Physiological Changes in Pregnancy

Introduction

• Pregnancy causes significant physiological changes in multiple organ systems to support fetal development and prepare the mother for labor and delivery, including increased cardiac output by 30-50% and increased renal blood flow by 40% 1

Cardiovascular Changes

• Cardiac output increases by 30-50% due to increased stroke volume and heart rate, as noted by the American Heart Association and the European Society of Cardiology 2, 1
• Systemic vascular resistance decreases significantly due to vasodilation mediated by progesterone, estrogen, and nitric oxide, as stated by the American Heart Association and the European Society of Cardiology 2, 1
• Uteroplacental blood flow increases from 50 to nearly 1000 mL/min during pregnancy, receiving up to 20% of maternal cardiac output at term 1
• The enlarging uterus can compress the aorta and inferior vena cava starting at approximately 12 weeks, significantly affecting hemodynamics when supine 1
• Heart rate increases by 10-20 beats per minute, particularly in the third trimester, according to the American Society of Anesthesiologists 3
• Blood pressure decreases by 10-15 mmHg by 20 weeks gestation, then returns to pre-pregnancy levels by term, according to the American Society of Anesthesiologists and the European Society of Cardiology 3, 2

Respiratory Changes

• Functional residual capacity decreases by 10-25% as the uterus enlarges and elevates the diaphragm, as reported by the American Heart Association 1
• Tidal volume increases by 20-40% above baseline by term, as noted by the American Heart Association 1
• Minute ventilation increases by 20-40% above pre-pregnancy levels, as stated by the American Heart Association 1
• Oxygen consumption increases by 20-33% above baseline by the third trimester, as reported by the American Heart Association 1
• Mild compensated respiratory alkalosis occurs, with arterial CO₂ pressure of 28-32 mmHg and plasma bicarbonate decreasing to 18-21 mEq/L, as stated by the American Heart Association 1

Renal and Gastrointestinal Changes

• Renal blood flow increases by 40% to accommodate maternal role in fetal detoxification 1
• Glomerular filtration occurs to manage increased circulatory volume 1
• Tubular function is altered to prevent wasting of glucose, amino acids, and proteins required by maternal and fetal metabolism 1
• Gastroesophageal sphincters are relaxed by progesterone, increasing aspiration risk 1
• Transit times are prolonged throughout the intestinal tract during second and third trimesters 1

Anesthetic and Medication Considerations

• Airway changes include upper airway edema and friability due to hormonal effects, potentially reducing visualization during laryngoscopy and increasing bleeding risk 1
• Aspiration risk is increased due to relaxed sphincters and prolonged transit times, emphasizing the need for precautions against pregnancy-associated gastroesophageal reflux during procedures, as noted by the American Society of Anesthesiologists 4, 1
• Drug metabolism is altered through multiple mechanisms, including gastrointestinal absorption and transit, protein binding changes, steroid-induced acceleration of hepatic P450 metabolism, and increased renal clearance, requiring adjustment of drug dosing based on altered pharmacokinetics, as recommended by the European Heart Association 2, 1
• Fluid dynamics favor narrowing of oncotic pressure-wedge pressure gradient, increasing tendency for pulmonary edema 1
• Rapid hypoxemia development occurs due to reduced functional residual capacity combined with increased oxygen consumption, underscoring the importance of prompt oxygen supplementation, according to the American Thoracic Society 5, 1

Management of Pregnancy-Related Complications

• For a pregnant woman presenting with acute onset dyspnea, immediate oxygen supplementation should be provided to maintain target saturation of 94-98%, followed by prompt assessment for life-threatening causes including pulmonary embolism, respiratory failure, and cardiac conditions, as recommended by the American Thoracic Society 5
• Positioning the patient in left lateral tilt is crucial to prevent aortocaval compression, according to the American Heart Association 6
• Pulmonary embolism is a common condition in pregnancy, with a 4x higher risk, as reported by the European Society of Cardiology, necessitating immediate bedside assessment, including bedside transthoracic echocardiography if hemodynamic instability is present, as recommended by the European Society of Cardiology 7, 8
• Cardiac conditions, such as heart failure, cardiomyopathy, and valvular disease, require early cardiology consultation and optimization of preload, afterload, and contractility, as recommended by the American College of Cardiology 8
• Early delivery may be considered if maternal condition deteriorates despite medical management, as suggested by the American College of Obstetricians and Gynecologists 9

Laboratory and Diagnostic Considerations

• BNP (B-type natriuretic peptide) is not elevated during normal pregnancy, as stated by the American Society of Anesthesiologists 3
• Troponin is not elevated during normal pregnancy, as reported by the American Society of Anesthesiologists 3
• D-dimer is not recommended for use in pregnancy due to physiological elevation, as noted by the American Society of Anesthesiologists 3
• Cholesterol can be elevated up to five times during pregnancy, as stated by the American Society of Anesthesiologists 3
• Arterial blood gas shows mild, fully compensated respiratory alkalosis, as reported by the American Society of Anesthesiologists and the American Heart Association 3, 1

Perioperative Considerations

• Position patients with left lateral tilt during procedures to improve exposure while maintaining hemodynamic stability, as recommended by the American Society of Anesthesiologists 4
• Early epidural analgesia with local anesthetics is preferred for pain relief, as recommended by the European Respiratory Society 9
• Avoiding systemic opioids is crucial, as they can suppress ventilation, according to the European Respiratory Society 9
• A multidisciplinary approach involving obstetrics, anesthesia, and critical care is essential, and modification of birth plan may be required for women with significant respiratory disease, as suggested by the European Respiratory Society 9
• Schedule elective procedures in early second trimester when possible to minimize miscarriage risk, as recommended by the American College of Obstetricians and Gynecologists 4
• Use regional anesthesia when appropriate to minimize fetal exposure to medications, as recommended by the American Society of Anesthesiologists 4
• Consider laparoscopic approach when feasible (shorter operative times, fewer fetal adverse effects) with low intra-abdominal pressure, limited procedure duration, open introduction technique, and experienced surgeon, as recommended by the American College of Surgeons 4

Postpartum Considerations

• Monitor for post-partum complications that may result from physiological changes during pregnancy, including pelvic floor disorders and genitourinary conditions, as reported by the World Health Organization 10
• Recognize that physiological changes during pregnancy can lead to medium and long-term maternal complications including pelvic floor disorders and genitourinary conditions, as noted by the World Health Organization 10

Genetic Counseling and Medication Use

• Provide genetic counseling for women with cardiovascular disease regarding inheritance risk for offspring, as recommended by the European Society of Cardiology 2
• Exercise caution with medication use, recognizing altered pharmacokinetics throughout pregnancy, as recommended by the European Heart Association 2
• Account for increased renal perfusion and hepatic metabolism when dosing medications, as reported by the European Heart Association 2
• Monitor glucose homeostasis changes and increased cholesterol levels (up to five times normal), as noted by the European Heart Association 2
• Adjust drug dosing based on altered pharmacokinetics which vary throughout pregnancy, as recommended by the European Heart Association 2