Learning Module Six
Intended Learning Outcomes
Explain the relationship between the Campbell diagram and the Rahn diagram
Draw changes in chest wall and pulmonary compliance using the Rahn diagram
Differentiate between plateau pressure, intrathoracic and transpulmonary pressures using the Rahn diagram
Reexamine how changes in intrathoracic pressure and transpulmonary pressure alter right ventricular hemodynamics
Explain both the driving pressure and stress index and how they may be utilized to optimize transpulmonary pressure.
Part A [complete alone] ----------------
Reconsider the cases that you worked through in learning module 5.
Bed 1 is a morbidly obese woman (BMI 47) intubated for airway protection. She suffered an intra-cerebral hemorrhage as a complication of Roux-en-Y gastric bypass. The respiratory therapist is calling you for plateau pressures in the low-40s even after administration of vecuronium. Her blood pressure is sagging as are her oxygen saturations.
Bed 2 is an elderly man with severe emphysema. He is cachectic (BMI 17) and was intubated for both transfusion-associated circulatory overload and aspiration pneumonia. He is 4-days post an ax-fem bypass for severe peripheral arterial disease. He too has plateau pressures in the low-40s even following the administration of vecuronium. His blood pressure and oxygen saturations are sagging.
Reflect on the differences you and your learning partner observed between these two patients with respect to their right ventricular preload and afterload. Which patient had their elevated plateau pressure driven by poor pulmonary compliance? Which had their plateau pressure driven by poor chest wall compliance? Have you ever observed the pressure-time waveform of such patients? If so, what have you noticed? Have you encountered the driving pressure? What is your understanding of this metric and how might it be different between these patients?
Part B [complete alone] ----------------
Read this 1000 word essay entitled: 'Driving Pressure & Stress Index'
Read this 1000 word essay entitled: 'Hidden Hemodynamics in Respiratory Mechanics'
Part C [complete together] ----------------
1. If you chose to explain patient 1 in the previous module, now describe patient 2. Your learning partner should similarly select their complimentary patient. Now draw your patient's Rahn diagram. Explain to each other your results; contrast the diagrams you have drawn and relate them to the Campbell diagrams that you drew in module 5.
2. Continue with the diagrams you have drawn and explain to each other where the esophageal, plateau and transpulmonary pressures would fall on the diagram. How is this different between the two patients? Recall that the esophageal pressure is a surrogate for pleural pressure.
3. Explain to your partner what your patient’s stress index would be and why. How would the stress index be visualized on a pressure time tracing on the ventilator [draw it!]? Importantly, provide the reason as to why the stress index can be utilized only when flow delivery is constant [i.e. square wave].
4. Now imagine what would happen if you increased the PEEP of your patient by 5 cm H2O – explain to your partner what would happen to each of the following: the stress index, the pulmonary compliance curve, the chest wall compliance curve, the respiratory system compliance curve and finally, the driving pressure. Draw the Rahn diagram to support your explanation.
5. Given your explanations from question 4, what would increasing the PEEP do to your patient’s RV preload, RV afterload and why? Try to integrate the Guyton [encountered in earlier modules] and Rahn diagrams here; how do they relate?
6. If increasing the PEEP in your patient led to hypotension, what would you expect to see on a bedside echocardiogram of the right ventricle and IVC? Would the correct hemodynamic intervention be to give fluids or lower the PEEP? Explain to your partner why this is so in terms of preload and afterload.
7. Review the intended learning outcomes with your partner. With your partner, write a 250 word abstract summarizing the intended learning outcomes; then meet with another learning pair to share your synopsis.
Heart-lung.org will provide a comprehensive, on-line tutorial in cardiovascular and respiratory physiology for the interested medical student, resident and fellow.
The first 4 chapters will cover basic physiology and pathophysiology with an emphasis on the Campbell and Guyton Diagrams.
The remaining 4 chapters will focus on clinically-relevant topics in the intensive care unit; the discussions will be largely drawn from the physiology covered in the first half of the textbook.