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[Cheng Wuji's Commentary]
"Alternating chills and fever" refers to the phenomenon where fever arises after chills subside, and chills return after fever subsides. This is because chills belong to Yin, while fever belongs to Yang; the interior belongs to Yin, and the exterior belongs to Yang. When pathogenic factors lodge in the exterior and contend with Yang Qi, chills manifest; when pathogenic factors enter the interior and contend with Yin, fever manifests. If exterior pathogenic factors are predominant, chills are severe and fever is mild; if interior pathogenic factors are predominant, fever is severe and chills are mild. When pathogenic factors reside in the half-exterior half-interior region, contending outwardly with Yang Qi causes coldness, and contending inwardly with Yin Qi causes heat. Since the disease location is not fixed strictly in the exterior or interior, and the manifestation of chills and fever has no fixed pattern, alternating chills and fever occur. Therefore, Xiao Chai Hu Tang (Minor Bupleurum Decoction) is used to establish various modification methods for harmonizing the Shaoyang stage.

Furthermore, "chills and fever resembling malaria" and "alternating chills and fever" appear similar but are distinct. "Chills and fever resembling malaria" have a regular onset and cessation time; they occur when healthy Qi contends with pathogenic factors and cease when healthy Qi retreats. In contrast, "alternating chills and fever" have irregular onset and cessation times, occurring intermittently, possibly three to five times or even more than ten times a day. This differs from malaria. Although both involve the half-exterior half-interior stage and require harmonization therapy, there is another scenario: if the disease persists for over ten days, with heat binding in the interior yet presenting with alternating chills and fever, Da Chai Hu Tang (Major Bupleurum Decoction) should be used to purge the interior heat.

[Zhao Sizhen's Commentary]
The 59th question of *Shanghan Baiwen Ge* states that when Yin and Yang mutually overcome each other, insufficient Yang leads to chills first followed by fever, while insufficient Yin leads to fever first followed by chills. This specifically discusses the mutual overcoming of Yin and Yang in mixed diseases and cannot be applied to explain Shanghan (Cold Damage).

In Shanghan, on the fifth or sixth day, whether due to Wind Strike or Cold Damage, symptoms such as alternating chills and fever, fullness and discomfort in the chest and hypochondrium, silence with lack of desire for food, irritability and frequent vomiting, or alternatively, chest irritability without vomiting, thirst, abdominal pain, hardness and stuffiness under the hypochondrium, palpitations with urination difficulty, absence of thirst with slight body heat, or coughing, are primarily treated with Xiao Chai Hu Tang. (Cheng Wuji notes: Diseases may be located in the exterior, interior, or between the two. Pathogenic factors between the exterior and interior constitute the half-exterior half-interior syndrome. On the fifth or sixth day, pathogenic factors transmit from the exterior to the interior. Whether it is Wind Strike or Cold Damage, these symptoms may appear. The *Yuyuan Jing* states: "Shanghan with alternating chills and fever," indicating either Wind Strike or Cold Damage, not necessarily Cold Damage re-infected by Wind or vice versa. The classic text says: "In Shanghan or Wind Strike, if Chai Hu patterns are present, observing one symptom is sufficient; all symptoms need not be present." This applies to either condition. Exterior pathogens cause chills; interior pathogens cause abdominal heat. Currently, with pathogens in the half-exterior half-interior, their position is undetermined, hence alternating chills and fever. Exterior pathogens do not cause heart/abdominal fullness; interior pathogens cause distension. Current chest diaphragm fullness indicates pathogens between exterior and interior. The Shaoyang meridian runs along the side of the body, and the chest/hypochondrium are Shaoyang regions. Hence, the text mentions pain in the chest/hypochondrium and deafness due to the meridian connecting the ear. Silence indicates pathogens moving from exterior to interior, residing between the two. Exterior pathogens allow eating; interior pathogens prevent it. Lack of desire for food indicates pathogens are between exterior and interior, not yet severe enough to completely prevent eating. Exterior pathogens cause no irritability/vomiting; interior pathogens cause fullness and vomiting. Irritability and frequent vomiting indicate recent transmission from exterior to interior. As pathogens initially enter the interior, their location is unstable, leading to various potential symptoms. "Observing one Chai Hu symptom" refers to these potential symptoms.)

