# Situation: A 48-year-old woman weighing 60 kg returns to the surgical ward after an open abdominal operation under general anesthesia. She has an indwelling urinary catheter and a wound drain. Her preoperative blood pressure was 126/80 mmHg and her heart rate was 80/min. Her daughter stays with her as a watcher. Several antiemetics are available for postoperative nausea and vomiting. Which one is contraindicated when a mechanical bowel obstruction is present?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=629772  
> language: ko  
> subject: Nursing Practice III — Care of Clients with Problems in Surgery, Oxygenation, Fluid and Electrolytes, Infectious, Inflammatory and Immunologic Response, Cellular Aberrations

## 문제

Situation: A 48-year-old woman weighing 60 kg returns to the surgical ward after an open abdominal operation under general anesthesia. She has an indwelling urinary catheter and a wound drain. Her preoperative blood pressure was 126/80 mmHg and her heart rate was 80/min. Her daughter stays with her as a watcher.

Several antiemetics are available for postoperative nausea and vomiting. Which one is contraindicated when a mechanical bowel obstruction is present?

## 보기

1. Transdermal scopolamine
2. Metoclopramide **✔ 정답**
3. Ondansetron
4. Dexamethasone

**정답: 2**

## 해설

Metoclopramide stimulates gastrointestinal motility and speeds gastric emptying, so it is contraindicated in mechanical bowel obstruction, perforation, or bleeding. The other antiemetics act on vomiting pathways without pushing the bowel against a blockage.

## 심화 해설

Why metoclopramide is the contraindicated antiemetic

A mechanical bowel obstruction means the intestinal lumen is physically blocked, so peristaltic waves cannot move contents forward. In this situation, any drug that increases gastrointestinal motility can raise intraluminal pressure proximal to the blockage, worsening distension, ischemia, and the risk of perforation. Metoclopramide is a prokinetic agent that enhances gastric emptying and small bowel transit, making it unsafe when a mechanical obstruction is present. Its dopamine D2 receptor antagonism and 5-HT4 agonism drive this motility effect, which is the exact opposite of what an obstructed bowel needs.

The other options do not push the bowel against a blockage. Transdermal scopolamine is an antimuscarinic that reduces acetylcholine-driven smooth muscle contraction and also acts on the vestibular and central vomiting pathways. Ondansetron is a 5-HT3 receptor antagonist that blocks serotonin-mediated vagal afferent signaling to the chemoreceptor trigger zone and vomiting center, without stimulating peristalsis. Dexamethasone suppresses inflammation and edema around a tumor or bowel wall, and it modulates central emetic pathways, but it does not directly increase motility. Key point! The contraindication is specific to the prokinetic mechanism, not to antiemetic efficacy.

| Drug | Primary antiemetic mechanism | Effect on GI motility | Safe in mechanical obstruction? |
| --- | --- | --- | --- |
| Metoclopramide | D2 antagonist, 5-HT4 agonist, 5-HT3 antagonist | Increases gastric emptying and small bowel transit | No — contraindicated |
| Transdermal scopolamine | Muscarinic antagonist, central vestibular suppression | Decreases or neutral | Yes |
| Ondansetron | 5-HT3 antagonist at vagal afferents and CTZ | Neutral | Yes |
| Dexamethasone | Anti-inflammatory, central emetic pathway modulation | Neutral | Yes |

In malignant bowel obstruction, symptom management focuses on reducing secretions, inflammation, and central nausea rather than forcing motility. Prokinetic agents such as metoclopramide are avoided because they can convert a partial obstruction into a complete one or precipitate perforation. Antisecretory and anti-inflammatory approaches, along with centrally acting antiemetics, are preferred when the obstruction cannot be relieved mechanically. Watch out! Metoclopramide is also contraindicated in suspected gastrointestinal perforation or hemorrhage for the same reason: increased motility worsens the underlying structural injury.

For a postoperative patient with an open abdominal procedure, the nurse must assess for signs of ileus versus mechanical obstruction before administering any prokinetic. Bowel sounds, distension, nausea, vomiting, and the character of nasogastric or drain output help differentiate the two. A functional ileus may respond to prokinetics, but a mechanical obstruction requires decompression and surgical evaluation, not motility stimulation. The presence of a wound drain and indwelling catheter does not change this contraindication; the decision rests on the underlying bowel pathology.

## 임상 시나리오

Antiemetic Selection in Suspected Bowel ObstructionAvoid prokinetics when a mechanical blockage is possible
In a postoperative patient with abdominal surgery, always assess for signs of mechanical bowel obstruction before choosing an antiemetic. Metoclopramide is contraindicated because its prokinetic action increases peristalsis against a physical blockage, raising the risk of perforation and ischemia.

Safer alternatives include ondansetron, a 5-HT3 antagonist that blocks vagal afferent signaling without stimulating motility, and transdermal scopolamine, an antimuscarinic that reduces smooth muscle contraction. Dexamethasone may also be used to reduce inflammation and modulate central emetic pathways.

CautionDo not assume all antiemetics are interchangeable. A prokinetic given in the presence of obstruction can convert a partial blockage into a surgical emergency. Review the surgical history and current bowel status before administration.

## 핵심 개념

- **Prokinetic** — A drug that increases gastrointestinal motility and accelerates gastric emptying, such as metoclopramide.
- **Mechanical bowel obstruction** — A physical blockage of the intestinal lumen that prevents forward passage of contents, making increased peristalsis dangerous.
- **Metoclopramide** — A D2 antagonist and 5-HT4 agonist antiemetic with prokinetic effects; contraindicated in obstruction, perforation, or GI bleeding.
- **Ondansetron** — A 5-HT3 receptor antagonist that suppresses nausea and vomiting without stimulating bowel motility.
- **Transdermal scopolamine** — An antimuscarinic antiemetic that reduces acetylcholine-driven smooth muscle contraction and acts on vestibular pathways.

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