Makindo Medical Notes

💊 Phytomenadione (Vitamin K₁) is a fat-soluble vitamin essential for hepatic synthesis of several clotting factors. It is used to treat or prevent bleeding due to vitamin K deficiency or reverse excessive anticoagulation (e.g. high INR on warfarin). Because it acts by enabling the γ-carboxylation of coagulation proteins, its effect develops over several hours rather than immediately.

📘 About

• Always check the BNF entry here for current dosing and administration guidance.
• Vitamin K₁ is the naturally occurring form of vitamin K, primarily derived from green leafy vegetables such as spinach, broccoli, Brussels sprouts, and kale.
• 90% of dietary vitamin K in Western diets is from vitamin K₁; bacterial synthesis of vitamin K₂ (menaquinones) in the gut contributes a smaller fraction.

⚙️ Mode of Action

• Acts as a cofactor for the γ-carboxylation of glutamic acid residues on prothrombin (factor II) and factors VII, IX, X, as well as proteins C and S.
• This modification allows calcium binding and proper activation of these coagulation factors in the liver.
• Without vitamin K, nonfunctional clotting factors are produced → prolonged PT/INR and bleeding tendency.
• Restores hepatic synthesis of functional clotting factors within 6–12 h (oral) or 2–6 h (IV).

🤰 Pregnancy & Neonatal Context

• Vitamin K is fat-soluble and crosses the placenta poorly.
• Newborns have low vitamin K stores because of poor placental transfer, sterile gut flora, and low levels in breast milk.
• Therefore, prophylactic vitamin K (usually 1 mg IM at birth) is given to prevent haemorrhagic disease of the newborn (HDN).
• Humans are unique among mammals in that breast milk is insufficient to protect against HDN, necessitating supplementation.

💊 Indications & Dose

• Reversal of warfarin over-anticoagulation / raised INR: Phytomenadione 1–10 mg PO or IV (dose depends on INR and bleeding severity).
• Treatment of vitamin K deficiency or malabsorption: 1–10 mg PO/IV daily as directed.
• Prophylaxis of HDN: 1 mg IM (preferred) or oral immediately after birth.

⚠️ Cautions

• High doses can antagonise warfarin for several days, delaying safe re-anticoagulation.
• Use IV route slowly and with monitoring — rare risk of anaphylactoid reaction.
• In liver failure, synthesis of clotting factors may remain impaired despite supplementation.

🚫 Contraindications

• Hypersensitivity to phytomenadione or excipients.
• Relative: when rapid re-anticoagulation with warfarin will be required — consider smaller doses (e.g. 0.5–1 mg).

💥 Adverse Effects

• Flushing, injection-site pain, and rare anaphylactoid reactions with IV use.
• Transient taste disturbance, dizziness, or nausea (uncommon).
• No chronic toxicity from oral doses within normal range.

🧠 Teaching Note

Vitamin K deficiency causes prolonged prothrombin time (PT) with normal fibrinogen and platelets — a classic teaching point in coagulation physiology. When reversing warfarin, combine phytomenadione with prothrombin complex concentrate (PCC) if there is life-threatening bleeding for rapid effect, as vitamin K alone takes several hours to act.

📚 References

• Vitamin K: Effects on health beyond coagulation — PMC 3321262
• BNF: Phytomenadione (Vitamin K₁)
• NICE NG24: Blood transfusion (2024 update)