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Refresher Courses - Sunday June 10 |
From the Department of Anesthesia, Sunnybrook Campus, Sunnybrook & Women's College, Health Sciences Centre, M3-208, 2075 Bayview Avenuem Toronto, Ontario, Canada.
Address correspondence to: Dr. Donald Penning, Department of Anesthesia, Sunnybrook Campus, Sunnybrook & Women's College Health Sciences Centre, M3-208, 2075 Bayview Avenue, Toronto, Ontario, Canada M4N 3M5. Phone: 416-480-4794; Fax: 416-480-6039; E-mail: penni008{at}mc.duke.edu
The clinical issues
Consider the following hypothetical patient: a 25-yr-old pregnant woman at 30 weeks gestation arrives in the emergency room (ER). (Already we are fortunate since this valuable information is usually not immediately available. The gestational age is very helpful in making obstetrical decisions regarding the timing of delivery – often ultrasound scanning is the only available option). She has been removed from a high-speed car crash. At the scene she was found wearing seat restraints and was conscious. Her obvious injuries are some facial lacerations and a fractured femur. When you meet her as the trauma team anesthesiologist in the ER she is in pain and slightly disoriented. Her vital signs are blood pressure (BP) 90/50 mmHg, heart rate (HR) 110 beats•min–1, room air oxygen saturation (SaO2) 95% and temperature 35.5°C. Many questions flash through your mind as you click into the routine of the ABC's of resuscitation (always the first step). What if she needs to be intubated? What should we do about preterm labour? How does one assess the fetus? If she goes to the OR, should I monitor the fetus? What drugs are safe? Is something else going on? If I need x-rays are they o.k. for the baby? What should we do if the fetus demonstrates the need for urgent delivery and the mother's condition makes that difficult? These and many other questions arise when anesthesiologists (or any physician) are called upon to care for these challenging patients in the operating room, intensive care unit (ICU), ER or delivery suite. Unfortunately, all these questions cannot be addressed in the space provided, and even if space were not limited all the answers are not available. Luckily, most situations can be managed by any competent anesthesiologist or critical care physician and obstetrician armed with a few basic principles.
The sight of an obviously pregnant woman, a trauma victim, being wheeled out of an ambulance into the ER is a troubling sight to any medical practitioner. There are many reasons for this. The image evokes many family images and emotional challenges; there is the care of two patients to consider. Nothing can reduce the stress of this situation to zero but being prepared can help. In this lecture we will examine some of those basic principles that are usually helpful. As well, there are some specific studies from the literature to provide an evidence basis for decision making. Despite a relative lack of studies in the field, several reviews on this topic have appeared in the last ten years.1–5
Epidemiology
Trauma is the most important cause of non-obstetric death in pregnant women.2 Reports list that trauma or accidental injury complicates as many as 6–8% of all pregnancies.5,6 It is estimated that 0.5% of all pregnant women require ICU admission, and many of these were as a result of trauma.7
A study was carried out involving patients at a large teaching hospital plus a large community hospital in North Carolina.2 Using the International Classification of Diseases, Clinical Manifestation, 9th ed. (ICD-9) codes there were 514 pregnancies in a six-year (1987–1993) period that were complicated by trauma. In that study most were motor vehicle accidents (MVA's) (55%) while domestic abuse and assaults (22%), falls (22%), burns, puncture wounds or animal bites (1%) accounted for most of the balance of injuries. The mean gestational age was 25.9 weeks with a mean gestational age at delivery of 37.9 weeks. Preterm labour occurred in 11.4% and abruption occurred in 2% of cases. Another large study from Maryland using ICD-9 code designations identified 80,311 hospitalisations for trauma in women considered of childbearing age. Two thousand one hundred and eighty-five (2,185) of these patients were pregnant. In this cohort of pregnant women the major causes of hospitalisation were poisoning (16.9%), fractures (14.7%), and sprains (10.9%).8 The differences between these two studies illustrates how variance in coding plus regional demographics can make comparisons confusing.
