Physiologic Changes in Pregnancy
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Virtually every organ system undergoes anatomic or physiologic change in pregnancy. It is critical that the emergency team have an appreciation of these changes in order to properly manage the pregnant trauma victim.
Respiratory
Increased levels of estrogen cause connective tissue in the respiratory tract to swell. This increase in mucosal congestion in the oropharynx predisposes the patient to bleeding during intubation. In addition, pharyngolaryngeal and vocal cord edema may make it difficult to pass an endotracheal tube. These combined changes, along with increased breast size and generalized weight gain, make the pregnant patient at risk for difficult airway management should intubation become necessary.
The increased metabolic demands of pregnancy increase oxygen consumption in the gravida. Minute ventilation increases approximately 40% as the maternal respiratory rate changes, largely as a result of increased tidal volume. The diaphragm moves upward approximately 4 cm and the anteroposterior diameter increases. These anatomic alterations, along with the enlargement of the uterus, can cause a decrease in expiratory reserve and residual lung volumes, which in turn decrease the functional residual capacity by about 20%. This, along with an increase in oxygen consumption in pregnancy, can result in rapid maternal desaturation with depressed respirations or apnea.
While the pregnant patient will desaturate very quickly, the fetus is even more vulnerable to hypoxia. The reason for this is that the umbilical vein and artery have a much lower partial pressure of oxygen than the maternal circulation. Fetal oxygenation will remain constant until the maternal paO2 drops below 60 mmHg. Once this occurs, the fetus has approximately 2 minutes of oxygen reserve. Some studies have shown that significant maternal hypoxia results in a 30% reduction in uterine blood flow, which can further compromise fetal outcomes.
The pregnant patient is also at risk for aspiration. Increased production of progesterone decreases gastrointestinal motility and encourages laxity in the lower gastroesophageal sphincter. This, in combination with the displacement of the stomach by the uterus, places the pregnant patient at greater risk for aspiration.
Cardiovascular
There are several changes in the cardiovascular system which need to be considered when caring for the pregnant trauma patient. As the patient begins to physiologically prepare for the blood loss associated with delivery, blood volume increases by 50% and there is a 30% increase in erythrocyte volume. There is also a corresponding increase in plasma volume relative to red blood cell volume resulting in a physiologic anemia. A pregnant patient can hemorrhage up to 2000ml of blood before there is any change in her heart rate or blood pressure. However, once the blood volume loss approaches 2500ml, rapid deterioration can occur. Tachycardia and hypotension should be considered late signs of severe hemorrhage and/or shock. Hypotension should be aggressively treated and the increase in blood volume should be considered when determining needs for resuscitation.
Cardiac output increases by up to 50% and peaks somewhere between 20 and 30 weeks gestation. Uterine blood flow comprises approximately 20% of the cardiac output (up to 600ml/minute) and can be a significant source of hemorrhage during trauma. Uterine blood flow has no autoregulation and is completely dependent on maternal blood pressure for blood flow. Any changes in blood pressure can negatively impact uterine blood flow and in turn, compromise fetal oxygenation. To preserve maternal circulation during hemorrhage, blood is shunted away from the uterus and fetus via uteroplacental vasoconstriction. This makes fetal distress, such as fetal decelerations or low variability in the fetal heart rate a subtle sign of compensated shock in the mother. Fetal distress equals maternal hemorrhage until proven otherwise.
When a pregnant patient is in the supine position, uterine compression of the inferior vena cava, abdominal aorta and iliac arteries can cause a significant decrease in venous return. This autocaval compression can result in a 30% decrease in cardiac output and can cause maternal pallor, sweating, nausea, vomiting, hypotension, tachycardia and mental status changes. Also of note is that this increased venous pressure can contribute to dangerous hemorrhage from lower extremity wounds. The use of the saphenous and femoral veins are also less preferred for medication administration due to the effects of this venocaval compression.
Pregnancy is also a hypercoaguable state with an increase in fibrinogen levels and most procoagulant factors. This may be of benefit to a trauma patient in achieving homeostasis after an injury, but can also place the patient at risk for thromboembolic complications including pulmonary embolism.
Gastrointestinal
The gradual growth and distention of the peritoneum as the uterus expands can desensitize the patient to peritoneal injury. Due to this, abdominal tenderness, rebound and guarding may not be present during an examination. The spleen becomes engorged with blood and is at greater risk for rupture. The small bowel is displaced toward the head and can increase the risk of bowel injury after penetrating trauma.
Genitourinary
The most significant genitourinary change during pregnancy is the growth and enlargement of the uterus. After 12 weeks gestation, the uterus rises above the pelvic brim and becomes an abdominal organ. The fetus is protected by a large amount of amniotic fluid and is well-cushioned. In the third trimester, the uterus becomes larger and more thinly walled, making it more susceptible to injury. Toward the end of the pregnancy, the fetal head may drop into the maternal pelvis, which makes it prone to injury in the event of a maternal pelvic fracture. The bladder is also more susceptible to injury as the uterus pushes it out into the abdomen. The increase in uterine mass and a concurrent increase in uterine blood flow place the pregnant patient at significant risk for exsanguination with a major uterine injury.
Later in pregnancy, as the pelvis changes in preparation for delivery, pelvic x-rays may be difficult to interpret. These changes include widening of the pubic symphysis and sacroiliac joint space which can appear abnormal on pelvic films.
Renal changes include a 60% increase in renal blood flow and an increase in the glomerular filtration rate (GFR). Due to this, the serum creatinine decreases by half. A "normal" creatinine in a pregnant trauma patient is an important sign of renal impairment. Also, an increased GFR results in increased frequency of urination which renders urine output a poor indicator of shock.