If the original Taiyang disease does not resolve and transmits to Shaoyang, presenting with hardness/stuffiness under the hypochondrium, dry retching, inability to eat, and alternating chills and fever, before any purgative or emetic treatment, and with a deep-tight pulse, Xiao Chai Hu Tang is indicated. (Cheng Wuji notes: Transmission from Taiyang to Shaoyang means exterior pathogens entering the interior. Symptoms indicate pathogens in the half-exterior half-interior. If emetics/purgatives were already used and the pulse is deep-tight, pathogens have entered the Fu organs forming interior excess. If not yet used, a deep-tight pulse indicates deep transmission but incomplete entry into Fu organs, with exterior unresolved. Thus, Xiao Chai Hu Tang harmonizes.)

If emesis, sweating, or warm needle therapy was incorrectly applied, resulting in delirium and the disappearance of Chai Hu patterns, this constitutes a damaged disease (Huai Bing). Treatment must address the specific iatrogenic error. (Cheng Wuji notes: Incorrect use of emesis, purgation, sweating, or warm needles damages fluids, drying the stomach. Wood pathogen attacks the stomach, causing delirium. If Chai Hu patterns persist, it is not an error. If they disappear, it is a damaged disease. Investigate the cause of the error and treat accordingly. To rescue damaged diseases by nourishing Ying-Wei and generating fluids, Guizhi Tang may be referenced.)

In Shanghan, on the fifth or sixth day, after sweating and subsequent purgation, if symptoms include slight constriction of chest/hypochondriac fullness, urination difficulty, thirst without vomiting, only head sweating, alternating chills and fever, and irritability, the disease is unresolved. Chai Hu Gui Zhi Gan Jiang Tang (Bupleurum, Cinnamon Twig and Dried Ginger Decoction) is the primary formula. (Nan Zhi notes: Using Chai Hu allows time for Qi and Blood to shift, leaving no place for pathogens, indicating resolution. After sweating and purgation on day 5-6, pathogens should resolve. Persistent symptoms indicate pathogens remain in the half-exterior half-interior. Chest/hypochondriac fullness and irritability confirm this location. Urination difficulty and thirst result from fluid loss due to sweating/purgation. If heat consumed fluids causing thirst, vomiting would likely occur. Thirst without vomiting rules out interior heat. Head-only sweating indicates insufficient fluids and deficient Yang above. Chai Hu Gui Zhi Gan Jiang Tang resolves exterior/interior pathogens and restores fluids/Yang.)

When blood is weak and Qi exhausted, pores open, and pathogens enter, contending with healthy Qi and knotting under the hypochondrium, Zheng-Xie struggle causes alternating chills and fever with periodic rest. Silence and lack of desire for food follow. Connected organs imply pain radiating downward; high pathology causing low pain induces vomiting. Xiao Chai Hu Tang is indicated. (Note: "Blood weak...knotting under hypochondrium" explains "chest/hypochondriac fullness"; "Zheng-Xie struggle..." explains "alternating chills and fever" using inverted syntax. "Silence..." relates to previous fullness/pain. "Connected organs..." explains "irritability and frequent vomiting".)

**Xiao Chai Hu Tang Formula:**
Chai Hu (8 liang), Ban Xia (half sheng, washed in boiling water), Ren Shen, Gan Cao, Huang Qin, Sheng Jiang (each 3 liang), Da Zao (12 pieces, broken).
Boil seven ingredients in 12 sheng of water to obtain 6 sheng, remove dregs, decoct again to obtain 3 sheng. Take 1 sheng warm, three times daily. (Cheng Wuji notes: *Nei Jing* states: "Heat rampant within, disperse with bitter." Chai Hu and Huang Qin's bitterness disperses transmitted heat. Interior deficiency requires sweet relaxation. Ren Shen and Gan Cao's sweetness moderates middle Qi. Pathogens halfway to interior cause reverse Qi flow; pungent herbs disperse. Ban Xia stops vomiting. Exterior pathogens cause Ying-Wei conflict; pungent-sweet herbs resolve. Sheng Jiang and Da Zao harmonize Ying-Wei.)

