Graphical Analysis Charles L. Webber, Jr., Ph.D.

Learning Objectives: You should be able to:

• Show how mass action shifts in the CO2 hydration reaction can explain both the direct and inverse couplings between [HCO3-] and pH.
• Draw the generalized Davenport diagram, dividing the graph into six labeled regions of acid-base disturbances and compensations.
• þThrow a dartþ at a calibrated Davenport diagram and diagnose the situation, following the proper rules of movement along the lines.
• Defend the necessity of taking several blood samples in the assessment and treatment of acid-base disturbances in a clinical setting.

Arterial PCO2 Isobars     Davenport's Diagram     Compensatory Processes     Base Excesses and Deficits     MainMenu

Arterial PC02 Isobars and the Normal Buffer Line

1. CO2 Hydration Reaction (Fig. 24)
   • mass action shifts to the left or right of reaction
   • respiratory control: CO2 (left side of equation)
     • fast responding system
     • [HCO3-] þ 1/pH
   • metabolic control: HCO3- and H+ (right side of equation) slow responding system
     • [HCO3-] þ pH

      

    

2. PaCO2 Isobars and Metabolic Disturbances (Fig. 25)
   • PaCO2 isobars (family of curves)
     • computation of bicarbonate buffer system using various constant PaCO2 values
   • alterations in [H+] or [HCO3-] will result in pH shifts along PaCO2 isobar
     • [HCO3-] þ pH
     • PaCO2 remains unchanged since the respiratory response to changes in acid drive is rapid

    

3. Normal Blood Buffer Line and Respiratory Disturbances (Fig. 13)
   • NBL is the titration curve for non-bicarbonate buffer systems
     • slope of line depends on hematocrit ([Hb] and buffer power)
     • vertical position of line depends on arterial oxygen tension
   • alterations in PaCO2 will result in pH shifts along the NBL
     • [HCO3-] þ 1/pH

Arterial PCO2 Isobars     Davenport's Diagram     Compensatory Processes     Base Excesses and Deficits     MainMenu

Davenport's Diagram

1. Introduction
   • there are at least 95 diagrams that graphically represent acid-base status
   • the Davenport Diagram is not necessarily used clinically, but is unsurpassable for teaching purposes

    

2. Quadrants of the pH-HCO3- Diagram (Fig. 25)
   • quadrant A: respiratory acidosis and metabolic acidosis (double insult)
   • quadrant B: respiratory alkalosis and metabolic alkalosis (double insult)
   • quadrant C: respiratory acidosis and metabolic alkalosis (compensation present)
   • quadrant D: respiratory alkalosis and metabolic acidosis (compensation present)
   • note: the central region of the diagram (N) represents normal acid base status

    

3. Sextants of the pH-HCO3- Diagram (Fig. 26)
   • sextant A: respiratory acidosis and metabolic acidosis (double insult)
   • sextant B: respiratory alkalosis and metabolic alkalosis (double insult)
   • sextant C1: 1ø respiratory acidosis and 2ø metabolic alkalosis
   • sextant C2: 1ø metabolic alkalosis and 2ø respiratory acidosis
   • sextant D1: 1ø metabolic acidosis and 2ø respiratory alkalosis
   • sextant D2: 1ø respiratory alkalosis and 2ø metabolic acidosis
   • note: it is the normal pH range that partitions both regions C and D into two compartments

Arterial PCO2 Isobars     Davenport's Diagram     Compensatory Processes     Base Excesses and Deficits     MainMenu

Compensatory Processes

1. Rules for Moving on the Davenport Diagram (Fig. 27)
   • plot point representing acid-base status of patient
   • move from normal acid-base status point N to patient point obeying these three rules
     • move on or parallel to NBL
     • move along CO2 isobar
     • never cross vertical line where pH = 7.40
       (compensation is never perfect)
   • types of compensation:
     • no compensation
       (patient point on NBL or on PaCO2 = 40 mm Hg isobar)
     • partial compensation
       (patient point in region C1, C2, D1 or D2 and pH is out of normal range)
     • full compensation
       (patient point in region where pH is within normal range)
     • perfect compensation
       (assumed physiologically impossible)
   • compass directions used
     • SW: southwest
     • SE: southeast
     • NW: northwest
     • NE: northeast

        

      

   • 2ø Respiratory Compensation for 1ø Metabolic Acidosis (point A)
     • move SW down reference CO2 isobar
       (identifies 1ø problem as metabolic)
       þ note  [HCO3-]
     • move SE parallel to NBL to point A (2ø compensation)
       þ note further [HCO3-]

      

   • 2ø Respiratory Compensation for 1ø Metabolic Alkalosis (point B)
     • move NE up reference CO2 isobar (identifies 1ø problem as metabolic)
       þ note  [HCO3-]
     • move NW parallel to NBL to point B (2ø compensation)
       þ note further  [HCO3-]

      

   • 2ø Metabolic Compensation for 1ø Respiratory Acidosis (point C)
     • move NW up NBL (identifies 1ø problem as respiratory)
       þ note  [HCO3-]
     • move NE up new CO2 isobar to point C (2ø compensation)
       þ note further  [HCO3-]

      

   • 2ø Metabolic Compensation for 1ø Respiratory Alkalosis (point D)
     • move SE down NBL (identifies 1ø problem as respiratory)
       þ note  [HCO3-]
     • move SW down new CO2 isobar to point D (2ø compensation)
       þ note further  [HCO3-]

Arterial PCO2 Isobars     Davenport's Diagram     Compensatory Processes     Base Excesses and Deficits     MainMenu

Base Excesses and Base Deficits

1. Importance of HCO3- Values
   • the goal of acid-base medicine is to restore arterial pH values within normal range
     • this is as much an art as it is a science
   • abnormal base concentrations help the physician establish proper corrective therapy
     • qualitative: what to give (agent)
     • quantitative: how much to give (dose)

    

2. Determination of Abnormal Base Concentrations (Fig. 28)
   • Tuller analysis
     • base excess: [HCO3-] > 24 mM
     • base deficit: [HCO3-] < 24 mM
   • Davenport analysis
     • þ base excess: [HCO3-] above NBL
     • þ base deficit: [HCO3-] below NBL
   • differences between Davenport and Tuller
     • what Tuller would call a large base excess, Davenport would call a small excess (left point L)
     • what Tuller would call a base deficit, Davenport would call an excess (right point R)
     • Davenport's analysis is more accurate that Tuller's since the NBL is taken into account

        

    

3. Specific Examples Using Davenport's Analysis
   • the following data show the time course of 2ø metabolic compensation for 1ø respiratory acidosis

      

   Data Point	pH (unit)	PaCO2 (mm Hg)	[HCO3] (mM)	Compen- sation	Base Excess (mM)
   A	7.17	80	28	none	28 - 28 = +0
   B	7.32	60	30	partial	30 - 26 = +4
   C	7.39	55	32	full	32 - 24 = +8
   D	7.60	20	25	--- incompatible data ---

    

   Time Course of Acid-Base Status (Fig. 29)

   • points A, B and C from Table above are plotted on a Davenport Diagram
   • the points are connected with a line to show time course changes in acid-base status
   • following the time course of corrective therapy cannot be stressed enough
     
     þ maintenance of pH in chronic patients is very touchy (an art)
     þ a one time administration of some agent will seldom correct the problem
     þ time-course plots enable the physician to continuously evaluate success of treatment program
     þ beware that 2ø compensation can become 1ø problem if treatment is too aggressive

Arterial PCO2 Isobars     Davenport's Diagram     Compensatory Processes     Base Excesses and Deficits     MainMenu