A 42-year-old woman with Crohn’s disease is admitted to the ICU following laparotomy for small bowel obstruction. She is persistently hypotensive despite vasopressor and volume administration. She is breathing spontaneously on high-flow nasal cannula. Ultrasound imaging of her IVC reveals a 1.5 cm vessel diameter and >50% decrease in vessel diameter during inhalation. The critical care fellow concludes that these findings indicate the patient would increase her cardiac output with intravascular volume administration.
Which of the following, if present, would confound that conclusion?
A. PneumothoraxCorrect Answer: C
Ultrasound assessment of the retro-hepatic IVC can provide insights into a patient’s volume status. The assessment hinges around changes in intrathoracic pressure throughout the respiratory cycle being transmitted to the IVC via the right atrium. As a nonmechanically ventilated patient inhales, pressure in the thorax is reduced. This pressure reduction is transmitted to the thin-walled right atrium and then to the IVC.
Patients who are volume-responsive tend to have low right-sided filling pressures and an IVC that has not reached its maximum distensibility. When the right atrial pressure decreases during inspiration, the IVC luminal pressure decreases relative to the intra-abdominal pressure and the vessel collapses. In patients who are not volume responsive, the rightsided filling pressures are higher relative to the changes in thoracic pressure and the vessel size changes less.
The exact cutoff of IVC diameter change for predicting volume responsiveness is not well defined. Some studies identify >40% to 45% collapse during tidal breathing as a cutoff for volume responsiveness in spontaneously breathing nonventilated patients. However, these studies exclude many patients where the technique is prone to drawing the wrong conclusion. If the intra-abdominal pressure is elevated, as in choice C, then the IVC may collapse even in a patient who would not be volume responsive. This would call the fellow’s conclusion into question and makes choice C the correct answer.
Both a hemodynamically significant pneumothorax and pericardial effusion would increase the right atrial pressure without changing the abdominal pressure, leading to a distended IVC even if a patient would indeed be volume responsive. A unilateral DVT would not alter venous return enough to be the correct answer in this case.
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