Important physiologic changes of pregnancy
The physiologic changes of pregnancy have been well described9 but there are several that must be emphasised here. In the respiratory system, weight gain including breast enlargement and soft tissue swelling in the mouth and upper airways may make tracheal intubation difficult. This problem is exaggerated in preeclampsia. Tissue friability is also increased and this makes nasal instrumentation increasingly hazardous. A decrease in functional residual capacity (FRC) and increased oxygen consumption lead to rapid maternal oxyhemoglobin desaturation if intubation is delayed. This is particularly true in the supine position as early airway closure can occur with the reduction of FRC.7 For these reasons, preoxygenation should be used and intubation attempted by the most skilled operator. Also, while high-inspired oxygen may have little effect on maternal oxygen saturation, the high PO2 developed can raise fetal PO2 2–10 mmHg. This can result in a large increase in fetal hemoglobin oxygen saturation since the fetus usually operates on the steep portion of its oxygen dissociation curve.7 A familiarity with the normal blood gas values is valuable particularly if mechanical ventilation is considered or required. The PO2 and pH are similar to the non-pregnant state but the PCO2 is usually around 32 mmHg. Thus, a PCO2 of 32 mmHg would not be considered hyperventilating and 40 mmHg would not be considered "o.k.". The increased respiratory drive leading to this normal degree of hypocapnia is thought to be due to a progesterone-mediated effect.
Important changes also take place in the cardiovascular system. Beginning in the 8th week of pregnancy, by the second trimester the cardiac output rises to 40% above normal. This increases further during labour and is at its highest in the minutes immediately post-partum – a particularly vulnerable period. Cardiac output remains at the high pre-labour values for two days post-partum and then slowly declines to pre-pregnant values over the next two weeks.7 Echocardiography is commonly used in critical care settings today and there are several normal changes of pregnancy that should be noted. By 32 weeks increases in left ventricular (LV) wall thickness (28%) and LV wall mass (52%) are measured. These changes return to normal over a 24-week period post-partum.7 The "supine hypotensive syndrome" is another pregnancy-related cardiovascular change. The enlarging gravid uterus can obstruct or occlude the inferior vena cava or the aorta when in the supine position. This condition can lead to a reduction in cardiac output and cause hypotension that the mother in the supine position or sleeping may not notice. The hypotension could however reduce uterine blood flow by reducing uterine arterial pressure. Also an increase in venous pressure below the obstruction could further reduce perfusion pressure in the uterus. This condition becomes important after the 20th week of gestation and increases steadily after that. Slight lateral tilt such as produced by a wedge under the hip or lying the patient in the lateral position should prevent this. This phenomenon should be considered when transporting patients or during radiological procedures.
There are a number of important hematological changes in pregnancy. The normal red cell mass increases but because of a greater increase in plasma volume there is a so-called anemia of pregnancy. The increase in blood volume allows the mother to easily tolerate the normal blood loss associated with vaginal (500 mL) or Cesarean birth (600–700 mL). An increase in coagulation factors and decreased fibrinolysis, especially at term, also plays a role in this adaptation.6
In the genitourinary system there is an increase in renal blood flow by about 60% leading to an increase in glomerular filtration rate. This has the effect of reducing serum blood urea nitrogen (BUN) and serum creatinine by about half. Thus, a relatively "normal" BUN and creatinine may reflect a seriously compromised renal function. The gravid uterus may obstruct or impede urine outflow thus ultrasound evidence of a dilated renal pelvis is not uncommon.7
Special topics: fetal monitoring
Several studies addressed the issue of prolonged fetal heart rate (FHR) monitoring after trauma. In the large North Carolina study cited previously2 there were no adverse fetal outcomes linked to the trauma when the FHR tracing was normal (97% of the cases) and if there were no early warning symptoms. The authors concluded that there were no indications for prolonged fetal monitoring beyond four hours if all other indicators were normal. A separate study investigated the identification of pregnant patients who can be safely monitored FHR for less than six hours following trauma.10 Using logistic regression analysis of 271 pregnant women sustaining blunt trauma they concluded patients without risk factors for preterm labour (gestational age >35 weeks, assaults, pedestrian collisions) or fetal loss (ejections, motorcycle or pedestrian collisions, maternal tachycardia, abnormal FHR, lack of restraints and injury severity score (ISS) >9) could safely be discharged after only six hours of FHR monitoring.
