At the Bedside: IVC Collapse and Right Atrial Pressure
The concept of maximal flow rate is clinically relevant in many clinical situations we experience every day. One of these is the estimation of right atrial pressure (RAP) using ultrasound.
Presume that you are examining a spontaneously breathing patient and you attempt a bedside echocardiogram. As part of the assessment, you evaluate the inferior vena cava (IVC). The ultrasound picture you capture is displayed below:
-
What can you conclude about the RAP?
-
What can you conclude about preload responsiveness?
Right Atrial Pressure (RAP)
When we consider the physiology of IVC collapse, it is important to recall the forces that dictate whether a vessel is patent or not. Patency of a venous vessel is contingent on a a positive transmural pressure (PTM > 0 mmHg) across the vessel. If the transmural pressure falls to 0 mmHg or below, the vessel will collapse and flow will cease.
In humans, venous vessels collapse at a PTM of 0 (zero).
When the PTM < or = to 0, the tube collapses
As blood flows to the heart, the intravascular pressure (red numbers) within the IVC decreases.
Adapted from heart-lung.org
In a spontaneously breathing patient, the pressure in a normal abdomen (PAB-blue) is ~0mmHg at end expiration. At this point in the respiratory cycle, the PTM within abdominal IVC remains above 0mmHg and the IVC remains patent.
During inspiration, as the diaphragm contracts, pleural pressure drops and abdominal pressure (blue number) increases slightly. The drop in pleural pressure is reflected across the RA wall, causing the RA pressure to drop too. Thus, the pressure gradient for blood flow toward the RA is increased (red numbers). In a healthy human, the increased gradient causes the PTM to drop below 0 mmHg. At this point the IVC collapses.
Preload Responsiveness
asdfasdfasdf