Fetal and maternal circulation interface at the placenta, driving gas exchange between mother and fetus. Maternal cardiopulmonary adaptation to pregnancy allows balanced delivery of oxygen to the mother’s tissues and the fetus. Synergistic physiologic changes protect oxygen delivery in normal pregnancy. Maternal plasma volume and red blood cell mass increase, augmenting blood volume by 40% (>1000 mL). The left ventricle dilates and becomes more compliant, increasing stroke volume and cardiac output by 40%. The high oxygen affinity of fetal hemoglobin facilitates oxygen exchange across the placenta. Uterine contractions during labor result in maternal autotransfusion, enhancing oxygen delivery when it is needed most. Clinical experience and animal experimentation indicate that during normal pregnancy, maternal systemic and uterine oxygen delivery far exceed the minimal level necessary to sustain maternal and fetal life—with a remarkable reserve to compensate for life-threatening conditions. During cardiopulmonary arrest, however, oxygen delivery to maternal tissue and the uterus is dramatically reduced or eliminated. Maternal or fetal adaptations to severe insult are insufficient to sustain tissue viability. Death happens in minutes.
Pathophysiologic Rationale for CPR Recommendations in Pregnancy
Some normal physiologic changes of late pregnancy may have deleterious effects on oxygen delivery. By approximately 20 to 24 weeks’ gestation, the gravid uterus begins to compress abdominal and pelvic blood vessels, particularly the inferior vena cava and aorta. This diminishes preload to the heart, decreases maternal stroke volume, and may decrease uteroplacental oxygen delivery. In about 10% of women, these effects are so profound that the patient will become hypotensive in a supine position, even in the absence of illness. Uterine aortocaval compression has 2 important clinical consequences during maternal arrest: (1) it must be temporarily relieved to optimize the effectiveness of CPR and (2) it provides an opportunity to definitively improve maternal survival, since emergent delivery can have a profound benefit on maternal hemodynamics.
In nonpregnant patients, chest compression is estimated to produce cardiac output approximately 30% of normal. Although the effectiveness of chest compressions in pregnancy is unknown, it is likely diminished by aortocaval compression. In healthy late-term pregnancy, the gravid uterus can be shifted off the inferior vena cava if the patient is positioned in left lateral decubitus position, increasing cardiac output by 25%. However, CPR force generation is reduced by left decubitus positioning during arrest. When performing CPR on patients with gestational ages more than 20 weeks, manual displacement of the uterus to the left is a better first option (Fig. 17-1). If left lateral positioning during CPR is attempted, a 27 to 30 degree angle is recommended—but this may be difficult to maintain during chest compressions.
Technique for manual displacement of the uterus to the left during CPR.
The most recent update to ACLS guidelines (2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care) emphasizes the importance of high-quality CPR. Rapid initiation of CPR increases survival to discharge. Even a few seconds of delay between interruption of chest compressions and shock significantly reduces defibrillation success rates. CPR should be started immediately once pulselessness is identified and should not be interrupted—even for intravenous line placement or cesarean delivery—except as briefly as possible (<10 seconds) when indicated by ACLS protocol (eg, to perform rhythm determination, to administer shock, or to rotate CPR providers). Hands should be placed slightly higher on the sternum than usual, accounting for the anatomy of the gravid abdomen and thorax. The optimal rate of compression is 100/min, with a compression depth of 2 in. Complete recoil of the chest should be allowed between compressions. If waveform capnography is used to monitor CPR effectiveness, the partial pressure of end-tidal CO2 should exceed 10 mm Hg. If an arterial line is present, diastolic pressures should exceed 20 mm Hg.
Pathophysiologic Rationale for Perimortem Cesarean Section—the 4-Minute Rule
Although the maneuvers discussed above may partially relieve aortocaval compression, perimortem cesarean delivery should theoretically be even more effective. It has been shown that cesarean delivery immediately increases cardiac output by 30% in healthy women. Delivery drastically reduces the perfusion demands of the uterus and placenta, which consume approximately 30% of maternal cardiac output near term; it also provides an approximate 500-mL autotransfusion. Delivery also allows resuscitative access to the infant.
Katz and colleagues have reviewed all published data on perimortem cesarean deliveries from 1900 through 2004. Their combined analyses show that 71% of babies who survived maternal arrest with good neurologic outcome were delivered in 5 minutes or less (Table 17-1) and that the rate of neurologic injury among survivors increases dramatically if delivery is further delayed. Maternal benefit of perimortem cesarean delivery is less well documented, but many case reports describe maternal recovery from refractory shock upon perimortem cesarean. Publication bias should be considered when interpreting these reports, though.
TABLE 17-1.Interval From Death of Mother Until Perimortem Cesarean Delivery of Surviving Infants: 1900 to 2004 ||Download (.pdf) TABLE 17-1. Interval From Death of Mother Until Perimortem Cesarean Delivery of Surviving Infants: 1900 to 2004
|Interval (min) ||All surviving infants ||% of total healthy infants ||% surviving with neurologic injury |
|0-5 ||54 (1 with CNS injury) ||71% ||2% |
|6-10 ||12 (3 with CNS injury) ||12% ||25% |
|11-15 ||9 (2 with CNS injury) ||9% ||22% |
|>15 min ||11 (5 with CNS injury) ||8% ||45% |
|Total ||86 ||100% || |
There is no rule that says delivery cannot be initiated before 4 minutes. If CPR is ineffective or the cause of the arrest is unlikely to be reversed within 4 minutes (eg, abruption, pulmonary embolism), it is prudent to proceed immediately to cesarean birth.