The issue of intraoperative or ICU fetal monitoring is more complex. Maternal injuries or surgical needs often make monitoring difficult or impossible. Although often possible at 18 weeks' gestation, FHR monitoring may not be reliable until 22 weeks' gestation.11,12 It is often argued that fetal surveillance is not warranted prior to fetal viability (a moving target) but several points argue in its favour. First, age is often difficult to establish and the zone from 22–24 weeks may be uncertain. Also, advances in neonatology are narrowing this gap. Another argument is that not all changes detected as abnormal must lead to delivery. Often simple measures such as position changes, fluid or oxygen administration or manipulations of blood pressure may improve the fetal condition even if delivery is impractical because of gestational age or maternal condition. Many pharmacological agents commonly used in trauma affect the FHR trace. Most sedative drugs and anesthetics completely suppress beat-to-beat FHR variability, a common sign in healthy fetuses greater than 28 weeks.11 In this common situation, one has to rely on changes of baseline heart rate and intervene if prolonged decreases in baseline FHR occur.
Radiology
The interpretation of pregnant and non-pregnant chest radiographs should apply the same criteria, however it should be noted that fullness of the cardiac silhouette, increased breast density and pulmonary vascularity are usual findings.6
Radiation risk to the fetus should not be ignored (e.g., there may be a slightly increased risk of childhood leukemias) but it would be misguided to jeopardise maternal well being by withholding necessary radiological procedures. That being said, every effort should be made to limit fetal exposure using good clinical judgement and appropriate shielding. It has been estimated that the exposure to the fetus of 5 rads or less of ionising radiation (e.g., pelvic computer tonography scan 5000 millirads; chest x-ray 1–8 millirads) carries a risk far less than the risk of spontaneous congenital malformations.7
Head injury and neuro-intensive care
A number of special topics arise when considering the neuro-trauma patient who is pregnant.13 The management of raised intracranial pressure using osmotic diuretics (e.g., mannitol) and hyperventilation has been studied. Normal plasma osmolarity in pregnancy is slightly reduced to about 280 mOsmol•kg–1 water.7 Mannitol has been administered to rabbits and shown to increase fetal plasma osmotic pressure.14 In this study it was demonstrated that water was drawn from the fetus to the mother resulting in severe fetal dehydration. The results of this are somewhat unclear since the osmotic pressure was increased to >400 mOsm•kg–1 water! In perhaps a more meaningful study, 200 g of mannitol were administered to pregnant women volunteers at term, one hour prior to delivery.15 This had the effect of raising their serum osmotic pressure from 290 to 320 mOsmol•kg–1 water. When the fetal levels were determined it showed a fetal osmotic pressure of 312 mOsmol•kg–1 water. In clinical usage, 100 g mannitol has been safely administered to a pregnant neurosurgical patient.15
Hyperventilation, while controversial,16 is a mainstay of the treatment of raised intracranial pressure. The normal arterial PCO2 is 32 mmHg thus hyperventilation may require levels of CO2 that are extremely low. It has been suggested that extreme hypocapnia causes direct uterine vasoconstriction possibly leading to fetal hypoxia.17 It is likely not so simple. In an elegant study it was demonstrated that mechanical ventilation did indeed lead to a reduction in uterine blood flow.18 However, if the inspired CO2 was increased to maintain normocapnia the same degree of reduction in uterine blood flow occurred. Thus the hyperventilation rather than the hypocapnia lead: to a reduction in uterine blood flow, likely through mechanical reduction in venous return and subsequent decrease in cardiac output. While it is nice to understand the mechanism for the reduction, it remains that the effective range for hyperventilation is reduced in pregnancy and caution must be exercised when approaching levels of arterial CO2 of 24 mmHg or less.
