MASSACHUSETTS INSTITUTE OF TECHNOLOGY

Transcription

1 Harvard-MIT Division of Health Sciences and Technology HST.54J: Quantitative Physiology: Organ Transport Systems Instructors: Roger Mark and Jose Venegas MASSACHUSETTS INSTITUTE OF TECHNOLOGY Departments of Electrical Engineering, Mechanical Engineering, and the Harvard-MIT Division of Health Sciences and Technology 6.0J/.79J/BEH.371J/HST.54J: Quantitative Physiology: Organ Transport Systems QUIZ 3 SOLUTIONS

2 These are normal values of physiological parameters for a 70 kg person. R rs (respiratory system R) = 4 mbar s/l C cw = 00 ml/mbar C lung = 00 ml/mbar V D (Anatomic) = 150 ml V O = 74 ml/min V CO = 0 ml/min RQ = 0.8 Q s /Q T (Shunt fraction) < 0.05 Q T (cardiac output) = 5 l/min P atm = 760 mmhg P vco = 46 mmhg P vo = 40 mmhg P aco = 40 mmhg P ao (at room air) = 100 mmhg (A a)d O 6-10 mmhg ph = 7.4 chb = 15 g/100ml-blood Hb O Binding capacity = 0.1 ml O /100ml blood FRC =.4 l The normal hemoglobin O saturation curve is also included and should be used only when there is no alternative data available. Figure 1: The first two problems are cases that include certain respiratory physiologic abnormalities. You can use the normal values as a reference, or in absence of additional information. 6.0j 004: Quiz 3 solutions

3 Problem 1 (Case 1) A patient comes to the emergency ward with shortness of breath and wheezing. He is breathing room air at a rate of 30 breaths per minute, and the pulse oximeter shows his arterial blood saturation to be S ao = Arterial and mixed venous blood samples are taken at arrival and reveal the following values: P vco = 44 mmhg P vo = 7 mmhg P aco = 39 mmhg P ao (at room air) = 0 mmhg The blood gas data comes with a computer generated caution questioning the validity of the measurements. A. Please identify which of the four blood gas values may have an error and explain your reasoning. (5%) P ao cannot be correct for two reasons: (i) P ao is always P vo (ii) P ao = 0 is not compatible with S ao = 0.8. In fact S ao B. You need to make a best guess to treat the patient with the knowledge available to you; can you find an approximate value of the erroneous blood gas? (5%) For S ao = 0.8, P ao = 45 (from the O saturation curve). 6.0j 004: Quiz 3 solutions 3

4 C. The patient is given 100% O by mask and one hour later his blood gases come back: This time without caution notes. P vco = 48 mmhg P vo = 47 mmhg P aco = 4 mmhg P ao = 60 mmhg What can you say about the cause of gas exchange impairment in this patient? (50%) Hint, you can ignore the oxygen carrying capacity of plasma in your calculations. From alveolar gas eq: P aco ( ) P AO F IO P atm P H O RQ 4 = 1(713) = Given that the paitent is breathing at F IO = 1, this large = (A a) DO gradient can only be explained by shunt. Using the approximate shunt equation This patient has a 56% shunt fraction. Q s 1 sat a,o = = = = 0.56 Q t 1 sat v,o j 004: Quiz 3 solutions 4

5 Problem (Case ) The same patient eventually develops respiratory failure and is placed on a mechanical ventilator adjusted to parameters matching his tidal breathing: VT = 390 ml f = 30 bpm T ins = 40% T exp = 50% F i O = 0.50 And his blood gases are measured as: P vco = 4 mmhg P vo = 45 mmhg P aco = 40 mmhg P ao = 75 mmhg V O = 74 ml/min V CO = 0 ml/min The ventilator output shows the following screen Figure : 6.0j 004: Quiz 3 solutions 5

6 A. Is this patient exhibiting dynamic hyper-inflation, and why or why not? (5%) Yes, because expiratory flow at the end of exhalation is non-zero. B. Can you estimate the patient s respiratory system mechanical parameters: Resistance and Compliance? (5%) P peak P plateau R = = = ( ) V V T 390 t insp 0.4 =.05 mbar s/l V T 390 C = = = 78 ml/mbar P peak Pini 1 16 RC = = 1.6 sec 6.0j 004: Quiz 3 solutions 6

7 C. The attending MD suggests decreasing frequency while keeping the inspiration (insufflation in Germanic English) and exhalation time % unchanged. What frequency and tidal volume would you choose? Assume that the VD physiologic remains unchanged. (50%) (Note: if you decide to use VD anatomic in your calculation, you will lose 5% of the question points.) To avoid hyperinflation: t exp 4 RC = = 6.4 sec 1 t exp % = 50% f = 60 = 4.7 bpm To keep a constant V A, we need to calculate a V T such that ( ) ( ) VT1 V D f 1 = VT V D f V T = ( VT1 V D ) f 1 + VD f We know that V D V CO P aco = 1 V T f F ACO where F ACO V T (P atm 47) V D 0 = 1 = V 40 T V D = = ml 30 V T = ( ) = 114 ml V T f = 1.14 L = 4.7 bpm 6.0j 004: Quiz 3 solutions 7

8 Problem 3 Pulmonary fibrosis is a debilitating disease of the lung characterized by replacement of elastin by collagen and resulting in a decrease of lung compliance. In severe cases, lung transplant is the only option for survival. To maximize organ availability and reduce post-operative mortality, usually unilateral lung transplant is conducted. A. First draw the normal chest wall and lung compliance curves. Then draw changes that result from pulmonary fibrosis ( reduced by 1/). Assume that compliances are linear and that the chest wall compliance does not change. What happens with FRC in pulmonary fibrosis? (5%) Volume Normal lung After transplant Fibrotic lung For CW: VCW = V CW0 + P PL C CW P PL = V CW V CW0 C CW For lungs: V L + P a0 P PL = P PL P PL = }{{} =0 V L FRC = Lung Volume, where V L = V CW and P PLCW = P PLL. 6.0j 004: Quiz 3 solutions 8

9 V CW V CW0 = C CW V L For V L = V CW = FRC FRC V CW0 C CW = FRC CL FRC = 1 + V CW0 C CW For normal lung For fibrotic lung CCW = FRC = V CW0 V CW0 V CW0 CL F = FRC = / B. Second, draw the effects of replacing one of the lungs with a normal donor lung. What will be the new FRC after surgery? You can assume that both right and left lungs have equal compliance before surgery. (5%) After transplant:,post = 4 fibrotic lung + transplanted normal lung = 3 4 So V CW0 3 FRC = 4 = V CW j 004: Quiz 3 solutions 9

10 C. How does the amount of pressure required to inspire a similar tidal volume compare between before and after surgery? (5%) Total lung compliance Before transplant: After transplant: 1 V T V T F = = P = ( CL ) P F 3,post = + = + = 4 ) 4 V T 4 ( V T P after = P 3 after = 3 C 4 L ( ) V T VT P before = ( ) = CL For the same V T 4 P after 3 = = P before 3 After transplant the person needs two-thirds the pleural pressure to generate the same total V T. D. In what proportions is the tidal volume distributed between both lungs? (5%) V T distributes with regional C. V Ttranpl = V Tfibrotic 6.0j 004: Quiz 3 solutions 10