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BIOM5100 - Final Exam Winter 2007

You have 80 minutes to complete this exam. The exam has four questions; you are required to answer any two (2) of them. All questions are worth 5 marks, unless indicated otherwise. This is a closed book exam; however, you are permitted to bring two (2) 8.5"×11" sheet of notes into the exam. You are permitted to use a non-programmable calculator. You may not communicate with anyone during the exam except the instructor.

1. Indicator dilution blood flow measurement

    1A. In an indicator dilution system, a 10 ml bolus of liquid indicator is injected (at Time=0) into the pulmonary artery (using a Swan-Ganz catheter) and indicator concentration (in units of mlindicator/ mlblood) is measured at the left ventricle output. The following graph shows the measured concentration. Calculate the average cardiac output.

    answer: Area under circulation 'A' is 1/2*(0.2*0.5s)*2 = 0.1 s
    Input is 10ml, so flow is 10ml/.1s = 100ml/s = 6L/min

    1B. Calculate the fraction of dye that has been absorbed into the system in the course of one cycle of circulation.

    answer: Area under circulation 'A' is 1/2*(0.2*0.5s)*2 = 0.1 s
    Area under circulation 'B' is 1/2*(0.005*15s)*2 = 0.075 s

    1C. What does indicator dilution measure? An average flow? An instantaneous flow? Discuss briefly.

2. Doppler Ultrasound

    2A. In order to measure the heart rate of a baby in utero, the most common instrument is a Doppler ultrasound probe. A cylindrical probe with a 2 cm diameter (designed for 3 MHz operation) is placed onto the mother's abdomen. Assume the speed of sound in tissue is 1500 m/s. If the maximum blood velocity in baby's aorta is 100 mm/s, calculate the maximum frequency in the Doppler ultrasound output signal

    answer: Δf = 2×fv/c = 2*3e6(1/s)*0.1(m/s)/1500(m/s) = 400 Hz

    2B. Sketch the probe placement and the zone to which the probe is sensitive. Show the near and far field regions and indicate the near field size.

    answer: λ = 1500m/s / 3e6/s = 0.5mm Near field size is D_NF= D^2/(4λ) = (20mm)^2/0.5mm/4 = 200mm

    2C. Another technique to measure the fetal heart rate uses ECG electrodes on the mother's abdomen. Discuss some difficulties of ECG measurement of the fetal heart rate. If possible, suggest a technique to improve measurement of fetal heart rate using the ECG.

3. Pulse Oxymeters

    3A. A pulse oxymeter is used to measure arterial oxygen concentration. However, arterial blood represents a small fraction of the material in the finger or ear lobe onto which the oxymeter is typically placed. Most of the optical path is filled with skin, tissue and venous blood. Explain how a pulse oxymeter is able to measure arterial blood concentration alone and to reject the contribution from other tissues.

    3B. (10 points) The pulse oxymeter based on the isobestic wavelength (such as we discussed in class) is known to give inaccurate measurements for patients with carbon monoxide (CO) poisoning. The following table gives the absorptivities of hemoglobin (from Zijlstra et al. 1991, Clin. Chem. 37:1633-38). The absorptivity units are in L/mmol/cm (although this should not make any difference for the calculations).

    Wavelength 800nm 660nm
    Hb (deoxyhemoglobin) 0.20 0.81
    HbO2 (oxyhemoglobin) 0.20 0.08
    HbCO (carboxyhemoglobin)    0.01 0.06

    Consider a patient with carbon monoxide poisoning with 40% HbO2, 10% Hb, and 50% HbCO in the arterial blood. What value would a pulse oxymeter calculate for HbO2 (assuming it doesn't account for HbCO?)

    answer: Theory:
    s_800 = k×( a_800_Hb f_Hb + a_800_HbO2 f_HbO2 )
    s_660 = k×( a_660_Hb f_Hb + a_660_HbO2 f_HbO2 )
    But a_800_Hb = a_800_HbO2, and f_O2Hb = 1- f_Hb
    s_800 = k× a_800_Hb (f_Hb + 1- f_Hb)
    k= s_800 / a_800_Hb
    a_800_Hb×s_660/s_800 = a_660_Hb×f_Hb + a_660_HbO2×(1-f_Hb)
    a_800_Hb×s_660/s_800 - a_660_HbO2 = (a_660_Hb - a_660_HbO2)×f_Hb
    f_Hb = (a_800_Hb×s_660/s_800 - a_660_HbO2 )/(a_660_Hb - a_660_HbO2)
    f_HbO2= 1-f_Hb

    Test given: k=.01; f_Hb=.10; f_HbO2= .90; a_800_Hb= 0.20; a_800_HbO2=0.20; a_660_Hb= 0.81; a_660_HbO2=0.08; s_800 = k*( a_800_Hb*f_Hb + a_800_HbO2*f_HbO2 ); s_660 = k*( a_660_Hb*f_Hb + a_660_HbO2*f_HbO2 ); f_HbO2 = 1- (a_800_Hb*s_660/s_800 - a_660_HbO2 )/(a_660_Hb - a_660_HbO2); % == .90 Ok

    Solution with HbCO; f_Hb=.10; f_HbO2= .40; f_HbCO= .50; a_800_HbCO= 0.01; a_660_HbCO= 0.06; s_800 = k*( a_800_Hb*f_Hb + a_800_HbO2*f_HbO2 + a_800_HbCO*f_HbCO ); s_660 = k*( a_660_Hb*f_Hb + a_660_HbO2*f_HbO2 + a_660_HbCO*f_HbCO ); f_HbO2 = 1- (a_800_Hb*s_660/s_800 - a_660_HbO2 )/(a_660_Hb - a_660_HbO2);
    Measured HbO2= 73.7%

4. Indirect Blood Pressure Measurement

    4A. An indirect blood pressure instrument is constructed based on an inflatable cuff on the arm. The cuff is inflated and slowly deflated and the cuff pressure measured and shown in the figure below (oscillations amplitudes are exaggerated from those in an actual system). From these data, estimate: i) systolic pressure, ii) diastolic pressure, iii) mean arterial pressure, and iv) the heart rate.

    answer: Systolic Pressure = 150mmHg
    Mean Arterial Pressure = 120mmHg
    Diastolic Pressure = MAP - (SP - MAP) = 90mmHg

    4B. What are Korotkoff sounds? Explain if measurement of Korotkoff sounds allows improved accuracy of measurement of blood pressure (compared to the instrument in 4A).

    4C. Describe one mechanical source of inaccuracies between the pressure in the inflatable cuff, and the arterial pressure in the arm.



Last Updated: $Date: 2008-11-07 06:41:50 -0500 (Fri, 07 Nov 2008) $