january 2023
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Dear First name / Colleague, 
 
Happy New Year! 
  • Check out Episode 5 of Partum the Interruption, “What To Do When You Get a Wet Tap,” with guest OB Anesthesiologist Dr. Amy Lee of Baylor College of Medicine!
  • Scroll to the end to read Partum Pearls. This month, we
    • simplify the pathophysiology of intraoperative nausea & vomiting in patients undergoing cesarean delivery
    • identify logic-based strategies for treating IONV and
    • explain why sniffing an alcohol pad can be such an effective rescue treatment
Register for this virtual & in-person workshop to learn from an interdisciplinary team of MFM/Intensivists & OB Anesthesiologists! 
 
Some of our most anticipated sessions:
Intrapartum fluid management
Acid/Base Norms in Pregnancy
Cardiac Biomarkers in Pregnancy
Arrhythmia Management on L&D
Anticoagulation for Patients With Valvular Disease
 
partum pearls
 When the nausea hits 🤢
olfactory distraction & isopropyl alcohol:
 an OB anesthesiologist’s best friend 
 
During a recent elective c-section on L&D, I fell behind while managing the patient's blood pressure five minutes after her spinal. I glanced down and immediately recognized that look of intense nausea, so l reached for an old trick passed down to me during residency. I handed her a soaked alcohol pad to sniff while I scrambled to bolus phenylephrine.
 
Abruptly, her urge to vomit was suppressed, which bought me the thirty seconds I needed to bring her blood pressure up. 
 
I found myself wondering, why does this trick work so well? Why exactly does hypotension cause nausea? And why are the standard anti-emetics often ineffectual during cesarean deliveries? 
 
 
Remember, nausea and vomiting are two distinct processes. Nausea is a sensory experience, while vomiting is the coordinated expulsion of stomach contents.
 
Intraoperative nausea/vomiting (IONV) is specific to patients undergoing awake surgery, like those undergoing cesarean delivery. IONV can be a uniquely distressing and frustrating experience for a mom who is just trying to enjoy the birth of her child. 
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Why do we vomit?
Teleologically, the emetic response is part of a protective reflex arc to sense and rid the body of poisons. To coordinate this, the brainstem has developed the area postrema, a loosely organized vomiting center at the floor of the fourth ventricle.
 
D & Red Arrow: Area Postrema
Left: Mirza M, M Das J. Neuroanatomy, Area Postrema. [Updated 2022 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. [Figure, Area postrema. Image courtesy S Bhimji MD] Available from: https://www.ncbi.nlm.nih.gov/books/NBK544249/figure/article-17808.image.f2/
Right:  Chan KH, Vorobeychik GArea postrema syndrome: a neurological presentation of nausea, vomiting and hiccups BMJ Case Reports CP  2020; 13: e238588.
 
The vomiting center, comprised of the chemoreceptor trigger zone (CTZ), reticular formation and the nucleus tractus solitarius, receives input from various areas of the body, including [1]:
  • Vagal afferents from the gastrointestinal tract
  • Stimuli from the vestibular (balance) and visual areas
  • The chemoreceptor trigger zone (CTZ), where offending agents in the blood and CSF are monitored
Which “offending” or “pro-emetic” agents trigger the vomiting center? Drugs and toxins, obviously, but ultimately, the vomiting center is activated by high levels of specific neurotransmitters. The primary neurotransmitters (and corresponding receptors) used by nerve endings of the vagus, vestibular system, and CTZ are:
  • Acetylcholine (Nicotinic receptor)
  • Dopamine (D2 & D3 receptors)
  • Histamine (H1 receptor)
  • Substance P (Neurokinin-1 receptor)
  • Serotonin (5-HT3 receptor)
The CTZ also contains opioid receptors. Each receptor monitors the amount of associated neurotransmitter in the blood, and triggers emesis at a particular threshold.
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A birds-eye view of the vomiting center: afferent and efferent circuits
The Pathophysiology of Nausea and Vomiting. From: Krakauer EL, Zhu AX, Bounds BC, Sahani D, McDonald KR, Brachtel EF. Case records of the Massachusetts General Hospital. Weekly clinicopathological exercises. Case 6–2005. A 58-year-old man with esophageal cancer and nausea, vomiting, and intractable hiccups. N Engl J Med. 2005 Feb24;352(8):817–25. doi: 10.1056/NEJMcpc049037. PMID: 15728815.
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How does nausea lead to emesis?
The act of vomiting is coordinated by the nucleus tractus solitarius (NTS), a series of purely sensory nuclei embedded in the medulla oblongata. The NTS is organized into subnuclei that control swallowing, gastric sensation, laryngeal and pharyngeal sensation, baroreceptor function, and respiration [2]. Once the NTS is activated by the CTZ, vagus, or vestibular system, its subnuclei coordinate the physical movements required for vomiting.
 
