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◉ Extrinsic pathway
This is the fastest pathway, triggered when blood is exposed to tissue factor (Factor III)from outside the blood vessel due to injury. Tissue factor and Factor VIIa form a complex that activates Factor X, initiating the common pathway.

➔ PT assessment: The PT test works by adding a reagent called thromboplastin (which contains tissue factor and phospholipids) to a patient's plasma sample. This immediately activates the extrinsic pathway. The time it takes for the plasma to clot primarily reflects the function of Factor VII.

◉ Intrinsic pathway
This pathway is slower and is triggered when blood comes into contact with a negatively charged surface, such as exposed collagen in an injured blood vessel. It involves Factors XII, XI, IX, and VIII.

➔ aPTT assessment: The aPTT test is designed to measure this pathway. An activating agent (like silica or kaolin) and phospholipids are added to the patient's plasma to start the intrinsic pathway. This allows the measurement of the time to clotting for all the intrinsic factors.

◉ Common pathway
This is the final stage where both the extrinsic and intrinsic pathways converge to form a stable clot. It begins with the activation of Factor X (Xa) and involves Factors V, II (prothrombin), and I (fibrinogen).

➔ Both PT and aPTT assessment: Since the common pathway is the final sequence of clotting regardless of the initial trigger, both tests are affected by deficiencies in the common pathway factors. If there is an issue with Factor V, Factor II, or Fibrinogen, both the PT and aPTT will be prolonged. The difference between the two tests lies in whether a problem is found in the intrinsic or extrinsic factors before the common pathway begins.

🧨Wearable Cardioverter-Defibrillator (WCD)

• A WCD is an external device (like a vest) worn by patients for a limited time when they are at risk of life-threatening ventricular arrhythmias, but either are not candidates yet for an implantable defibrillator, or are in a “bridge period” (following a myocardial infarction, or while waiting for recovery or implantation).
• It monitors the heart rhythm and delivers shocks if dangerous arrhythmias (ventricular tachycardia or fibrillation) occur. The patient is alerted and can abort an inappropriate shock if conscious.

◉ Implications for anaesthetic practice
From what is known about similar devices (implantable defibrillators, subcutaneous ICDs, etc.), some probable recommendations and important considerations are:
• Preop planning: Identify that patient has a WCD; check its settings; ensure it’s functioning and charged; understand how it responds to interference, whether the wearer can disable alarms or shocks if needed.
• Electromagnetic interference (EMI): Electrocautery (especially monopolar), diathermy, other surgical instruments can generate EMI, which might trigger inappropriate detection or shocks from the WCD. Might need to minimize use, or use bipolar instruments, or use short bursts, put the return pad far from the device, etc.
• Backup safety: Even if the WCD is in place, one must have external defibrillator/pads ready, because the WCD might be removed, fail, or be temporarily disabled.
• Intraoperative handling: The WCD garment might interfere with surgical field access or sterile draping; it may need to be adjusted. Also, the device’s electrodes might interfere with other monitoring leads.
• Postop care: After surgery, ensure the WCD is resumed if disabled; check for any shocks or arrhythmias during surgery; re-evaluate its function.

MAC in hypothyroidism and hyperthyroidism

● Hypothyroidism

MAC Value: Unchanged. The partial pressure of the anaesthetic gas required at the brain to prevent movement in response to a surgical stimulus is not directly affected by the level of thyroid hormone.

Clinical Reality: Patients with hypothyroidism are clinically very sensitive to the effects of all anaesthetic agents, including volatile anaesthetics. They will appear to require less anaesthetic.

Explanation: The sensitivity is not due to a change in brain requirement (MAC) but rather due to changes in drug delivery and systemic effects (pharmacokinetics).

→ Reduced Cardiac Output: A lower cardiac output means that the anaesthetic gas taken up from the lungs is delivered more slowly to the rest of the body's tissues. This causes the partial pressure in the blood and brain to rise more quickly toward the inspired concentration, leading to a faster induction of anaesthesia.
→ Altered Systemic Effects: Hypothyroid patients often have blunted baroreceptor reflexes and are prone to significant hypotension and bradycardia with anaesthetic agents.
→ Associated Hypothermia: Severe hypothyroidism can be associated with hypothermia. Hypothermia itself is a potent cause of decreased MAC. So, if a hypothyroid patient is cold, they will require less anaesthetic, but the cause is the low temperature, not the thyroid state directly.

🛑 Clinical Approach: You do not change your target MAC on the vaporizer based on thyroid status. However, you must be extremely cautious, titrate the agent very slowly, and be prepared to manage significant haemodynamic instability.

● Hyperthyroidism

MAC Value: Unchanged. Similar to hypothyroidism, the intrinsic requirement of the brain for the anaesthetic is not altered by excess thyroid hormone.

Clinical Reality: Anaesthetising a patient with untreated or poorly controlled hyperthyroidism can be very challenging. They have a hypermetabolic, hyperdynamic state.

Explanation:

→ Increased Cardiac Output: A high cardiac output means anaesthetic gas is rapidly taken up from the lungs and distributed throughout the body. This can slow the rate of rise of the partial pressure in the brain, potentially leading to a slower induction of anaesthesia.
→ Hyperdynamic State: The main challenge is managing the tachycardia, hypertension, and risk of arrhythmias caused by the high sympathetic tone. The focus is on controlling this sympathetic output with beta-blockers and ensuring adequate anaesthetic depth to blunt surgical stimulation, not on simply increasing the MAC value.
→ Associated Hyperthermia: Hyperthyroidism can cause a mild increase in core body temperature. Hyperthermia is a cause of increased MAC. If the patient is hyperthermic, they will require more anaesthetic, but again, this is due to the temperature change itself.

🛑 Clinical Approach: The primary goal is to ensure the patient is euthyroid (in a normal thyroid state) before any elective surgery. If emergency surgery is required, the focus is on aggressively managing the hemodynamic effects with beta-blockers and ensuring deep anesthesia to prevent a catastrophic thyroid storm, rather than focusing on a specific MAC value.