Consider the Following…
Our crew was dispatched to an outlying facility by fixed wing aircraft for a patient with respiratory difficulty secondary to COVID. What made this case interesting from a transport standpoint wasn’t the COVID diagnosis or the respiratory distress diagnosis but that this patient was also 32 weeks pregnant.
The patient had no significant past medical or surgical history. This was her second pregnancy with good prenatal care and no complications during this pregnancy or her pervious pregnancy. She was being supported at the outside facility on a high flow nasal cannula at 50 LPM blended to 60%. The level of support with the high flow nasal cannula (liter flow and the FIO2 adding up to more than 100) is enough to give us pause. The consideration of physiologic changes during pregnancy also had to be considered to complete this transport safely.
Physiologic Changes in Pregnancy
In a non-pregnant female, approximately 2% of cardiac output goes to the uterus. During pregnancy, the uterus and placenta take around 30% of cardiac output. To compensate for this change in blood distribution, total cardiac output increases 30-50% during pregnancy (recall that cardiac output (CO) is heart rate (HR) x stroke volume (SV)). During pregnancy, the heart rate increases by around 20% and stroke volume increases 20-35%. This increase in stroke volume is also associated with thickening of the left atrium and ventricle. Cardiac output is also affected by preload and afterload. If the gravid uterus is positioned where it can compress the IVC, the decreased venous return can drop cardiac output by 25%. Afterload is reduced during pregnancy due to the vasodilation caused by progesterone.
Total blood volume is increased during pregnancy. An adult on average, has a blood volume around 60 ml/kg. During pregnancy the blood volume increases to 75-95 ml/kg. Most of this extra volume is in the form of plasma meaning that our pregnant patient will have a lower hemoglobin/hematocrit values compared to a non-pregnant patient. One advantage of having a lower hematocrit is a decrease in blood viscosity reducing some of the cardiac workload to pump this additional volume. The downside is there is less oxygen carrying capacity. Hemoglobin levels are the most important variable in the oxygen carrying capacity formula. In order to maintain adequate oxygen delivery in the setting of lower oxygen carrying capacity, the cardiac output has to increase.
Pregnancy is associated with functional changes to the respiratory system. As the pregnancy progresses the diaphragm elevates by 4-7 cm from the non-pregnant state. It seems that this would cause a decrease in tidal volumes but during pregnancy tidal volume actually increases around 150 ml giving our patient an average tidal volume of 600 ml. Normal respiratory rates do not change during pregnancy; however, the increased tidal volumes result in increased minute ventilation. This increase in minute ventilation causes a reduced PaCO2 (28-32 cmH20) and is compensated by decreased HCO3 values (18-21 meq/L). A normal PaCO2 level (35-40 cmH20) in the later stages of pregnancy may indicate impending respiratory failure. The fetus requires oxygen delivery and carbon dioxide removal via the placenta. The placenta is essentially a semipermeable membrane and gas exchange operates under the Fick’s law. Fick’s law states that gas exchange across a semipermeable membrane is determined by the surface area of the membrane, the thickness of the membrane and the relative concentration gradients of the dissolved gasses. The thickness of the placenta is essentially fixed. Vasodilation caused by progesterone results in the blood vessels in the placenta being in a maximally dilated state resulting in a fixed surface area. This means that the only factor of the Fick’s law that can be changed is the concentration of dissolved gasses on either side of the membrane. A normal fetal pCO2 measured from the umbilical artery is around 50. With the mother’s increased minute ventilation and decreased pCO2, this causes a larger concentration gradient allowing increased CO2 diffusion from the fetus to the maternal circulation.
Changes during pregnancy result in decrease functional reserve capacity, decreased chest wall compliance and increased oxygen demands. All of these factors contribute to decreased oxygen reserves and potential for rapid desaturations. Over the course of the pregnancy the Mallampati score typically worsens. There are airway changes from pregnancy related weight gain along with edema of the tongue, pharynx, vocal cords, arytenoids and glottis. A smaller than expected ETT may be required to successfully intubate the patient. Further complicating matters is the increased risk of aspiration due to delayed gastric emptying, higher gastric volumes and decreased GI peristalsis. Just how much of a concern should we have for these physiologic changes? The failed intubation rate is 8-10 times higher and the probability of fatal aspiration during intubation is 7 times higher than the non-pregnant population.
Preparation for Transport
We had a joint discussion between the outside hospital, the receiving hospital, the flight team and our OB team to form a management plan. Although the patient was relatively stable, considering the physiologic issues with pregnancy and the potential for deterioration during transport, the decision was made that she should be intubated prior to transport. She was transported to the OR where she was electively intubated by anesthesia where a wide variety of alternative airway adjuncts were readily available in case of a failed intubation attempt. The patient was intubated on the first attempt without issues. During transport we adjusted the ventilator minute volume to keep an end tidal CO2 around 30. To monitor the fetus, we performed frequent fetal ultrasounds to assess the heart rate. The remained for the transport occurred without incident and our patient continued to make a full recovery. She delivered her child as scheduled without any complications.
Take Home Points
Most obstetrical transports that we are tasked with involve some sort of “obstetrical emergency,” requiring rapid transport to a facility that specializes in high-risk obstetrics. In this case, the primary problem was a respiratory diagnosis. Having said that, our understanding of the physiologic changes during pregnancy assisted us in making the best decisions possible in order to provide a safe transport for the mother and her unborn child. More aggressive interventions that may not have been required had the patient not been pregnant were necessary due to the physiology and resultant pathophysiology that this patient was experiencing.
References
Fundamental Critical Care Support: Obstetrics. Society of Critical Care Medicine. (2017).
ALSO provider manual (June 2017).
Up to date Reference range for umbilical artery blood gas values in preterm newborns and term newborns.