Cardiovascular System Changes in Pregnancy

Cardiovascular System Changes

• Blood volume increases progressively from 6-8 weeks, reaching a 1-1.5 liter increase by delivery.
• Plasma volume increases more than red blood cell mass, resulting in relative anemia.
• Clotting components like fibrinogen and factors VII, X, and XI increase to enhance clotting and reduce bleeding during delivery.
• Increased blood volume meets the mother and fetus's metabolic demands, allowing the mother to tolerate blood loss during delivery.
• Average blood loss during vaginal delivery is 400-500 ml, while cesarean sections result in 800-1000 ml blood loss.

Cardiac Output

• Cardiac output increases up to 40% at term, primarily during the first and second trimesters.
• During labor, cardiac output peaks due to increased heart rate and stroke volume.
• The myocardium and chambers enlarge to accommodate the increased blood volume.
• Cardiac output returns to normal 2 weeks after delivery.
• Cardiac output can decrease after 28 weeks of pregnancy due to mechanical changes.

Blood Pressure

• Blood pressure does not usually increase from pre-pregnancy levels unless there are abnormalities like pregnancy-induced hypertension.
• A decrease in blood pressure occurs by about 8 weeks of gestation.
• Diastolic blood pressure and mean arterial pressure reach their lowest point (16-20 mmHg below pre-pregnancy values) by mid-pregnancy.
• Blood pressure returns to pre-pregnancy levels by term.
• Overall decrease in diastolic blood pressure and mean arterial pressure is 5-10 mmHg.

Venous System

• The venous system has an increased capacity for distension and dilation (up to 150%).
• This can reduce blood flow, delaying the absorption of subcutaneous or intramuscular medications.
• Distention of vessels within the epidural space may increase the risk of vascular damage and bleeding during neuraxial blockade.
• This, along with hormonal changes, reduces the required amount of local anesthetics by 30%.

Anesthetic Considerations due to Cardiovascular Changes in Pregnancy

• Never place the pregnant patient supine. Use a wedge or roll under the right hip to "tip" the patient to the left, preventing supine hypotension syndrome/aortocaval compression.
• Pregnant patients are dependent on sympathetic outflow to maintain systolic blood pressure. Pre-load patients with 1-2 liters of crystalloid fluids before neuraxial blockade.
• Vessel distention in the epidural space increases the risk of vessel damage during neuraxial blockade.
• Vessel distention also decreases the intrathecal and epidural spaces. Decrease the dose of local anesthetics by 30% to avoid a high neuraxial block.
• Delayed absorption of subcutaneous and/or intramuscular medication.

Respiratory System Changes

Lung Volumes

• As the gravid uterus grows, it places pressure on the abdomen, leading to thoracic breathing over abdominal breathing at term.
• Functional residual capacity (FRC) decreases by 20% at term, returning to normal 48 hours after delivery.
• Decreased FRC reduces the patient's reserve, potentially causing rapid hypoxia in case of apnea.
• Tidal volume increases by 40% in pregnant patients.

Respiratory Gas Exchange

• Minute ventilation increases by 50% by the second trimester.
• Respiratory rate increases by 15% (2-3 breaths per minute).
• These changes accelerate the uptake of inhaled anesthetics.
• Alveolar ventilation increases by 70% at term.
• Oxygen consumption increases by 20-50% throughout pregnancy.
• The combination of a decreased FRC and increased oxygen consumption can lead to hypoxia.

Respiratory Tract

• Venous vascular engorgement leads to a swollen respiratory tract, making visualization during laryngoscopy difficult.
• This makes intubation in pregnant patients more challenging.
• A smaller-than-usual endotracheal tube might be required.
• Manipulation during laryngoscopy can cause bleeding, obscuring the view of the glottic opening.

Anesthetic Considerations due to Respiratory Changes in Pregnancy

• Patients undergoing regional anesthesia should receive supplemental oxygen.
• Patients undergoing general anesthesia should be pre-oxygenated with 100% O2 before induction.
• Patients may desaturate despite pre-oxygenation due to increased oxygen consumption and decreased FRC.
• Be prepared for difficult intubation. Swollen mucous membranes can decrease visualization. Ensure the patient is positioned optimally for laryngoscopy.
• Have smaller endotracheal tubes available for intubation.
• Be gentle during laryngoscopy as bleeding can obstruct the view.

Renal System Changes

• Renal plasma flow and glomerular filtration rate increase by 50-60% at term, correlating with increased cardiac output and blood volume.
• Increase in renal plasma flow and glomerular filtration rate leads to increased clearance of blood urea nitrogen and serum creatinine, which may be reduced by 40%.
• Obstructive changes to the renal system can occur due to the enlarging uterus, potentially resulting in increased urinary tract infections and decreased blood flow to the kidneys.

Gastrointestinal System Changes

• Mechanical and hormonal alterations cause several changes in the gastrointestinal system.
• Pressure on the stomach from the enlarging uterus and the effect of progesterone result in an incompetent lower esophageal sphincter.
• Placental gastrin stimulates increased gastric acid secretion.
• These changes lead to reflux of gastric acid into the esophagus, delayed gastric emptying, and an increased risk of aspiration during anesthesia.

Anesthetic Considerations due to Gastrointestinal Changes in Pregnancy

• Pregnant patients should be considered to have "full stomachs" regardless of fasting.
• If available, administer medications to reduce gastric acidity and volume before anesthesia.
• Use a non-particulate antacid like sodium citrate, Metoclopramide (Plasil) 10 mg IV 30-60 minutes before anesthesia to stimulate gastric emptying and increase lower esophageal sphincter tone.
• Use of histamine H2 blockers 30-60 minutes before surgical intervention may help reduce the acidity of stomach contents.
• Position the patient with a roll under the right hip.
• A slight reverse Trendelenburg position may also help prevent passive reflux.
• Apply cricoid pressure until the patient is intubated and confirm that the endotracheal tube has been placed in the trachea before releasing it.
• Do not routinely administer positive pressure ventilation with a mask before intubation; only use it if the patient's pulse oximetry reading declines or a difficult airway is encountered. Unnecessary positive pressure ventilation can lead to gastric distention and an increased risk of aspiration.

Hepatic System Changes

• Overall function and blood flow to the liver remain unchanged during pregnancy.
• There's a 25-30% decrease in pseudocholinesterase function at term.
• This should not produce clinically significant prolongation of succinylcholine, mivacurium, or ester local anesthetics in the immediate delivery period.

Central Nervous System Changes

• Changes in hormones result in a decrease of up to 40% in minimal alveolar concentration (MAC).

This quiz explores the adaptations of the cardiovascular system during pregnancy, focusing on blood volume increases, cardiac output changes, and their implications for delivery. Understand how these changes support both maternal and fetal health throughout gestation and labor.