Anesthesiology APPLIED Exam › OSCE: application of ultrasound

Transversus abdominis plane

T1OSCE: application of ultrasound·959 words · ~5 min read·v0.1✓ Sourced · 3DRAFT

Why examiners ask this

The TAP block is a fascial plane block: the target is not a nerve but a layer, and success depends on recognizing three muscles and putting local anesthetic in the right gap between two of them. TAP is one of the outline's nerve-block targets;¹ the examiner asks me to show the plane, name two layers, demonstrate where the needle tip deposits local anesthetic,² and say what the block covers and what it does not. It tests application of knowledge and judgment, especially dose arithmetic when the block is bilateral.

The 60-second answer

I use a linear probe at 6–15 MHz, or a curvilinear probe in an obese patient. For a lateral TAP I place the probe transverse in the mid-axillary line between the costal margin and the iliac crest. I name the layers from the top: skin, subcutaneous fat, external oblique, internal oblique, transversus abdominis, then the transversalis fascia, peritoneum and moving bowel. The target is the plane between the internal oblique and the transversus abdominis. I advance in-plane from anterior to posterior, confirm the tip with 1–2 mL of saline, and watch a lens-shaped hypoechoic pocket open between the two muscles. For upper abdominal incisions I add a subcostal TAP along the costal margin. I state that it gives somatic analgesia of the abdominal wall only, not visceral pain.

Decision points and the "why"

Approach	Probe position	Target	Coverage
Lateral TAP	Mid-axillary line, between costal margin and iliac crest	Between internal oblique and transversus abdominis	About T10–T12 (variably to L1), infraumbilical wall
Subcostal TAP	Oblique along the costal margin, medial	Between rectus and transversus abdominis, or along the plane laterally	T6–T9, supraumbilical wall
Posterior TAP	Posterior axillary line, toward the lumbar fascia	Plane near the quadratus lumborum edge	Wider spread, sometimes some visceral benefit

Identify the layers by moving them — the peritoneum and bowel slide with breathing and peristalsis below the transversus abdominis. That deepest moving layer tells me where the plane ends and the abdomen begins.
Hydrodissection before the full dose — a small test volume that spreads as a lens confirms the plane; a volume that disappears into muscle means the tip is intramuscular, and one that spreads below the transversus means it is preperitoneal or intraperitoneal.
Match the approach to the incision — a lateral TAP covers a Pfannenstiel or lower port sites; a midline laparotomy above the umbilicus needs subcostal injections; a full midline incision needs both bilaterally, or a different block.
Dose before volume — a bilateral four-quadrant TAP uses 60–80 mL. I calculate the maximum dose first, about 3 mg/kg for ropivacaine, and dilute to 0.2–0.25% to reach the volume. Plasma levels peak about 30 minutes after a TAP, later than after a brachial plexus block, so I watch for toxicity in recovery.
Part of a multimodal plan — TAP blocks do not treat visceral pain, so I pair them with acetaminophen, an NSAID if allowed, and a small opioid plan. Expecting a TAP block alone to control pain after bowel surgery sets up a failure.

The complication they will inject

The examiner asks me to perform a right subcostal TAP in a thin patient and then shows a needle tip below a bright line that moves with breathing. I recognize that I am deep to the transversus abdominis, at or through the peritoneum, and near the liver. I stop advancing, withdraw until the tip is in the plane, and confirm with a small volume before injecting. I tell the team and watch for bleeding or bowel injury: hypotension, abdominal pain, a falling hemoglobin. The second variant is a patient 30 minutes after bilateral TAP blocks with 80 mL of 0.375% ropivacaine who complains of tinnitus and a metallic taste and then has a seizure. That dose exceeds 4 mg/kg in a 70 kg patient. I treat local anesthetic toxicity: stop, call for help, airway and oxygen, benzodiazepine for the seizure, 20% lipid emulsion, small epinephrine doses (under 1 mcg/kg) if needed, and avoid large propofol doses, vasopressin, calcium-channel blockers and beta-blockers.³

Pitfalls that cost points

Injecting into a muscle belly and calling it the plane.
Using a lateral TAP alone for an upper midline incision.
Ignoring the cumulative dose of bilateral blocks.
Promising visceral analgesia from a TAP block.
Advancing without seeing the needle tip when the bowel is 1 cm below it.

Rapid-why

Q: Why the plane between internal oblique and transversus abdominis? A: The lower intercostal (T6–T11), subcostal, iliohypogastric and ilioinguinal nerves run in it before supplying the anterior abdominal wall.

Q: Why do TAP plasma levels peak late? A: Fascial planes absorb slowly but steadily across a large surface, so the peak arrives about 30 minutes after injection.

Q: Why add a subcostal injection for a laparotomy? A: The lateral TAP spreads mostly to the lower thoracic segments (about T10–T12, variably L1) and misses the upper abdominal wall.

Q: Why use a curvilinear probe in obesity? A: The muscles can lie more than 5 cm deep, beyond useful linear-probe penetration.

Numbers to know

Item	Value
Volume per side	15–20 mL
Ropivacaine concentration	0.2–0.25%
Ropivacaine maximum	~3 mg/kg
Peak plasma level	~30 min after injection
Lateral TAP coverage	About T10–T12
Subcostal TAP coverage	T6–T9
Lipid emulsion 20% bolus	~1.5 mL/kg over 2–3 min (>70 kg: ~100 mL); maximum 12 mL/kg³

Sources

1.APPLIED Exam Objective Structured Clinical Examination (OSCE) Content Outline · ABA · theaba.org
2.OSCE Example Scenarios (all stations) · ABA, 2023 · theaba.orgNerve block task template: probe orientation, two structures, needle-tip location for local anesthetic.
3.Checklist for Treatment of Local Anesthetic Systemic Toxicity · ASRA, 2020 · asra.com

Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.