What are the common causes of rigors during labor?

Causes of Rigors in Labor

Rigors during labor are primarily caused by epidural analgesia and maternal fever, with over half of all parturients experiencing at least one episode of shaking, and epidural use being the strongest predictor of both occurrence and duration.

Primary Etiologies

Epidural-Associated Rigors

• Epidural analgesia is the most significant risk factor for shaking rigors during labor, with epidural usage directly related to the risk, duration, and number of rigor episodes 1
• Approximately 57% of all parturients experience at least one rigor episode during labor, with epidural administration being a clear antecedent event 1
• Women receiving epidural analgesia are substantially more likely to develop both hyperthermia and clinical fever compared to those using other forms of analgesia or unmedicated labor 2

Maternal Fever and Hyperthermia

• Increased maximum maternal temperature is independently associated with an increased probability of rigors and a greater number of episodes 1
• Maternal fever occurs as an antecedent event to rigors, with epidural-associated fever being the most common pathway 1
• The gradual development of hyperthermia in women with epidural analgesia is not observed in those using other analgesic methods 2

Mechanisms and Pathophysiology

Thermoregulatory vs. Non-Thermoregulatory Shivering

• Approximately 15-18% of shivering episodes during labor are non-thermoregulatory, occurring when women are simultaneously normothermic (>37°C) and vasodilated 3
• High pain scores and vasoconstriction are significant predictors of thermoregulatory shivering during labor 3
• The phenomenon is multifactorial, with both thermoregulatory and non-thermoregulatory mechanisms contributing to peripartum tremor 3

Inflammatory Mechanisms

• The most likely etiology of epidural-associated fever is inflammation, most commonly manifesting as chorioamnionitis in the placenta and membranes 2
• Altered thermoregulation and potential antipyretic effects of opioids in non-epidural groups may partially explain temperature differences 2

Timing and Clinical Patterns

Temporal Relationships

• Epidural administration, birth events, and maternal fever all serve as antecedent events preceding rigor episodes 1
• Shivering-like tremor is observed in approximately 18% of 30-minute observation periods before delivery 3
• Postdelivery rigors occur in 16% of observation epochs, with 28% being non-thermoregulatory 3

Clinical Implications and Consequences

Maternal Outcomes

• The combination of inflammation and hyperthermia adversely impacts uterine contractility, increasing the risk for cesarean delivery and postpartum hemorrhage by 2- to 3-fold 4
• Obstetricians are more likely to intervene surgically in laboring women with fever 2

Neonatal Outcomes

• Maternal fever >101°F is associated with increased neonatal morbidity, including higher rates of 1-minute Apgar scores <7 (22.8% vs 8.0% in afebrile women) and neonatal hypotonia (4.8% vs 0.5%) 5
• Infants exposed to maternal temperatures >101°F require bag-and-mask resuscitation more frequently (11.5% vs 3.0%) and oxygen therapy in the nursery (8.2% vs 1.3%) 5
• The combination of intrapartum fever and fetal acidosis is particularly detrimental, with fever potentially lowering the threshold for hypoxic brain injury 4
• Maternal inflammatory fever is associated with neonatal brain injury, including cerebral palsy, encephalopathy, and learning deficits in later childhood 2

Prevention and Management Considerations

Risk Reduction Strategies

• Maintaining labor progress is critical to reduce both the duration of epidural exposure and the length of labor, both significant risk factors for intrapartum fever 4
• High-dose oxytocin regimens (6×6 mU/min) compared to low-dose regimens (2×2 mU/min) led to clinically meaningful reductions in intrapartum fever rates (10.4% vs 15.6%) 4

Treatment Limitations

• There are currently no safe and effective means to inhibit epidural-associated fever 2
• Acetaminophen may not be effective in reducing maternal temperature once fever develops 4
• When fever occurs, antibiotic treatment should be initiated promptly 4

Critical Clinical Pitfalls

• Intrapartum fever alone is not an indication for cesarean delivery to improve neonatal outcomes, as there is no evidence that reducing fetal exposure duration prevents known adverse neonatal outcomes 4
• Clinicians must be prepared for increased risk of postpartum hemorrhage when fever occurs and have uterotonic agents immediately available at delivery 4
• Selection bias can confound the epidural-fever association, as women at risk for fever (longer ruptured membranes, longer labor, more cervical examinations) are also more likely to select epidural analgesia 2