Conclusion
An exhaustive review of the care of the pregnant trauma patient would fill a book. Nevertheless, an understanding of the normal physiologic changes of pregnancy is virtually all there is to go on except in a few special situations such as listed above. Close communication with critical care, surgical and obstetrical colleagues are essential. This is certainly a clinical situation where a truly multi-disciplinary approach brings positive results. With this is mind it should be possible to confidently care for these special patients.
References
1 Pearlman MD. Blunt trauma during pregnancy. N Engl J Med 1990; 323: 1609–13.[Medline]
2 Connolly A, et al. Tauma and pregnancy. Am J Perinatol 1997; 14: 331–6.[Medline]
3 Shah, KH, et al. Trauma in pregnancy: maternal and fetal outcomes. J Trauma 1998; 45: 83–6.[Medline]
4 Aitokallio-Tallberg A, Halmesmaki E. Motor vehicle accident during the second or third trimester of pregnancy. Acta Obstet Gynecol Scand 1997; 76: 313–7.[Medline]
5 Kissinger DP, et al. Trauma in pregnancy - predicting pregnancy outcome. Arch Surg 1991; 126: 1079–86.[Abstract]
6 Clark SL, et al. Critical Care Obstetrics 3rd ed. Malden, Massachusetts: Blackwell Science, 1997: 763.
7 Lapinsky S, Kruczynski K, Slutsky A. Critical care in the pregnant patient. Am J Respir Crit Care Med 1995; 152: 427–55.[Medline]
8 Greenblatt JF, Dannenberg AL, Johnson CJ. Incidence of hospitalized injuries among pregnant women in Maryland, 1979-1990. Am J Prevent Med 1997; 13: 374–9.[Medline]
9 Conklin KA. Physiologic changes of pregnancy. In: Chestnut DH (Ed.). Obstetric Anesthesia: Principles and Practice. St. Louis, MO: Mosby, 1994: 17–42.
10 Curet MJ, et al. Predictors of outcome in trauma during pregnancy: identification of patients who can be monitored for less than 6 hours. J Trauma 2000; 49: 18–25.[Medline]
11 Biehl DL. Foetal monitoring during surgery unrelated to pregnancy. Can Anaesth Soc J 1985; 32: 455–9.[Medline]
12 Liu PL, et al. Foetal monitoring in parturients undergoing surgery unrelated to pregnancy. Can Anaesth Soc J 1985; 32: 525–32.[Medline]
13 Penning DH. Fetal effects of anesthesia. In: Loftus CM (Ed.). Neurosurgical Aspects of Pregnancy. Park Ridge, Illinois: American Association of Neurological Surgeons, 1996: 29–38.
14 Bruns PD, et al. The placental transfer of water from fetus to mother following the intravenous infusion of hypertonic mannitol to the maternal rabbit. Am J Obstet Gynecol 1963; 86: 160–7.
15
Battaglia F, et al. Fetal blood studies XIII. The effect of the administration of fluids intravenously to mothers upon the concentrations of water and electrolytes in plasma of human fetuses. Pediatrics 1960; 25: 2–10.
16 Dexter F. Research synthesis of controlled studies evaluating the effect of hypocapnia and airway protection on cerebral outcome. J Neurosurg Anesth 1997; 9: 217–22.[Medline]
17 Morishima HO, et al. Effects of positive pressure ventilation of the mother upon the acid-base state of the fetus. Am J Obstet Gynecol 1965; 93: 269–73.
18 Levinson G, et al. Effects of maternal hyperventilation on uterine blood flow and fetal oxygenation and acid-base status. Anesthesiology 1974; 40: 340–7.[Medline]
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