Cardiovascular Alterations in the Parturient Undergoing Cesarean Delivery With Neuraxial Anesthesia

Katherine W Arendt; Jochen D Muehlschlegel; Lawrence C Tsen

Expert Rev of Obstet Gynecol. 2012;7(1):59-75.

Cardiovascular Changes During Pregnancy

Pregnancy is associated with a 20% decrease in systemic vascular resistance (SVR),^[6] which likely triggers the renin–angiotensin–aldosterone system to retain sodium and increase plasma volume. This increase in plasma volume exceeds a simultaneous increase in red blood cell mass, resulting in a lower hemoglobin level than in the nonpregnant state.^[7] Plasma volume increases 50% over pre-pregnant values by 32 weeks gestation (Table 1).^[8] SVR decreases to 30% below pre-pregnancy values by 20 weeks gestation; this change is reflected in a decrease in mean arterial pressure (MAP) and an increase in heart rate (HR) of 15 beats/min.^[9] Overall arterial compliance increases from pregnancy onset until second trimester, then decreases from the second trimester through the remainder of the pregnancy.^[10] Cardiac output (CO) increases by 30 to 60% until approximately 32–36 weeks gestation.^[11–15] Throughout pregnancy, pulmonary vascular resistance decreases, pulmonary blood flow increases, and pulmonary mean arterial pressures remain unchanged.^[16] With supine positioning after 20 weeks gestation, the gravid uterus can compress the vena cava, reducing cardiac output, and compress the aorta proximal to the uterine artery further compromising uterine blood flow. A more in-depth review of the hemodynamic changes of pregnancy has been previously published.^[17]

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Table 1. Hemodynamic variables in pregnancy.
Variable	% change (gestational age by which change occurs)	Ref.
Peripheral vascular resistance	30% decrease (20 weeks)	[6,11]
Plasma volume	50% increase (30 weeks)	[8]
Cardiac output	30–50% increase (32–36 weeks)	[11,15,165]
Heart rate	29% increase (33–36 weeks)	[15]
Stroke volume	18% increase (36–39 weeks)	[15]

Table 2. Cardiac output measurements during epidural anesthesia for elective cesarean delivery.
Study (year)	Measurement technique	Confounding factors	Epidural drug	CO after prehyd (l/min)	CO after block (l/min)	CO after placental delivery (l/min)	CO 1 h after birth (l/min)	Ref.
Ueland et al.(1972)	Dye dilution	Prehyd: NA Level: T2–T10 Position: supine LUD performed if significant drop in BP occurred	2% mepivacaine (13.5–17.5 ml)	5.88 ± 1.31	5.52 ± 1.77 ↓6%^†	7.34 ± 1.84 ↑25%^† ↑33%^‡	6.71 ± 1.95 ↑14%^† ↑22%^‡ ↓9%^§	[96]
Maruta (1982)	Echo	Prehyd: <1 l Level: NA Position: NA	NA	5.95 ± 1.49	5.05 ± 1.59 ↓15%^†	6.08 ± 1.31 ↑2%^† ↑20%^†	5.41 ± 0.91 ↓9%^† ↑7%^‡ ↓11%^§	[89]
James et al. (1989)	Suprasternal Doppler (aortic orifice measured by cross-section)	Prehyd: 15 ml/kg LR Level: T4–T8 Position: LUD	2% lidocaine + 1:200,000 epi	6.63 ± 1.33	7.88 ± 1.6 ↑19%^†	9.05 ± 1.39 ↑36%^† ↑15%^‡	7.11 ± 1.7 ↑7%^† ↓10%^‡ ↓21%^§	[97]
Milsom et al. (1985)	Impedance cardiography	Co-load: 1.5–2.0 l Level: NA Position: LUD Atropine 0.5 mg iv. prior to epidural Subjects requiring ephedrine excluded	0.5% bup (23–28 ml)	NA	Before to after block: increased by 0.7 ± 0.2 ↑^¶	Before to after delivery: increased by 1.1 ± 0.3 (33%) ↑^¶	NA	[39]
Robson et al. (1992)	Suprasternal Doppler (aortic orifice measured by cross-section)	Prehyd: 1 l crystalloid 1.2 ± 0.2 l total fluids Ephedrine administered 60 mg/l in iv. fluids after prehyd Level: T2–T6 Position: LUD	0.5% bup + 1:200,000 epi (18–30 ml)	7.83	10 min after 8.20 ↑5% 30 min after 7.96 ↑2%	NA	NA	[52]
Robson et al. (1989)	Suprasternal Doppler (aortic orifice measured with cross-sectional echo)	Prehyd: 500 ml crystalloid Level: >T5 Position: LUD Subjects requiring ephedrine excluded	0.5% bup (21 ± 3 ml)	7.14	7.08 ↓<1%^†	7.69 ↑8%^† ↑9%^‡	6.65 ↓7%^† ↓6%^‡ ↓14%^§	[98]
Robson et al. (1989)	Suprasternal Doppler (aortic orifice measured with cross-sectional echo)	Prehyd: 0.8 l mean Level: T2–T6 Position: LUD Subjects requiring ephedrine excluded	0.5% bup (20 ± 5 ml)	7.82	10 min after 7.34 ↓6%^† 30 min after 7.12 ↓9%^†	NA	NA	[53]

