module - 4

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Recognize the physiological differences between patients who are breathing spontaneously, those who are paralyzed and adapted with a mechanical ventilator, and those who are mechanically ventilated but triggering a breath.

Understand and explain the mechanism of pulse pressure variation in the aforementioned patients; relate this to the detection of a pulsus paradoxus at the bedside.

Critique the use of pulse pressure variation from the perspective of a clinical physiologist.

Predict the effects of various hemodynamic interventions on pulse pressure variation.

Expand upon one’s knowledge of the physiology of IVC diameter change in the aforementioned patients.

A 32 year old woman with primary pulmonary hypertension [on epoprostenol] presents to the emergency department with 10 days of fevers, chills and dyspnea which have accelerated over the last 2 days.  She has neglected taking her diuretics due to malaise and, as a result, over the last two weeks she has gained 5 kg of lower extremity and sacral edema.  The patient appears toxic, is febrile and has a saturation of 82%.  Her influenza A swab is positive and her CXR reveals bilateral patchy infiltrates.  When her blood pressure falls to 70/45, the resident in the emergency department quickly places a radial arterial line, sends an arterial blood gas and performs an ultrasonographic assessment of her inferior vena cava.

-Do you expect her IVC to be diminutive and collapsing or engorged and unvarying?  Why?
-Do you expect her to have significant pulse pressure variation in her radial arterial line tracing?
-Would you administer a fluid challenge empirically?  What would convince you not to give fluids?  Why?

The resident notes significant pulse pressure variation in the patient’s arterial line tracing, but an IVC that is more than 2 cm in diameter and does not change with inspiration; she is unsure as to whether fluids are appropriate to treat the patient’s hypotension.  At that moment the ABG returns with the following [taken on 3 L via nasal cannula] pH = 7.1, PaCO2 = 68 mmHg, PaO2 = 48 mmHg; the patient looks exceptionally unwell and preparations are made for rapid sequence intubation [RSI]. 

Following intubation, the patient is delivered a 500 mL tidal volume with initial PEEP of 5 cm H2O and her blood pressure falls further.  While the patient is paralyzed [following RSI], pulse pressure variation remains in her arterial line tracing and her IVC continues to be engorged.  Given the significant variability in the patient’s pulse pressure, the residents opts to give a 500 mL crystalloid challenge.

-Do you think a fluid challenge is the appropriate intervention?  Why?
-How would you describe the patient’s volume status now?  Volume responsiveness?

Minutes later, the paralytic wears off and the patient begins to trigger ventilator breaths with significant inspiratory effort.  The patient’s blood pressure begins to fall further and the resident plans an epinephrine infusion – the ICU team has been activated. 

When the ICU fellow arrives, he performs a limited ultrasound noting inspiratory collapse of the IVC.  He demands a fluid bolus immediately.   

Reflect on a patient you've cared for in whom you've noticed pulse pressure variation in either an arterial line tracing or oximetry tracing.  What have you thought of this physiology?  What is your understanding of how this relates to volume responsiveness?  Does pulse pressure variation play a role in how you think about pulsus paradoxus?  How would you explain this physiology to a curious medical student?

1. Please meet with your partner – one of you draw the arterial line tracing for the spontaneously breathing [without assistance from a ventilator] patient and the other draw the arterial line tracing for a passive, mechanically ventilated patient.  While you draw, explain to each other the mechanisms of dDOWN and dUP.

2. With your partner, jointly determine how you would use the above physiology to explain a pulsus paradoxus to a medical student.  Also explain how one detects it using a sphygmomanometer.

3. With your partner, use the Guyton Diagram to explain why the patient had pulse pressure variation when breathing spontaneously and when passive [i.e. paralyzed] with the ventilator.  Explain how this reflects volume responsiveness?  Does it speak to volume status?

4. Critique the use pulse pressure variation as a marker of volume responsiveness and volume status.

5. While the patient was paralyzed and fully adapted with the ventilator, predict what would happen to her pulse pressure variation after each of the following interventions: Tidal volume is decreased?  Inhaled nitric oxide is begun?  An epinephrine infusion is started?  A bolus of IV Lasix is administered?  She is placed in prone position?  If her epoprostenol infusion became disrupted?

6. How would you explain to the ICU fellow why the patient has inspiratory IVC collapse when paralysis wears off?  How would all of the interventions mentioned in question 5, alter inspiratory IVC collapse?

7. Reconsider the questions posed during the case above in part A; with your partner(s), use the Guyton diagram/concept map to answer each of these questions.

8. With your partner, write a scientific abstract [250-300 words] explaining the physiology of pulse pressure variation in the passive, mechanically ventilated patient and in the spontaneously breathing patient.  Explain its use in determining fluid responsiveness and its caveats.  Afterwards, find another learning team and give each other feedback on your respective abstracts.