Study Notes for Blood Gas Sampling Errors Chapter

Chapter 3: Blood Gas Sampling Errors

Introduction

  • Improper sampling technique or blood specimen handling can introduce marked errors in blood gas measurements.

  • Incorrect results can be detrimental, sometimes worse than having no result at all.

  • Blood gas values are inherently unstable and subject to significant changes due to minor sampling flaws.

  • The likelihood of sampling error increases with inexperienced clinicians in charge of the blood draw.

  • The importance of proper education regarding potential sampling errors is emphasized.

  • Blood gas sampling techniques must be thoroughly understood and practiced with care.

Basic Physics of Gases

Molecular Behavior

  • Gases consist of minute molecules in rapid, continuous, random motion, known as Brownian movement.

  • Kinetic energy (the energy of motion) leads to force generation with collisions between molecules and surfaces.

  • The pressure generated by a gas is defined as the force per unit area, measured using a manometer.

  • Water molecules can also be present in the gas phase and contribute to pressure; this is known as water vapor pressure.

Atmospheric Pressure

  • The air around Earth is called the atmosphere, with a total atmospheric pressure at sea level approximately 760 mm Hg (1 atmosphere).

  • The unit "torr" is synonymous with millimeters of mercury, and both units can be used interchangeably.

  • Atmospheric pressure changes with altitude: it is higher than 760 mm Hg below sea level and lower above sea level.

  • Atmospheric pressure can be measured using a barometer.

Partial Pressure

  • Each gas in a mixture contributes to the total pressure; this contribution is termed partial pressure (P).

  • The specific pressure exerted by carbon dioxide, for example, is referred to as PCO2.

  • Dalton’s Law states that the sum of partial pressures in a gas mixture equals the total pressure.

Fractional Concentration

  • Fractional concentration of a gas (F) is the percentage of total gas molecules occupied by that particular gas, excluding water vapor.

  • It's expressed as a decimal (e.g., FO2 = 0.21 for 21% O2).

  • In a scenario with only O2 and water vapor, the FO2 can be 1.0, but the actual concentration accounts for the presence of H2O.

Calculation of Partial Pressure

  • The equation to calculate partial pressure is as follows:
    ext{Partial Pressure} = ( ext{Total Pressure} - ext{Water Vapor Pressure}) imes ext{Fractional Concentration}

  • Example: If total pressure is 240 mm Hg and PH2O is 40 mm Hg, with FO2 at 1.0:
    PO2 = (240 - 40) imes 1.0 = 200 ext{ mm Hg}

Symbols

  • FIO2 represents the percentage of inspired oxygen.

  • Capital letters indicate gas measurements related to lung function (e.g., I, inspired; E, expired; A, alveolar; T, tidal).

  • Lowercase symbols indicate blood measurements (e.g., a, arterial; v, venous; c, capillary).

  • A bar over the symbol denotes the mean (e.g., P–vO2 is the average PO2 in veins).

Composition of Atmospheric and Alveolar Air

  • Major gases present in dry atmospheric air and alveolar air, with their partial pressures and percentages, are listed in Table 3-1:

    • Nitrogen (N2):

    • Atmospheric: 590.0 mm Hg (78.09%)

    • Alveolar: 569 mm Hg (74.8%)

    • Oxygen (O2):

    • Atmospheric: 158.0 mm Hg (20.95%)

    • Alveolar: 104 mm Hg (13.7%)

    • Carbon Dioxide (CO2):

    • Atmospheric: 0.2 mm Hg (0.03%)

    • Alveolar: 40 mm Hg (5.3%)

    • Argon and others:

    • Atmospheric: 7.8 mm Hg (0.93%)

    • Alveolar: negligible (<0.1%)

    • Water vapor in alveolar air is 47 mm Hg and 6.2% in composition.

Humidification

  • Water vapor pressure depends on temperature and humidity. Higher temperatures can hold more moisture.

  • Relative humidity (RH) compares actual humidity to potential humidity at a given temperature; 100% RH indicates saturated air.

  • At body temperature (37° C), the water vapor pressure (PH2O) is 47 mm Hg, meaning air in alveoli has a higher water vapor pressure than atmospheric air.

  • This process changes the partial pressures of other gases in alveoli due to increased PH2O.

External Respiration

  • External respiration involves the exchange of O2 and CO2 between alveoli and blood:

    • O2 diffuses from alveoli to blood.

    • CO2 diffuses from blood to alveoli.

  • Thus, the alveolar PO2 is less than atmospheric PO2, and alveolar PCO2 is greater than atmospheric PCO2.

Body Temperature and Pressure Saturated (BTPS)

  • Clinical measurements of gases must be standardized to allow accurate comparisons, typically at BTPS conditions: 37° C, ambient pressure, and PH2O of 47 mm Hg.