^†From CO after prehydration.
^‡From CO after block.
^§From CO after delivery of placenta.
^¶Not enough data available to calculate percent change.
BP: Blood pressure; bup: Bupivacaine; CO: Cardiac output; epi: Epinephrine; iv.: Intravenous; LR: Lactated Ringers; LUD: Left uterine displacement; NA: Data not available in the paper; Prehyd: Prehydration.

Table 3. Cardiac output measurements during spinal anesthesia for elective cesarean delivery.
Study (year)	Measurement technique	Confounding factors	Intrathecal drug	CO after prehyd (l/min)	CO after block (l/min)	CO after delivery of placenta (l/min)	CO 1 h after CD (l/min)	Ref.
Ueland et al. (1968)	Dye dilution	Prehyd: NA Level: T2–T8 Position: supine	Tetracaine 7–10 mg + epi 200 mcg	5.4 ± 1.8	3.5 ± 1.2 ↓35%^†	8.4 ± 2.6 ↑61%^† ↑140%^‡	NA	[66]
Niswonger et al. (1970)	Dye dilution	Prehyd: NA Level: T4–T9 Position: NA 9 patients: 8 with spinal, 1 with epidural anesthetic 2 out of 9 were less than 36 weeks gestation	NA	6.29 ± 1.75	6.95 ± 2.43 ↑10%^†	8.81 ± 3.26 ↑40%^† ↑27%^‡	6.83 ± 2.50 ↑9%^† ↓2%^‡ ↓22%^§	[61]
Robson et al. (1992)	Suprasternal Doppler (aortic orifice measured by cross section)	Prehyd: 1.39 ± 0.18 l crystalloid Level: T1–T4 Position: LUD Ephedrine administered 60 mg/l in iv. fluids, and in boluses	HB bup 10–12.5 mg	7.90	5 min after block: 6.73 ↓15%^† 15 min after block: 7.36 ↓7%^†	NA	NA	[52]
Tihtonen et al. (2005)	Whole-body impedance cardiography	Prehyd: 10 ml/kg 6% hydroxyethyl starch Level: NA Position: LUD Hypotension treated with 50 mg/100 ml ephedrine infusion Mean dose 0.53 mg/kg	HB bup 12–13.5 mg	CI: 3.6 ± 0.2 l/min/m²	CI: 4.1 ± 0.5 l/min/m² ↑11%^†	CI: 4.7 ± 0.2 l/min/m² ↑31%^† ↑15%^‡(↓39% SVRI and stable MAP)	CI: 3.7 ± 0.3 l/min/m² ↑3%^† ↓10%^‡ ↓21%^§	[40]
Langesaeter et al. (2008)	LiDCOplus	Prehyd: None Co-load of 750 ml over 20 min Level: NA Postion: LUD Prophylactic phenylephrine infusion 0.25 mcg/kg/min	IB bup 7 mg	6.1 ± 0.3	Initial immediate ↑32.8%^† in first 200 s, then ↓16.8%^† approximately 6 min after block	NA	NA	[63]

