水腫和一些影響呼吸肌肉的疾病（如漸凍症、格林─巴利症候群）。
On the other hand, factors that contribute to increased plant gain include: Obesity and supine body position (which leads to a decrease in lung volume and functional residual capacity (FRC)), and reduced blood-gas damping (weakened buffering effect on blood-gas changes).
Reduced blood-gas damping can be driven by several physiological and pathological conditions: Chronic obstructive pulmonary disease (COPD), asthma, pulmonary edema, and diseases affecting respiratory muscles (e.g. amyotrophic lateral sclerosis (ALS), Guillain-Barre syndrome).
高代謝需求（甲狀腺功能亢進、發燒、感染、重大創傷或手術後、高強度運動後，血中二氧化碳大量增加）以及代謝性酸中毒（糖尿病酮酸中毒、乳酸中毒、嚴重腹瀉或腎衰竭），則是因為超出了緩衝範圍，使plant gain升高。
High metabolic demands (hyperthyroidism, fever, infection, major trauma or surgery, high-intensity exercise, which increases CO2 production) and metabolic acidosis (diabetic ketoacidosis, lactic acidosis, severe diarrhea or kidney failure) can overwhelm the body's damping mechanisms and increase plant gain.
上呼吸道阻力和塌陷性（高 Pcrit，睡眠呼吸中止症的另一種亞型）會降低換氣效率，而血量顯著下降會導致組織灌流的不穩定，這些都是導致plant gain增加的原因。
Upper airway resistance and collapsibility (high Pcrit, another endotype of OSA) influence the effectiveness of ventilation, while a significant drop in blood volume (hypovolemia) causes fluctuations in tissue perfusion; both can lead to increased plant gain.
自律神經系統可透過兩種方式導致high loop gain，一種是交感神經過度活化（增加controller gain），以及誘發血管收縮影響肺血流量（增加plant gain），常發生在緊張、寒冷或急性壓力下，屬於身體「戰或逃」的反應；另一種是迷走神經（屬於副交感神經系統）調節氣道張力，使支氣管收縮。不過，副交感神經系統對於loop gain的影響較微弱。
The ANS contributes to a higher loop gain through two primary branches, one involves sympathetic hyperactivation (increases controller gain) and induces vasoconstriction, affecting pulmonary blood flow (increases plant gain). This often occurs under stress, cold, or acute pressure and is a "fight or flight" response of the body.
The other involves vagus nerve (from the parasympathetic nervous system) that modulates airway tone and narrows the bronchi. Nevertheless, the role of the parasympathetic system in regulating respiratory loop gain is considered very weak.
𓂃◌𓈒𓐍𓂃𓈒𓂂
我都有刻意避免用晶晶體，因為感覺上、好像有些人對中英夾雜很反感，另一方面是希望減少隔閡，醫學用詞我都盡量把中文找出來。不過有些詞，翻成中文反而會不知道那是啥，loop gain就是其中一個。
這篇是我一直想整理的筆記，可能蠻枯燥的。曾經以為我未來的工作範圍可能是會更常去RCC(呼吸照護中心)看病人的，當時為了對抗中醫小R踏進西醫重症中心，到底能做什麼的劣等感，只要聽到旁邊的VS和護理師說了什麼詞我不懂的，回去都會努力查清楚。我想要會診的時候，可以從旁邊的monitor上跳動的數字得到更多資訊，知道呼吸治療師的處置計畫和如何設定呼吸器的模式和壓力。
我覺得不同胸腔科醫師的titration手法也帶著個性（而且跟平常外表給人的感覺不一樣），有的會大刀闊斧、進逼能力的極限，有的是循循善誘、溫柔呵護。只要醫師和病人相性合就很好。
Loop gain主要還是看中樞神經的化學敏感性(controller gain)和肺部的氣體交換效率(plant gain)，不過我想說呼吸也是自律神經調節的一部分，所以還蠻好奇HRV test是不是有什麼指標能看出這個人有high loop gain傾向的？睡眠呼吸中止症不同分型治療方法會不一樣，有些分型中醫可以做，有些分型中醫能幫忙的範圍就很有限，但至少，我想知道我實際能改變的有哪些，大概是這樣的心情。
我覺得HRV test水很深，不能只看LF/HF就判斷是交感還是副交感過盛，因為你看到的表現，可能是踩油門的結果，也可能是煞車壞掉的結果，或是兩者都有；你在事件剛發生看到這個表現，或是它已經這樣很久了，你看到的是代償已經發生的樣子。而且它還持續在變動中。
我發現很多對自己的健康過度焦慮的人，都好喜歡看醫學知識節目，然後越看越焦慮。我跟他說，可能是他們都列一堆症狀，你總會中個兩三個，你就會以為自己有那種病，實際上診斷不是只在對症狀的。後來我發現，可能這些人會焦慮，是因為缺乏找答案的能力，才會一直擔憂卻不去弄清楚他擔心的事情。
又有些時候，他們一直跟你說他的擔憂，只是希望從你這裡得到一個保證和安心，這種向外求的安心會變成一個無底洞，永遠無法填滿，還會把任何靠近洞的人都拉下去。
𓈒𓂂 𓈒自律神経のバランス...𓈒𓍼
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