**Modifications (for Shaoyang pathogens in the chest):**
If chest irritability without vomiting: Remove Ban Xia and Ren Shen; add Gua Lou Shi (one piece). (Irritability without vomiting indicates heat accumulation without Qi reversal. Sweet herbs cause fullness; useless here. Pungent herbs disperse reversal; unnecessary without vomiting. Use bitter-cold Gua Lou Shi to drain chest heat.)
If thirst: Remove Ban Xia; increase Ren Shen to total 4.5 liang; add Gua Lou Gen (4 liang). (Ban Xia dries fluids, unsuitable for thirst. Ren Shen is sweet-moistening; Gua Lou Gen is bitter-cool, clearing heat and generating fluids.)
If abdominal pain: Remove Huang Qin; add Shao Yao (3 liang). (Remove Huang Qin to avoid damaging middle Jiao cold; add Shao Yao to unblock stagnation and relieve pain.)
If hypochondriac hardness/stuffiness: Remove Da Zao; add Mu Li (4 liang). (Sweet herbs cause fullness; remove Da Zao. Salty herbs soften hardness; add Mu Li.)
If epigastric palpitations and urination difficulty: Remove Huang Qin; add Fu Ling (4 liang). (Fluid retention causes palpitations. *Nei Jing*: "Kidney desires firmness, feed with bitter to firm it." Bitter hardens Kidney/Water, so remove Huang Qin. Fu Ling's bland-sweet nature drains retained water.)
If no thirst but slight exterior heat: Remove Ren Shen; add Gui Zhi (3 liang); cover warmly for slight sweating. (No thirst indicates interior harmony; remove Ren Shen. Slight exterior heat indicates unresolved exterior; add Gui Zhi to induce sweating.)
If cough: Remove Ren Shen, Da Zao, Sheng Jiang; add Wu Wei Zi (half sheng) and Gan Jiang (2 liang). (Cough indicates reverse Qi flow. Sweet herbs obstruct Qi; remove Ren Shen/Da Zao. *Nei Jing*: "Lung desires gathering, feed with sour to gather it." Wu Wei Zi's sourness gathers reverse Qi. Cough stems from cold; disperse with pungent-hot. Replace warm Sheng Jiang with hot Gan Jiang.)

For pathogens in the exterior, induce sweating via soaking; for pathogens in the interior, purge via bowel movement. For pathogens in the half-exterior half-interior, harmonization is appropriate. Xiao Chai Hu Tang harmonizes exterior and interior. *Nei Jing*: "Heat rampant within, disperse with bitter." Half-exterior half-interior heat requires dispersal. Anticipate internal transmission by clearing heat first, using bitter-cold as main agents. Chai Hu is Sovereign, Huang Qin is Minister. For interior transmission disrupting interior Qi, Ren Shen and Gan Cao support healthy Qi. Initial interior transmission causes Qi reversal; Ban Xia assists with pungency to descend Qi. Stabilized interior Qi prevents deep invasion. These three assist Chai Hu in harmonizing interior. *Nei Jing*: "Pungent-sweet disperses Yang." Unresolved exterior pathogens gradually transmitting internally but not yet solid require dual resolution. Pungent-sweet (Sheng Jiang, Da Zao) assists Chai Hu in harmonizing exterior. The seven ingredients form a proper dual-resolving agent. Modifications address varying transmission: remove Ren Shen/Ban Xia if no Qi reversal; add Gua Lou Shi for heat; adjust for thirst, pain, hardness, palpitations, or cough based on specific mechanisms (fluid loss, stagnation, water retention, lung cold).