^†From CO after prehydration.
^‡From CO after block.
^§From CO after delivery of placenta.
bup: Bupivacaine; CD: Cesarean delivery; CI: Cardiac index; CO: Cardiac output; epi: Epinephrine; HB: Hyperbaric; IB: Isobaric; iv.: Intravenous; LUD: Left uterine displacement; MAP: Mean arterial pressure; NA: Data not available in the paper; Prehyd: Prehydration; SVRI: Systemic vascular resistance index.

Box 1. Techniques to decrease hypotension with neuraxial anesthesia for cesarean delivery.
Leg wrapping [134] Prehydration or co-load with intravenous colloid solution [57] Co-load with crystalloid intravenous solution [57] Lower dose intrathecal local anesthesia supplemented with opioid [86] Maternal left uterine displacement positioning [128] Consider epidural instead of spinal anesthesia [95] Phenylephrine infusion with rapid crystalloid co-load [160] Phenylephrine infusion with low-dose intrathecal bupivacaine [63] Phenylephrine infusion or boluses titrated to maintain a consistent heart rate [41,63]

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Relatively noninvasive and accurate modes of cardiac output (CO) measurements in the parturient undergoing cesarean delivery (CD) include photometric dye dilution techniques, Doppler, 2D or 3D echocardiography, and a lithium dilution (LiDCO) technique. Bioimpedance and LiDCO techniques allow a beat-to-beat determination of CO.
The major hemodynamic changes during CD with neuraxial anesthesia occur during prehydration, block onset and birth.
Neuraxial anesthesia-induced hypotension in parturients undergoing CD is frequent. The maternal and fetal morbidity and mortality associated with hypotension has investigators seeking clinically feasible and reliable ways to predict which parturients are at greatest risk, and which treatment regimens are the most appropriate.
The factors that influence the degree of neuraxial anesthesia-induced hypotension include hydration; maternal positioning during and after the block; block type, height and density; the addition of epinephrine to epidural local anesthetic; the presence of hypertensive disorders of pregnancy; gestational age; the number of gestations; the presence of labor; and the prophylactic or therapeutic use of vasoactive substances. Vascular tone and reactivity to neuraxial anesthesia is a complex system unique to each individual woman.
The most effective strategies for prevention of spinal-induced hypotension include prehydration with a sufficient amount of colloid or co-loading with sufficient crystalloid or colloid, positioning the parturient for surgery in the left uterine displacement position, using a lower dose of intrathecal bupivacaine and, most notably, administering a prophylactic vasoactive agent such as phenylephrine.
Hydration prior to (i.e., 'prehydration') or during (i.e., 'co-loading') neuraxial anesthesia placement can not reliably prevent post-spinal hypotension. Crystalloid solutions are more beneficial in preventing hypotension when used as a co-load instead of as prehydration, whereas colloid solutions can be as beneficial when used in either of these time frames.
As the uterus achieves a contracted state following fetal delivery, aortocaval decompression and an autotransfusion of uterine blood occur, resulting in an increase in cardiac preload. This preload augmentation typically results in an increase in CO through increases in heart rate and stroke volume. A decrease in systemic vascular resistance typically results in an unchanged mean arterial pressure at the time of delivery.
Neuraxial anesthesia, including via the spinal technique, are now considered acceptable methods for providing anesthesia for stable, noncoagulopathic severe preeclamptic parturients undergoing cesarean delivery.
Multiple recent studies have indicated that phenylephrine is more effective with lesser fetal acid base alterations than ephedrine when used either as prophylaxis or treatment for hypotension in both elective and nonelective cesarean deliveries.
The prophylactic administration of phenylephrine, as either an infusion or bolus, has been found to be effective in preventing post-spinal hypotension. Maternal bradycardia may occur.

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Cardiovascular Alterations in the Parturient Undergoing Cesarean Delivery With Neuraxial Anesthesia

Cardiovascular Changes During Pregnancy

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