Although, most pregnant patients are young and healthy and less than 1% will require admission to the intensive care unit, the overall acuity of this patient population remains high.Most admissions will be secondary to obstetric complications such as hypertensive disorders from eclampsia/severe preeclampsia and hemorrhage. However, there are many other conditions for which a critical care practitioner should be prepared. To fully understand how to take care of this vulnerable patient population, it is important to understand the basic physiologic adaptations that occur in the mother in response to the demands of pregnancy (Table 66.1).
OXYGENATION AND RESPIRATORY SUPPORT DURING PREGNANCY
Normal physiologic changes of pregnancy increase oxygen delivery to the placenta and the fetus. Fetal oxygenation is dependent on maternal oxygen status. It is important to note that pregnancy is a chronically compensated state of respiratory alkalosis with a maternal pH of 7.40 to 7.47. Respiratory alkalosis is secondary to the increase in minute ventilation, which leads to a decrease in PaCO2. This is compensated for by increased renal excretion of bicarbonate (normal value in pregnancy: 18 to 22 mEq/L).
Pregnancy itself increases oxygen consumption by 15% to 20%. Bearing in mind the physiologic changes in ventilation during pregnancy and the fetal oxygen dissociation curve, tighter parameters for oxygenation are required to maintain maternal and fetal well-being. The same respiratory support devices used in nonpregnant patients may be used in pregnant patients. Respiratory support should be aggressive, and the goal should be to maintain maternal SpO2 ≥95% to 96% as to sustain adequate oxygen perfusion to the fetus. When determining necessity of intubation, airway edema related to pregnancy and decreased maternal reserve should be considered.
RESPIRATORY FAILURE
Diagnosis of respiratory distress may be difficult in the gravid patient, because about half of all pregnant women will have complaints of shortness of breath, fatigue, and decreased exercise tolerance. A careful evaluation of these symptoms is imperative to discern between normal pregnancy complaints and respiratory compromise. The most common causes of respiratory failure in pregnancy are pulmonary embolus, pulmonary edema (secondary to preeclampsia, cardiomyopathy, or tocolytic induced), infection, and asthma. Clinical recognition and treatment of a pregnant patient in respiratory failure are extremely important, as maternal oxygen status affects fetal oxygen status. A change in fetal heart rate pattern may be one of the first signs of maternal respiratory failure. A general rule of thumb is to stabilize the maternal condition before considering delivery. If indicated, initiating mechanical ventilation in the mother will likely improve fetal condition (Algorithms 66.1 and 66.2).
TABLE 66.1 Physiologic Changes of Pregnancy
Cardiovascular
Blood pressure
Heart rate
Cardiac output
Stroke volume
Systemic vascular resistance
Decreased (reaches a nadir in the second trimester)
Increased (17%)
Increased (40%)
Increased (25%)
Decreased (20%)
Hematologic
Blood volume
Coagulation factors
Red blood cell mass
White blood cell count
Hemoglobin
Platelets
Increased (40%-50%)
Increase in fibrinogen, vWF, clotting factors II, VII, VIII, IX, and X, and protein C resistance. Decrease in protein S
Increased
Increased
Decreased (12.5 g/dL at term)
Decreased (secondary to hemodilution)
Renal
Glomerular filtration rate
Renal plasma flow
BUN and creatinine
Urinary protein excretion
Increased (50%)
Increased (50%-75%)
Decreased
Increased
Pulmonary
Tidal volume
Respiratory rate
Minute ventilation
Vital capacity
FEV1
Residual volume
FRC
Inspiratory capacity
Increased (40%)
Little to no change
Increased (40%)
Unchanged
Unchanged
Decreased (20%)—secondary to elevated diaphragm
Decreased (20%)—secondary to elevated diaphragm
Increased (5%-10%)—secondary to decreased FRC
vWF, von Willebrand factor; BUN, blood urea nitrogen; FEV1, forced expiratory volume in the first second of expiration; FRC, functional residual capacity.
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