Anesthesia Pharmacology Chapter 34:  Anesthesia in the Asthmatic Patient and Related Issues   Co-existing Disease

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5Case study and analysis: chronic obstructive pulmonary disease

  • 5"A 45-year-old man with cholelithiasis was scheduled for cholecystectomy.

  • He had a long history of asthma developed dyspnea with only moderate exertion.

  • He slept on two pillows.

  • There was no peripheral edema. Arterial blood gases showed the following: pH, 7.36; PaCO2, 60 mm Hg; PO2, 70 mm Hg, CO2content, 36 mEq/liter"


5Lung Volumes and Capacities

5,6Normal Subject: Flow Volume Loop

V75, V59, and V25 indicate flow at 75%, 50% and 25% of vital capacity; RV = residual volume


5,6Flow Volume Loops in Normal subjects and subjects with obstructive and restrictive defects


5Closing Volume Measurement: Single-Breath Nitrogen Test



7Single Breath Nitrogen Elimination Test

  •   "A. Definition- evaluates characteristics within four portions of  the airway, particularly evenness of distribution within the alveoli.

  •  B. Measured with a spirometer and a nitrogen analyzer.

  • C. The graph plots percent Nitrogen against volume.

  • D. It measures the following:

    • 1. VC- from Phase I to Phase IV.

    •  2. Closing Volume (CV) - the volume at which closure of the airway begins. Reported as a percentage of vital capacity. On the graph, from the onset of Phase IV to the end of Phase IV.

    • 3. Closing capacity (CC)- the sum of the closing volume added to the residual volume (CC = CV + RV

  • E. Technique

    • 1. Patient exhales to residual volume (maximum expiration), then slowly inhales to TLC (maximum inspiration) a breath of 100% oxygen.

    • 2. Patient then immediately exhales back to residual volume NOTE: There is no breath hold occurring after the inhalation of the oxygen.                

    • 3. The graph can be divided into four phases:

      • Phase I- the beginning of expiration. This gas is coming from the anatomic deadspace and is pure oxygen. The phase is barely seen on the tracing.

      • Phase II- seen as the first steep slope on the tracing. The gas is coming from both the bronchial and alveolar areas and shows an abrupt increase in N2%. (50% deadspace and 50%  alveolar)

      •  Phase III- the plateau phase in which the gas is coming  from thealveoli. The percent of nitrogen changes slowly and evenly producing a flatter line.

      • Phase IV- the percent of nitrogen once again increases > sharply due to closure of the airway and emptying of the apices (high in amount of nitrogen). 

      • Closing capacity can be calculated by adding the closing volume to the residual volume NOTE: to measure the residual volume, the total lung capacity must first be measured by other methods

      • Significance

        • The steeper the slope of Phase III, the worse the distribution of ventilation

        • Increased CV or CC occurs with

          • Old age, restrictive disease where there is loss of  FRC, early small airway disease,  CHF (edema changes the size of airway diameter)

        • Severe small airway obstruction may show no clear break between Phase III and Phase IV."


1/lung compliance + 1/chest wall compliance = 1/Total compliance


8Shunt and Dead Space Definitions and Calculations

  • Shunt and dead space describe extreme conditions where either blood flow or ventilation does not meet the other in the lung in a manner to support normal gas exchange. These terms refer not only to extreme conditions but also to describe areas or effects corresponding to improper matching of blood flow and ventilation. Ventilation/perfusion mismatch states which are less extreme than absolute shunt or dead space may be described as shunt-effects or dead space-effects.

  • Shunt is defined as that part of the cardiac output that returns to the left heart without passing through capillaries of  ventilated alveoli.

    • The oxygen content of mixed arterial blood (CaO2) is determined by the content of oxygen in the blood that reached ventilated alveoli (CcO2), the content of oxygen in blood that bypassed ventilated alveoli (CvO2), and the proportion of the two in accord with the relationship: 

      QS/QT = (CcO2 - CaO2) / (CcO2- CvO2)

    • Normal shunt fraction (QS/QT) is less than 0.05 (<5%). Remember CxO2=1.39 x Hb x SxO2 + 0.003 x PxO2

  • CcO2 represent oxygen content of pulmonary capillary blood, estimated using 100% as the saturation (1.00 in the equation) since  PcO2 (pulmonary capillary PO2) is reasonably assumed high enough result in 100% saturation. PcO2 itself is not subject to direct measurement so PAO2 from the alveolar air equation is used.  Therefore, 

    CcO2="1.39" x Hb + 0.003 x PAO2

  • Dead space: is the inspired air component which is exhaled without being exposed to perfused alveoli

  • CO2 content of mixed exhaled gas (PECO2) is determined by the 2 content of the gas that came in contact with perfused alveoli (PACO2), the content of the gas that did not come in contact with perfused alveoli, and the proportion of the two. 

    • Assumption 1:   no carbon dioxide in inspired air and thus no carbon dioxide in that part of the inspired volume that does not come in contact with perfused alveoli. 

    • Assumption 2:  that gas that does contact perfused alveoli will equilibrate to the carbon dioxide content in the perfusing blood PACO2="PcCO2" or arterial blood PcCO2="PaCOv" since arterial carbon dioxide content is not greatly influenced by shunting). The following formula results: 

    VD/VT=(PaCO2- PECO2) / (PaCO2 PICO2), and assuming that PICO2="0"

    VD/VT=(PaCO2- PECO2) / PaCO2 [Normal VD/VT < than <33%).]

  • Example calculation using the following data:

    • Hb 10 gm%; PaO2 75 mmHg ; SaO2 97%; Pb 760 mmHg;PvO2 33 mmHg; SvO2 65%;PaCO2 45 mmHg; PECO2 15 mmHg; FIO2 40%

    • Calculate the shunt and dead space:

      • Dead space:

        1. VD/VT=(PaCO2- PECO2) / PaCO2

        2. VD/VT=(45- 15) / 45

        3. VD/VT=30/45

      • The shunt equation is multistep. Development of contents to plug into the shunt equation: QS/QT=(CcO2- CaO2) / (CcO2-CvO2

        •  arterial content:

          1. CaO2="Hb" x SaO2 x 1.39 + 0.003 x PaO2

          2. CaO2="10" x .97 x 1.39 + 0.003 x 75

          3. CaO2="13.48" + 0.225 = "13.705" Vol %

        • mixed venous content:

          1. CvO2="Hb" x SvO2 x 1.39 + 0.003 x PvO2

          2. CvO2="10" x .65 x 1.39 + 0.003 x 33

          3. CvO2="9.035" + 0.099= "9.134" Vol %

        • Prior to calculating CcO2, PAO2 must be calculated (PH2O = in the alveoli is assumed to be 47 mmHg)

          1. PIO2=FIO2 x (Pb - 47)

          2. PIO2=0.4 x 713=285.2 mm Hg

          3. PaO2=PIO2 - PaCO2 x 1.25

          4. PaO2=285.2 - (45 x 1.25)

          5. PaO2=285.2 - 56.25=228.95 mm Hg

        • Next the pulmonary capillary content must be calculated:

          1. CcO2=Hb x 1.39 + 0.003 x PAO2

          2. CcO2=10 x 1.39 + 0.003 x 229

          3. CcO2=13.9 + 0.687=14.59 Vol %

        • Final shunt equation:

          • QS/QT=(CcO2 - CaO2) / (CcO2 - CvO2)

          • QS/QT=(14.6 - 13.7) / (14.6 - 9.1) 

          • QS/QT=0.9 / 5.5= 0.16 %


Preoperative Issues

5Intraoperative management

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