What are the contraindications for using nitrous oxide (N2O) inhalation in pregnant women with low ejection fractions (reduced left ventricular ejection fraction)?

Contraindications for Nitrous Oxide Inhalation in Pregnant Women with Low Ejection Fraction

Nitrous oxide should be used with extreme caution or avoided entirely in pregnant women with ejection fraction <40%, as severe systemic ventricular dysfunction (LVEF <30%, NYHA III-IV) represents a WHO Class IV contraindication to pregnancy itself, and the hemodynamic effects of nitrous oxide may further compromise already limited cardiac reserve. 1

Absolute Contraindications Based on Cardiac Function

Severe Ventricular Dysfunction

• Ejection fraction <30% with NYHA Class III-IV symptoms is classified as WHO Class IV risk, where pregnancy itself is contraindicated 1
• Women with LVEF <40% should be counseled against pregnancy due to substantially increased risk of maternal cardiovascular events 2
• The CARPREG risk score identifies reduced systemic ventricular systolic function (EF <40%) as a major predictor of maternal cardiovascular complications 2

Associated High-Risk Cardiac Conditions

• Pulmonary arterial hypertension of any cause represents an absolute contraindication to pregnancy (WHO Class IV) and would preclude nitrous oxide use 1
• Previous peripartum cardiomyopathy with any residual impairment of left ventricular function is WHO Class IV 1
• Severe symptomatic aortic stenosis or severe mitral stenosis are absolute contraindications 1

Hemodynamic Concerns with Nitrous Oxide in Cardiac Dysfunction

Cardiovascular Effects of Nitrous Oxide

• Nitrous oxide inhalation causes significant hemodynamic changes including decreased heart rate, decreased cardiac output, and decreased arterial pressure, particularly at higher concentrations (70% N2O) 3
• A 30% nitrous oxide concentration decreases maternal central vascular resistance and causes cerebral vasodilation 4
• These circulatory effects persist even during intervals between contractions when N2O is not being actively inhaled 3

Why This Matters in Low EF Patients

• Pregnant women with reduced ejection fraction have limited cardiac reserve and cannot compensate for the decreased cardiac output and blood pressure changes induced by nitrous oxide 1, 3
• Normal pregnancy already increases cardiac output by 30-50%, heart rate by 15-20 bpm, and stroke volume significantly—demands that a failing heart cannot meet 2
• The additional hemodynamic stress of labor further increases cardiac output, and immediate postpartum volume shifts dramatically increase cardiac filling pressures 1

Risk Stratification Framework

WHO Class IV (Absolute Contraindication to Nitrous Oxide)

• LVEF <30% with NYHA III-IV symptoms 1
• Any pulmonary arterial hypertension 1
• Previous peripartum cardiomyopathy with residual dysfunction 1

WHO Class III (Relative Contraindication—Use Only with Extreme Caution)

• LVEF 30-40% with close hemodynamic monitoring required 1
• Mechanical valves or complex congenital heart disease 1
• These patients require monthly or bimonthly cardiology review and should have delivery at tertiary centers with advanced cardiac anesthesia capabilities 1

WHO Class II (May Consider with Monitoring)

• LVEF 40-49% (borderline reduced) with frequent monitoring 2
• Mild valvular disease without significant hemodynamic compromise 1

Critical Management Considerations

If Nitrous Oxide Must Be Used Despite Concerns

• Limit concentration to 30% or less (lower concentrations produce less hemodynamic depression than 70% mixtures) 3, 4
• Continuous invasive hemodynamic monitoring with pulmonary artery catheter should be strongly considered 1
• Have immediate access to inotropic support (dobutamine) and vasopressors 1
• Ensure supplemental oxygen delivery to maintain maternal oxygen saturation 1

Safer Alternatives for Labor Analgesia

• Carefully titrated epidural anesthesia may be preferable as it can be incrementally adjusted and provides more predictable hemodynamic effects 1, 5
• Regional anesthesia avoids the systemic cardiovascular depression associated with nitrous oxide 3
• For patients with pulmonary hypertension specifically, inhaled nitric oxide (NOT nitrous oxide) has been successfully used as it selectively vasodilates the pulmonary bed without systemic effects 5

Common Pitfalls to Avoid

• Do not confuse nitrous oxide (N2O) with nitric oxide (NO)—they have completely different pharmacologic effects; nitric oxide is therapeutic in pulmonary hypertension while nitrous oxide causes hemodynamic depression 5
• Do not assume that because nitrous oxide is "safe" in normal pregnancy, it is safe in cardiac disease—the hemodynamic effects that are well-tolerated in healthy parturients can be catastrophic in those with limited cardiac reserve 3, 1
• Do not rely on intermittent administration to minimize effects—cardiovascular changes persist between contractions even when inhalation is stopped 3
• Avoid the supine position during nitrous oxide administration in any pregnant patient with cardiac disease, as this further compromises venous return 1

Monitoring Requirements

Essential Parameters

• Continuous pulse oximetry (maintain SpO2 >95%) 1
• Continuous cardiac monitoring with immediate access to advanced cardiac life support 1
• Frequent blood pressure monitoring (every 1-2 minutes during active inhalation) 3, 4
• Consider invasive arterial line for beat-to-beat blood pressure monitoring in LVEF <40% 1

When to Stop Immediately

• Maternal oxygen saturation <95% 1
• Systolic BP drop >20 mmHg or heart rate <60 bpm 3
• Development of chest pain, severe dyspnea, or signs of pulmonary edema 1
• Fetal heart rate abnormalities suggesting compromise 6