Hiccups are sudden, involuntary spasms of the diaphragm, the large muscle below the lungs that controls breathing. The spasm causes a rapid intake of air that is suddenly halted by the closure of the vocal cords, producing the characteristic “hic” sound. Anesthesia and the perioperative experience can trigger this reflex, often in the immediate post-operative period as the patient recovers from general anesthesia. While usually transient, hiccups can cause discomfort, interfere with breathing, and sometimes signal irritation of the nervous system.
The Physiological Link Between Anesthesia and Hiccups
The mechanism behind a hiccup is a complex, involuntary reflex arc involving several nerve pathways and a central processing center in the brainstem. The reflex begins with the afferent, or sensory, nerves, which include branches of the vagus nerve, the phrenic nerve, and the sympathetic chain. These nerves transmit irritation signals from the chest and abdomen up to the hiccup center in the medulla.
Once triggered, the signal returns via the efferent, or motor, pathway, primarily through the phrenic nerve, which is the sole motor supply to the diaphragm. This efferent signal causes the sudden, synchronous contraction of the diaphragm and the intercostal muscles, initiating the sharp inhalation. Simultaneously, a branch of the vagus nerve causes the glottis—the opening between the vocal cords—to snap shut, creating the sound.
General anesthesia procedures commonly introduce physical and chemical irritants that can activate this reflex arc. During surgery, a breathing tube is often placed in the airway, and the presence of this tube can cause mechanical irritation to nearby laryngeal and phrenic nerve branches. Furthermore, gastric distention, often caused by air or gas during ventilation or laparoscopic surgery, can stimulate the vagus nerve in the stomach lining. This visceral stimulation directly excites the afferent limb of the hiccup reflex, which may persist as the patient wakes up.
Specific Factors That Increase Hiccup Risk
Certain anesthetic agents and patient conditions increase the likelihood of developing hiccups in the perioperative setting. Several intravenous induction and sedative medications, including propofol, midazolam, and methohexital, have been implicated in triggering the reflex arc. Benzodiazepines like midazolam, for example, are sometimes associated with a dose-dependent increase in hiccup incidence.
The type of surgical procedure is also a significant factor, particularly those involving the abdomen or chest. Laparoscopic procedures use carbon dioxide gas to inflate the abdominal cavity for better surgical visibility. This direct mechanical stimulation of the diaphragm and the vagus nerve is a potent trigger for the hiccup reflex.
Patient-specific factors also play a role, with pre-existing conditions that affect the gastrointestinal tract being particularly relevant. Individuals with Gastroesophageal Reflux Disease (GERD) are at a higher risk because stomach acid backing up into the esophagus can irritate the vagus nerve, making the reflex arc more sensitive. Additionally, factors like pre-operative anxiety or stress, and electrolyte imbalances, can lower the threshold for the hiccup reflex, making the patient more susceptible to a reaction from even minor irritation.
Resolving Post-Anesthesia Hiccups
The vast majority of post-anesthesia hiccups are transient, resolving spontaneously without intervention as the patient fully recovers from the effects of the anesthetic agents. When hiccups persist and cause discomfort or interfere with recovery, clinicians will often initiate steps to interrupt the nerve reflex arc. Non-pharmacological approaches attempt to stimulate the vagus nerve, such as controlled breathing exercises, which involve holding the breath for a short period or performing a controlled Valsalva maneuver.
If the hiccups are persistent, pharmaceutical treatments are utilized, often targeting the pathways involved in the reflex. Metoclopramide is a common first-line agent, acting as a prokinetic that empties the stomach to reduce vagal nerve irritation caused by distention. Chlorpromazine is another option, and it is the only medication specifically approved by the Food and Drug Administration for the treatment of intractable hiccups, though its use is reserved due to potential side effects like low blood pressure.
Hiccups that last less than 48 hours are generally considered transient, but those that continue for 48 hours or more are classified as persistent and warrant closer medical evaluation. Prolonged hiccups are concerning because they can disrupt wound healing, interfere with a patient’s ability to eat or sleep, and increase the risk of aspirating stomach contents. If the spasms interfere with breathing or last longer than two days, medical staff will investigate more serious underlying causes and escalate treatment.