What is the most hydrophilic opioid

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Morphine is the most hydrophilic opioid in common clinical use.

Here is a full breakdown, sourced directly from the textbooks:

Hydrophilicity Among Opioids

Opioids span a wide range of lipid solubility. At one end sit highly lipophilic agents like fentanyl and sufentanil; at the other end sit hydrophilic agents, with morphine being the prototype and most hydrophilic of the clinically used opioids. Hydromorphone is also considered hydrophilic but is less so than morphine.

Key Properties of Morphine (Hydrophilic Prototype)

Property	Morphine (Hydrophilic)	Fentanyl/Sufentanil (Lipophilic)
Onset of analgesia	Delayed (30-60 min)	Rapid (5-10 min)
Duration of analgesia	Long (6-24 hr)	Shorter (2-4 hr)
CSF spread (neuraxial)	Extensive	Minimal
Primary site of action	Spinal > supraspinal	Spinal < systemic
Delayed respiratory depression	Yes (>6 hr possible)	Minimal
Metabolism	Hepatic conjugation (not CYP450)	CYP3A4

Why Hydrophilicity Matters Clinically

Slow BBB penetration - Morphine's poor lipid solubility slows its crossing of the blood-brain barrier, leading to a slower onset compared to fentanyl.
Neuraxial use - When given intrathecally or epidurally, morphine stays in the CSF much longer and spreads rostrally (upward toward the brain). This explains its risk of delayed respiratory depression hours after administration - a rare but serious complication.
Longer duration - Because it is not rapidly taken up by neural tissue or fatty tissue, morphine persists in the CSF and provides analgesia for up to 24 hours with a single intrathecal dose.
High intrathecal:IV potency ratio - The relative intrathecal-to-intravenous potency of morphine is 200-300:1, whereas for fentanyl it is only 10-20:1, reflecting how effectively morphine works at the spinal level.
Drug interactions - Because morphine is metabolized by hepatic glucuronidation rather than the CYP450 system, it has fewer pharmacokinetic drug-drug interactions than many other opioids.

Summary

Morphine = most hydrophilic opioid. Hydromorphone is the other clinically important hydrophilic opioid, though less so than morphine. All other commonly used opioids (fentanyl, sufentanil, alfentanil, remifentanil, oxycodone) are significantly more lipophilic.

Miller's Anesthesia, 10e - Table 77.3 (Neuraxial Opioids) and Hydrophilic Opioids section
Lippincott Illustrated Reviews: Pharmacology - Morphine hydrophilicity and metabolism

At what stage of labour intrathecal fentanyl can be given

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Based on authoritative textbook sources and established clinical knowledge, here is a complete answer:

Intrathecal Fentanyl in Labour - When Can It Be Given?

Short Answer

Intrathecal fentanyl can be given at any stage of labour, but it is most commonly and appropriately used in the first stage of labour, particularly as part of a Combined Spinal-Epidural (CSE) technique. It can also be given just before delivery (end of first stage / approaching second stage) as a saddle block adjunct.

Stage-by-Stage Breakdown

First Stage of Labour (Most Common Indication)

This is the primary indication for intrathecal fentanyl in obstetrics. It is especially useful:

Early first stage - when the patient has severe pain early in labour and desires fast-onset relief
Active first stage - as part of a CSE technique, providing almost immediate analgesia while the epidural catheter is placed for subsequent top-ups

The CSE technique injects intrathecal opioid + small dose local anaesthetic, then threads an epidural catheter for continued labour analgesia. Typical dose: fentanyl 10-12.5 mcg intrathecally (or sufentanil 5 mcg), often combined with bupivacaine 2.5 mg or ropivacaine 3-4 mg.

Key advantage: the intrathecal component provides essentially immediate pain control with minimal effect on early labour progress, while the epidural catheter handles the rest of labour and potential caesarean section.

Approaching / Just Prior to Delivery (Second Stage / Operative Vaginal Delivery)

A saddle block given just before delivery can include fentanyl 12.5-25 mcg or sufentanil 5-7.5 mcg as an adjunct to intrathecal local anaesthetic (hyperbaric bupivacaine, tetracaine, or lidocaine). This significantly potentiates the block quality for operative vaginal delivery (forceps/vacuum).

Key Clinical Points

Feature	Detail
Typical IT fentanyl dose (CSE)	10-12.5 mcg
Typical IT fentanyl dose (saddle block)	12.5-25 mcg
Onset	Near-immediate (~5 min)
Duration	~1-2 hours (short, hence need for epidural catheter)
Effect on labour progress	Does NOT prolong labour or increase operative delivery when used in dilute combinations
Motor block	Minimal - patient can often ambulate ("walking epidural")
Side effects	Pruritus (most common), nausea, rare respiratory depression, fetal bradycardia (rare)

Important Caveat: Fetal Bradycardia

There is a well-recognised association between intrathecal opioids (especially fentanyl/sufentanil) and fetal bradycardia, thought to be due to rapid pain relief causing a sudden drop in maternal catecholamines, leading to unopposed uterine activity (hyperstimulation) and reduced uteroplacental blood flow. This is a consideration at any stage but is monitored for with continuous CTG after administration.

Bottom line: Intrathecal fentanyl in labour is primarily a first-stage technique used via CSE for rapid pain relief, and also used as an adjunct in saddle blocks near delivery. It is not typically used as a standalone agent for the entire labour due to its short duration.

Morgan and Mikhail's Clinical Anesthesiology, 7e - Combined Spinal & Epidural Analgesia section
Miller's Anesthesia, 10e - Spinal Additives and Neuraxial Opioids

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