module 3

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Understand the conditions under which the inferior vena cava [IVC] will dilate in response to a mechanical breath; explain the reasoning as a clinical physiologist.

Hypothesize why IVC dilation will vary as a function of tidal volume.

Hypothesize why IVC dilation will vary as a function of chest wall compliance.

Critique IVC dilation as a marker of volume status and volume responsiveness.

Understand the IVC compliance curve.

You are a fellow in cardiac anesthesiology participating in a routine coronary artery bypass grafting [CABG]; it is early in your fellowship and you are getting used to intra-operative transesophageal [TEE] monitoring.  Following induction of anesthesia you get your bearings with the TEE and wonder if you can identify the inferior vena cava [IVC].  You readily find it and you note to yourself that with each ventilator breath, there is significant IVC distention.  You recall from your ICU rotation that this patient would likely respond to a fluid challenge.

As you recall the physiology, the attending cardiac anesthesiologist strolls in and wonders why you have set such a large tidal volume; she immediately cuts the tidal volume in half.  Simultaneously, the inspiratory IVC distention goes away. 

T or F: the patient is no longer fluid responsive
T or F: the patient is now hypervolemic
T or F: the patient is no longer paralyzed

Preparations continue and you notice an enthusiastic medical student scrubbed in, standing on a stool before the drape.  As you predict, the ventilator starts to alarm with rising plateau pressure.  Before you chastise the student for leaning on the patient’s chest you look back to the IVC and note that inspiratory distension is once again present.  You calmly tell the student to ‘back off’ and the IVC inspiratory engorgement, again, vanishes.

T or F: when the surgeons open the thorax, inspiratory IVC distension will return

Think back to any patient fully passive with the ventilator [i.e. heavily sedated, paralyzed, not triggering the ventilator] whom you have evaluated.  Have you ever performed ultrasonographic evaluation of the IVC?  In this situation would you expect to see IVC collapse on inspiration?  Why?  What about ultrasonographic assessment of the jugular vein?

1. Explain to each other how the cardiac function curve [on the Guyton Diagram] shifts as a result of an increase in intra-thoracic pressure in a paralyzed, mechanically ventilated patient.  How does tidal volume magnitude affect the degree of shift?  How does chest wall stiffness affect the degree of shift?  Use the concept map that you created in module 1 here.

2. Explain how the central venous pressure affects IVC dimensions.  With your partner, create a ‘mind map’ of the determinants. 

3. Hypothesize what might happen to the IVC of an intubated, volume-overloaded heart failure patient in whom the paralytic wears off.

4. Imagine that you and your partner are designing a clinical trial to examine the relationship between inspiratory change in jugular venous diameter and volume responsiveness in mechanically-ventilated patients.  One person describe what one would look for if the patient population were paralyzed.  The other person describe what one would look for if the patient population were triggering breaths?  Each person should defend his or her reasoning.

5. Reconsider the true of false questions presented in the case above.  Use the Guyton diagram and your concept map from questions 1 & 2 as an explanatory tool.

6. Reflect on the open access paper on the IVC.  Does it help you meet the intended learning outcomes mentioned at the outset of this module?  If not, which outcomes does it not specifically address?  Have you and your partner met all the learning outcomes?  If not, create a plan on how to do so and meet again to discuss your progress.  Do this iteratively until you are both satisfied with your understanding.