Gas Laws: Volume, Pressure, and Temperature

  • Gay-Lussac’s Law: At constant volume and mass, pressure varies directly with absolute temperature (Kelvin).

  • Boyle’s Law: At constant temperature, volume varies inversely with pressure.

  • Charles’ Law: At constant pressure, volume varies directly with temperature.

Gases in Liquids

  • Gases dissolve freely in liquids; the interaction is physical, reflecting equilibrium between phases.

  • Henry’s Law: The partial pressure of a gas in a liquid equilibrates with the gas's partial pressure in the gaseous phase.

Change in Altitude

  • Barometric pressure decreases with altitude, affecting the partial pressure of gases, including O2.

  • For instance, at the summit of Mount Everest, PO2 drops to approximately 42 mm Hg despite FIO2 remaining at 0.21.

Potential Sampling Errors

  • Discusses five types of arterial blood sampling errors:

    1. Air in blood sample

    2. Inadvertent venous sampling or admixture

    3. Anticoagulant effects

    4. Changes due to metabolism

    5. Alterations in temperature

  • Proper labeling of samples is essential to avoid mismatched laboratory results.

Air in the Blood Sample

  • Effects of Air Contamination:

    • Major effect of an air bubble in blood gas sample is a change in PaO2; presence of air increases major oxygen measurements erroneously.

    • According to Henry's Law, if PaO2 is less than 158 mm Hg, exposure to an air bubble increases PaO2 due to oxygen moving from blood to the bubble.

    • Conversely, if PaO2 exceeds 158 mm Hg, oxygen diffuses back into the bubble, resulting in a low reading.

Clinical Guidelines

  • The presence of an air bubble in a sample invites significant error.

  • Sampling should avoid agitation or prolonged exposure to air to minimize oxygen contamination.

  • Samples should be analyzed promptly: stable results can be obtained if air bubbles are expelled within 2 minutes.

Venous Sampling or Admixture

  • Venous Samples:

    • Inadvertent punctures are a risk, especially in hypotensive patients.

    • Characteristics of arterial samples include a flash of blood and pulsation during filling.

    • Peripheral venous blood should not be used for oxygenation assessments due to poor reliability.

  • Venous Admixture:

    • Contamination can occur from small amounts of venous blood mixing with arterial blood, significantly lowering measured PaO2.

    • Example: Mixing 0.5 mL of venous blood with 4.5 mL of arterial blood can decrease PaO2 significantly (as much as 25%).

Recognition of Venous Error

  • Venous contamination should be suspected when clinical status does not align with blood gas data.

  • Example: A healthy patient appearing normal yet presenting abnormal gas values suggests a sampling error.

Anticoagulant Effects

  • Anticoagulants like lithium heparin are essential in preventing clot formation in samples but may introduce their own errors:

    • Nature of Anticoagulant: Sodium heparin is sometimes used but with risks of contamination and electrolyte distortion.

    • Dilution Error: Overfilling with heparin can lead to false results, significantly affecting bicarbonate and other measures.

Metabolism

  • Blood cellular metabolism continues post-sampling, consuming O2 and producing CO2.

  • Changes in blood gas values depend on sample temperature; higher temperatures speed up metabolic reactions.

  • Table 3-7 quantifies blood gas changes over time at 37°C.

Plastic versus Glass Syringes and Icing

  • Plastic syringes allow gas diffusion, potentially influencing blood gas values, but with clinical relevance often underestimated.

  • Iced samples stabilize gas values, but cooling can also lead to unexpected results, especially in plastic syringes due to increased solubility.

Clinical Guidelines for Sample Handling

  • Room temperature samples should ideally be analyzed within 30 minutes.

  • Iced samples stabilize measurements such as PCO2 and pH but caution with initial PaO2 values is warranted.

Leukocyte Larceny

  • Refers to the consumption of O2 by leucocytes in the blood sample, particularly severe with high counts leading to erroneous hypoxemia results.

  • Suggested precautions include the use of glass syringes for samples from leukemic patients to minimize errors

Summary of Sampling Errors

  • Table summarizing effects of preanalytical errors on sample results.

Measurement of Blood Gases and Electrolytes from a Single Sample

  • Increasingly, blood gas machines measure blood gases, electrolytes, and other values from the same sample.

  • Error sources include anticoagulants and sample handling practices.

Anticoagulant Errors

  • The use of liquid heparin can dilute and bind calcium, affecting ionized calcium measurements.

  • Dry (lyophilized) heparin minimizes dilution issues but requires time for proper mixing.

Transporting/Icing Samples

  • Blood samples prepared incorrectly may lead to hemolysis, influencing potassium and ionized calcium levels.

  • Maintaining integrity during transport is critical to ensure accurate measurement.

Exercises

  • Various exercises prompt students to recall definitions and apply understanding to calculations relating to the concepts discussed.