As pregnancy progresses, increasing aortocaval compromise by the enlarging uterus and improving viability of the fetus leads to 3 clinically important pathophysiologic states, as seen in Table 17-2. At less than 20 weeks, significant aortocaval compression is unlikely, the fetus is not viable, and perimortem cesarean delivery is unlikely to benefit mother or child. From 20 to 24 weeks, maternal hemodynamic benefit and fetal viability are both questionable. At greater than 24 weeks, perimortem cesarean delivery is most likely to benefit mother and infant.
TABLE 17-2.Relationship Between Gestational Age and Possible Maternal and Fetal Benefit of Perimortem C-Section ||Download (.pdf) TABLE 17-2. Relationship Between Gestational Age and Possible Maternal and Fetal Benefit of Perimortem C-Section
|Gestational age ||Aortocaval compression ||Fetal viability ||Perimortem C-section |
|<20 wk || || ||Not indicated |
|20-24 wk ||+ ||+ ||Consider |
|>24 wk ||+++ ||+++ ||Indicated |
If the patient’s obstetrical dates are unknown, they can be estimated by bedside examination. The uterine fundus is usually palpable at the level of the umbilicus at approximately 20 weeks’ gestation and grows cephalad at a rate of 1 cm/wk. Therefore, perimortem cesarean is supported when the uterine fundus is palpable at least 4 cm above the umbilicus in the supine position. Fundal height can be misleading in many cases (eg, multiple gestation, intrauterine growth retardation, oligohydramnios, fibroids, etc) but may be the best available data in a dire emergency. Portable ultrasound can be useful for determining gestational age but should not interfere with resuscitation.
Exact measurements are not necessary in a code situation; the American Heart Association supports intervention on an “obviously gravid uterus,” which describes situations when the clinician decides that the uterus is causing aortocaval compression and the fetus is viable.
A great deal of preparation before and during a code is necessary to successfully perform a perimortem cesarean within 5 minutes, and it is likely that a few achieve this goal. However, maternal survival has been reported after up to a 15-minute delay to perimortem cesarean and infant survival has been noted after up to a 30-minute delay. It is important to remember that when rapid delivery is impossible to achieve, a delayed procedure might still provide benefit.
Pathophysiologic Rationale for Early Endotracheal Intubation
Perimortem cesarean delivery usually requires maternal intubation. The physiologic changes of late pregnancy increase the risk for life-threatening complications during endotracheal intubation roughly 10-fold. Maternal oxygen consumption is significantly increased, lung compression by the gravid uterus reduces functional residual capacity by 20%, and intrapulmonary shunting increases 3-fold. These factors result in rapid oxygen desaturation if gas exchange is interrupted. Edema and hyperemia of the upper airway make airway bleeding more common and visualization of the vocal cords more difficult. Furthermore, if preeclampsia is present, airway edema can be further exacerbated with fluid overload. Additionally, progesterone causes decreased gastric motility while simultaneously relaxing esophageal sphincter tone, thereby increasing risk for aspiration.
Thus, the decision to forego bag-mask ventilation and proceed directly to endotracheal intubation is often more appropriate in the pregnant patient. Intubation should be performed by the most experienced person available and equipment for difficult airways needs to be at the bedside. Aspiration risk can be reduced by avoiding prolonged bag-masking, which causes gastric distention, and by application of cricoid pressure during intubation, which can diminish passive aspiration. Use of neuromuscular blocking agents and rapid sequence intubation can help prevent vomiting that leads to aspiration. Proper “sniff position” of the head and neck facilitates direct laryngoscopy. Airway edema may necessitate the use of slightly smaller endotracheal tubes, and the Bougie endotracheal tube introducer can be helpful when laryngoscopic view is difficult.
Nonemergent intubation during pregnancy may actually cause arrest if airway difficulties arise. Therefore, the obstetric team and a physician experienced in airway management should be notified prior to intubation of all pregnant women. Assessment of the airway for potential difficulty during endotracheal tube placement should be performed and a plan should be formulated for the 3 most favorable methods to secure the airway. Equipment necessary to carry out these alternatives, such as a laryngeal mask airway, video laryngoscope, and percutaneous cricothyrotomy kit, should be present prior to proceeding. Additionally, placement of a nasogastric tube to reduce the risk of aspiration pneumonitis (Mendelson syndrome); preoxygenation with 100% FIO2 should also be performed prior to intubation.
Code Pharmacology and Cardioversion
Little information is available regarding the use of code medications in pregnancy. Vasopressor agents commonly used in ACLS protocols, such as norepinephrine and vasopressin, are known to decrease uterine blood flow. However, the infant’s best chance of survival is maternal survival. Therefore, in full arrest, ACLS pharmacology protocol is not altered by pregnancy. Standard defibrillation/cardioversion energies and electrical impedence of the chest wall are not altered by pregnancy. Moreover, deleterious effects of maternal defibrillation/cardioversion have not been reported in infants.