In patients with untreated hypotension after a spinal anesthetic, the vestibular system quickly activates the NTS, whose outputs: 
  1. Activate the rapid vomiting reflex of the upper GI tract, and
  2. Project to the mid and forebrain causing the perception of nausea [3,4]
TL; DR: Strategic Prevention & Treatment of IONV
 
The key to selecting an effective treatment (and avoiding unnecessary medications) is to identify the underlying triggers and specific pathways that are contributing to nausea in your patient. 
 
Simply put, is it the vagus, vestibular, or CTZ? And what is the root cause? 
 
The primary causes of IONV in cesarean deliveries under neuraxial are:
  1. Hypotension/hypoperfusion of the brainstem (vestibular & vagal)
  2. Pregnancy associated impaired gastric and intestinal motility (vagal)
  3. Surgical manipulation (vagal)
  4. Drug (uterotonic) side effects (CTZ, vagal)
Treatments & Prevention of IONV, by cause:
  1. Hypotension:
    • Vasopressors (phenylephrine, ephedrine, norepinephrine)
    • Olfactory distraction (isopropyl alcohol)
    • Adequate preload & minimal NPO time
    • Consider left uterine displacement. 30-45 degree tilt is needed to decompress the IVC & aorta [5].
  2. Impaired gastric/intestinal motility:
    • Observe NPO guidelines for elective cases
    • Consider GI “prophylaxis” with:
      • H2 antagonist (famotidine, ranitidine)
      • 5-HT3 antagonist (ondansetron)
      • pro-motility agent if not appropriately NPO (metoclopramide)
    • Refractory nausea or high-risk patients
      • H1 antagonist (diphenhydramine, promethazine) - sedating
      • Dopamine-2 antagonist (metoclopramide; chlorpromazine) - extrapyramidal effects; sedation
      • Anticholinergic (scopolamine transdermal patch) – dry mouth, blurry vision, sleepiness, other CNS effects
  3. Surgical manipulation
    • Avoid uterine exteriorization and excessive manipulation of the peritoneum
  4. Drug side effects
    • Use the minimum dose of uterotonic required
    • Avoid unnecessary medications
Now that we have a better understanding of the complex circuitry of n/v, there is a simple explanation for why isopropyl alcohol works as a rescue anti-emetic. The answer is olfactory distraction [6]. Inhalation of strong aromas such as isopropyl alcohol rapidly activate the olfactory pathway, which, like so many of the cranial nerve sensory pathways, converges on the NTS. The intense olfactory stimulation competes with the nausea pathway, reducing amplification and transmission of the nausea signal! 
1. Heckroth M, Luckett RT, Moser C, Parajuli D, Abell TL. Nausea and Vomiting in 2021: A Comprehensive Update. J Clin Gastroenterol. 2021 Apr 1;55(4):279-299. doi: 10.1097/MCG.0000000000001485. PMID: 33471485; PMCID: PMC7933092.
2. AbuAlrob MA, Tadi P. Neuroanatomy, Nucleus Solitarius. [Updated 2022 Jul 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK549831/
3. Zhong W, Shahbaz O, Teskey G, Beever A, Kachour N, Venketaraman V, Darmani NA. Mechanisms of Nausea and Vomiting: Current Knowledge and Recent Advances in Intracellular Emetic Signaling Systems. Int J Mol Sci. 2021 May 28;22(11):5797. doi: 10.3390/ijms22115797. PMID: 34071460; PMCID: PMC8198651.
4. Wickham R.J. Revisiting the physiology of nausea and vomiting—Challenging the paradigm. Support. Care Cancer. 2019;28:13–21. doi: 10.1007/s00520-019-05012-8. 
5.  Hideyuki Higuchi, Shunichi Takagi, Kan Zhang, Ikue Furui, Makoto Ozaki; Effect of Lateral Tilt Angle on the Volume of the Abdominal Aorta and Inferior Vena Cava in Pregnant and Nonpregnant Women Determined by Magnetic Resonance Imaging. Anesthesiology 2015; 122:286–293 doi: https://doi.org/10.1097/ALN.0000000000000553
6. Corona AGL, Chin J. Olfactory Distraction for Management of Nausea in Palliative Care Patients. Am J Hosp Palliat Care. 2022 Mar;39(3):388-393. doi: 10.1177/10499091211015957. Epub 2021 May 6. PMID: 33955274.
 

until next time,
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