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).