Respiratory
Increased levels of estrogen cause connective tissue in the respiratory tract to swell. This increase in mucosal congestion in the oropharynx predisposes the patient to bleeding during intubation. In addition, pharyngolaryngeal and vocal cord edema may make it difficult to pass an endotracheal tube. These combined changes, along with increased breast size and generalized weight gain, make the pregnant patient at risk for difficult airway management should intubation become necessary.
The increased metabolic demands of pregnancy increase oxygen consumption in the gravida. Minute ventilation increases approximately 40% as the maternal respiratory rate changes, largely as a result of increased tidal volume. The diaphragm moves upward approximately 4 cm and the anteroposterior diameter increases. These anatomic alterations, along with the enlargement of the uterus, can cause a decrease in expiratory reserve and residual lung volumes, which in turn decrease the functional residual capacity by about 20%. This, along with an increase in oxygen consumption in pregnancy, can result in rapid maternal desaturation with depressed respirations or apnea.
While the pregnant patient will desaturate very quickly, the fetus is even more vulnerable to hypoxia. The reason for this is that the umbilical vein and artery have a much lower partial pressure of oxygen than the maternal circulation. Fetal oxygenation will remain constant until the maternal paO2 drops below 60 mmHg. Once this occurs, the fetus has approximately 2 minutes of oxygen reserve. Some studies have shown that significant maternal hypoxia results in a 30% reduction in uterine blood flow, which can further compromise fetal outcomes.
The pregnant patient is also at risk for aspiration. Increased production of progesterone decreases gastrointestinal motility and encourages laxity in the lower gastroesophageal sphincter. This, in combination with the displacement of the stomach by the uterus, places the pregnant patient at greater risk for aspiration.
Cardiovascular
There are several changes in the cardiovascular system which need to be considered when caring for the pregnant trauma patient. As the patient begins to physiologically prepare for the blood loss associated with delivery, blood volume increases by 50% and there is a 30% increase in erythrocyte volume. There is also a corresponding increase in plasma volume relative to red blood cell volume resulting in a physiologic anemia. A pregnant patient can hemorrhage up to 2000ml of blood before there is any change in her heart rate or blood pressure. However, once the blood volume loss approaches 2500ml, rapid deterioration can occur. Tachycardia and hypotension should be considered late signs of severe hemorrhage and/or shock. Hypotension should be aggressively treated and the increase in blood volume should be considered when determining needs for resuscitation.
Cardiac output increases by up to 50% and peaks somewhere between 20 and 30 weeks gestation. Uterine blood flow comprises approximately 20% of the cardiac output (up to 600ml/minute) and can be a significant source of hemorrhage during trauma. Uterine blood flow has no autoregulation and is completely dependent on maternal blood pressure for blood flow. Any changes in blood pressure can negatively impact uterine blood flow and in turn, compromise fetal oxygenation. To preserve maternal circulation during hemorrhage, blood is shunted away from the uterus and fetus via uteroplacental vasoconstriction. This makes fetal distress, such as fetal decelerations or low variability in the fetal heart rate a subtle sign of compensated shock in the mother. Fetal distress equals maternal hemorrhage until proven otherwise.
When a pregnant patient is in the supine position, uterine compression of the inferior vena cava, abdominal aorta and iliac arteries can cause a significant decrease in venous return. This autocaval compression can result in a 30% decrease in cardiac output and can cause maternal pallor, sweating, nausea, vomiting, hypotension, tachycardia and mental status changes. Also of note is that this increased venous pressure can contribute to dangerous hemorrhage from lower extremity wounds. The use of the saphenous and femoral veins are also less preferred for medication administration due to the effects of this venocaval compression.
Pregnancy is also a hypercoaguable state with an increase in fibrinogen levels and most procoagulant factors. This may be of benefit to a trauma patient in achieving homeostasis after an injury, but can also place the patient at risk for thromboembolic complications including pulmonary embolism.
Gastrointestinal
The gradual growth and distention of the peritoneum as the uterus expands can desensitize the patient to peritoneal injury. Due to this, abdominal tenderness, rebound and guarding may not be present during an examination. The spleen becomes engorged with blood and is at greater risk for rupture. The small bowel is displaced toward the head and can increase the risk of bowel injury after penetrating trauma.
Genitourinary
The most significant genitourinary change during pregnancy is the growth and enlargement of the uterus. After 12 weeks gestation, the uterus rises above the pelvic brim and becomes an abdominal organ. The fetus is protected by a large amount of amniotic fluid and is well-cushioned. In the third trimester, the uterus becomes larger and more thinly walled, making it more susceptible to injury. Toward the end of the pregnancy, the fetal head may drop into the maternal pelvis, which makes it prone to injury in the event of a maternal pelvic fracture. The bladder is also more susceptible to injury as the uterus pushes it out into the abdomen. The increase in uterine mass and a concurrent increase in uterine blood flow place the pregnant patient at significant risk for exsanguination with a major uterine injury.
Later in pregnancy, as the pelvis changes in preparation for delivery, pelvic x-rays may be difficult to interpret. These changes include widening of the pubic symphysis and sacroiliac joint space which can appear abnormal on pelvic films.
Renal changes include a 60% increase in renal blood flow and an increase in the glomerular filtration rate (GFR). Due to this, the serum creatinine decreases by half. A "normal" creatinine in a pregnant trauma patient is an important sign of renal impairment. Also, an increased GFR results in increased frequency of urination which renders urine output a poor indicator of shock.