[Wu Qian et al.'s Commentary]
Why is Xiao Chai Hu Tang used indiscriminately for fever in modern Shanghan treatment? Chai Hu is bitter-pungent and neutral, treating Shaoyang fever and resolving half-exterior half-interior heat, including alternating chills/fever and slight afternoon tidal fever. Huang Qin clears heat; Ban Xia/Sheng Jiang dispel cold; Ren Shen/Da Zao support Qi, resolve thirst, and generate fluids, harmonizing Yin-Yang and resolving pathogens. However, it does not resolve Taiyang exterior heat or Yangming branch heat. Indiscriminate use can be harmful. Specifically, in cases of Yang deficiency with cold, red face, fever, deep pulse, and cold feet, it can be fatal. It is also contraindicated in internal deficiency-cold with loose stools, weak pulse, and postpartum fever.

If Chai Hu Tang indications are present but purgatives are mistakenly used, and Chai Hu patterns persist, administer Chai Hu Tang again. The patient will experience vigorous shaking (Zhen) followed by fever and sweating, leading to resolution. (Cheng Wuji notes: Pathogens in half-exterior half-interior constitute Chai Hu patterns. Purging is incorrect unless interior excess exists. If patterns persist, purging is not detrimental. Re-administering Chai Hu Tang returns pathogens to the exterior. Pre-existing interior deficiency from purging causes shaking as pathogens exit. Healthy Qi prevails, Yang generates, fever/sweating ensues, and disease resolves.)

In Shanghan or Wind Strike, if Chai Hu patterns exist, observing one symptom is sufficient; all symptoms need not be present. (Chai Hu patterns indicate pathogens between exterior and interior. Potential symptoms include chest irritability without vomiting, thirst, abdominal pain, hypochondriac hardness, epigastric palpitations with urination difficulty, no thirst with slight heat, or cough. Observe one symptom and treat with Chai Hu Tang accordingly, without waiting for all symptoms to appear.)

If Shanghan persists for over ten days with heat binding in the interior and recurring alternating chills and fever, administer Da Chai Hu Tang. If Jie Xiong (Chest Binding) presents with no significant heat, it is water binding in the chest/thorax, with only slight head sweating; Da Xian Xiong Tang (Major Thoracic Trap Decoction) is indicated. (See Jie Xiong section.)

**Da Chai Hu Tang Formula:**
Chai Hu (8 liang), Ban Xia (half sheng, washed), Huang Qin, Shao Yao (each 3 liang), Sheng Jiang (cut, 5 liang), Da Zao (12 pieces, broken), Zhi Shi (roasted, 4 pieces).
Boil seven ingredients in 12 sheng of water to obtain 6 sheng, remove dregs, decoct again to obtain 3 sheng. Take 1 sheng warm, three times daily. Some versions include Da Huang (2 liang). Without Da Huang, it arguably ceases to be Da Chai Hu Tang.

[Cheng Wuji's Commentary on Da Chai Hu]
Formulas vary in potency (harsh vs. mild). Physicians must judge clinically. Severe excess/fullness with dry stool requires harsh purgatives like Da/Xiao Cheng Qi Tang. For severe heat requiring purgation but lacking severe fullness, milder agents like Da Chai Hu Tang are used. Classics state: "Shanghan fever for 7-8 days, even with floating-rapid pulse, may be purged with Da Chai Hu Tang." Also: "Taiyang disease passing its period (>10 days), wrongly purged multiple times, then Chai Hu patterns persist. First try Xiao Chai Hu Tang. If vomiting persists, epigastric urgency, and depression/irritability remain, use Da Chai Hu Tang to purge." Thus, Da Chai Hu Tang is a mild purgative. Shanghan purgation targets excess heat (Fire/Heart). Bitter enters Heart first to reduce heat. Chai Hu (bitter-pungent-mild cold) is Sovereign; Huang Qin (bitter-cold) is Minister. *Nei Jing*: "Sour-bitter涌泄 (upward/downward discharge) is Yin." Sour-bitter (Shao Yao, Zhi Shi) aids discharge. Pungent disperses reverse Qi (Ban Xia, Sheng Jiang); sweet moderates healthy Qi (Da Zao). Da Huang (General) is added in some versions to scour accumulated heat.

[Xu Shimwei's Commentary]
A patient had Shanghan with irritability, frequent vomiting, and alternating chills/fever. Xiao Chai Hu Tang failed. I diagnosed a洪大实 (surging-large-excess) pulse, indicating heat binding in the interior. Xiao Chai Hu Tang cannot clear this. Zhongjing states: "Shanghan >10 days, heat binding in interior with alternating chills/fever, use Da Chai Hu Tang." Three doses cured him. Da Huang scours accumulated heat and is essential. Wang Shuhe noted: "Without Da Huang, it is not truly Da Chai Hu Tang." Use raw, wine-washed Da Huang for efficacy.

[Zhang Yuansu's Commentary]
Why use Da Chai Hu Tang for vomiting/diarrhea? Treatment depends on Deficiency vs. Excess. Interior Deficiency prohibits purging even with constipation. Interior Excess permits purging even with vomiting/diarrhea. *Nei Jing*: "Excessive sweating causes constipation; slow pulse indicates insufficiency/interior not yet solid; do not purge." Here, Da Chai Hu Tang is used. Vomiting/diarrhea with soft abdomen indicates Interior Deficiency. Vomiting/diarrhea with epigastric hardness indicates Interior Excess. Purge accordingly. In the case of prolonged Taiyang disease, wrongly purged, with persistent Chai Hu patterns, epigastric urgency, and irritability, Da Chai Hu Tang purges the interior heat binding in the Stomach. Both cases share Interior Excess, justifying Da Chai Hu Tang.

[Chai Hu Gui Zhi Gan Jiang Tang Section]
After sweating and purgation on day 5-6, if chest/hypochondriac fullness with slight constriction, urination difficulty, thirst without vomiting, head-only sweating, alternating chills/fever, and irritability persist, the disease is unresolved. Chai Hu Gui Zhi Gan Jiang Tang is indicated. (Sweating then purging is improper timing. Early purging causes binding, but since pathogens were weakened by sweating, binding is slight. Cheng Wuji's explanation was unclear; this clarifies.)

**Chai Hu Gui Zhi Gan Jiang Tang Formula:**
Chai Hu (8 liang), Gua Lou Gen (4 liang), Gui Zhi (remove bark, 3 liang), Mu Li, Gan Jiang, Huang Qin (each 2 liang), Gan Cao (roasted, 1 liang).
Boil seven ingredients in 12 sheng of water to obtain 6 sheng, remove dregs, decoct again to obtain 3 sheng. Take 1 sheng warm, three times daily. Initial dose may cause slight irritability; subsequent doses induce sweating and cure. (Cheng Wuji notes: *Nei Jing*: "Heat within, disperse with bitter." Chai Hu/Huang Qin clear transmitted interior heat. "Pungent-sweet disperses Yang." Gui Zhi/Gan Cao disperse exterior pathogens. "Salty softens hardness." Mu Li resolves chest/hypochondriac fullness. "Pungent moistens." Gan Jiang secures Yang-deficient sweating. Thirst from fluid deficiency requires "bitter to firm." Gua Lou Gen generates fluids.)

[Pulse Diagnosis Section]
Patients with faint (Wei) and choppy (Se) pulses suffer from iatrogenic error: excessive sweating and repeated purging led to blood loss. They experience chills followed by unremitting fever. In summer heat, they desire heavy clothing; in winter cold, they desire nudity. Faint Yang causes chills; weak Yin causes fever. Sweating depleted Yang; purging depleted Yin. In May (Yang in exterior), stomach is cold due to deficient Yang, unable to resist cold, hence desiring warmth. In November (Yang in interior), stomach is hot due to deficient Yin, unable to resist heat, hence desiring coolness. Choppy/cdelayed Yin pulses confirm blood loss. (Faint pulse from sweating causes chills even in summer. Choppy pulse from purging causes fever even in winter. This reflects blood loss, Yang deficiency, and Yin deficiency, not seasonal delay.)