Arterial Blood Gas (ABG)

Arterial Blood Gas (ABG) Arterial Blood Gas (ABG) is defined as: "The measurement of the acidity and the levels of oxygen and carbon dioxide in the blood from an artery." ABG are tests which are used to check how well a patient's respiratory efforts are able to move oxygen into the blood and remove carbon dioxide from the blood.. Arterial Blood Gas (ABG), Components ABG are broken down into 5 main components, which are then further grouped together into their areas of use. List of Components 1) pH * The hydrogen concentration in the Arterial Blood * ≈ 7.35 - 7.45 2) PaCO2 * Partial Pressure (PP) of CO2 within the Arterial Blood * ≈ 35 - 45 mmHg 3) PaO2 * Partial Pressure of O2 within the Arterial Blood * ≈ 80 - 100 mmHg 4) HCO3 * Concentration of Bicarbonate within the Arterial Blood * ≈ 24 - 30 mEq/l 5) SaO2 * Saturation of Hemoglobin with Oxygen within the Arterial Blood ** Sometimes referred to as: % O2-Hgb * ≈ 93 - 100% - ABG components are either measured directly from the sample or are calculated based off of measurements or other component values. 1) Measured * pH * PaCO2 * PaO2 2) Calculated * HCO3 * SaO2 00:05 01:29 Arterial Blood Gas (ABG), Components, Groupings The components of ABGs can be generally grouped into 3 sections 1) Acid-Base Calculations - Based on the components: * pH, PaCO2, HCO3 2) Ventilation Efficiency - Based on the component: * PaCO2 3) Oxygenation Efficiency - Based on the components: * PaCO2, PaO2, SaO2 Arterial Blood Gas (ABG), ABG vs. VBG There is always debates about which set of values to use: * Venous Blood Gas (VBG) - or - * Arterial Blood Gas (ABG) - VBG are not able to provide information about the oxygen status of the patient. * But are viewed as being an acceptable substitute ** For A/B status analysis Common VBG Values: - Venous pH ≈ Arterial pH - 0.03 - PvCO2 ≈ PaCO2 - 4 Arterial Blood Gas (ABG), Effect of Temperature ABG analysis is temperature control, but the temperature of the blood at which is was collected and the temperature of the lab equipment can impact the gathered results - Typically lab machines will report 2 ABG results for patients, one with the temperature of the blood when it was taken, and one with temperatures at the normal set point of the equipment. * ≈ 37°C Effects of Temperature: 1) Increased Temperature - Causes more Gas to dissolve into the blood, increasing readings 2) Decreased Temperature - Causes more Gas to evaporate out of the blood, decreasing readings The Journey of Oxygen The fundamental principle of human physiology is the delivering of oxygen to cells and removing of CO2 from cells to enable Aerobic Respiration. - All organ systems and individual biochemical processes exist to support this purpose - As oxygen is the critical element in this equation, its movement into the body, through the body, and use in the body are thoroughly evaluated. Journey of Oxygen can be summarized as: 1) Ventilation - Oxygen enters the Alveoli - Dependent on the Communication between the Lungs and Brain - Summarized by the PaCO2 Equation 2) Diffusion - Oxygen moves from within the Alveoli into the Blood stream - Dependent of the Tissue of the Lungs - Summarized by the Equations of Oxygenation * Alveolar Gas Equation * A-a Gradient 3) Transport & Delivery - Oxygen is: * Bond to Hgb * Taken to the tissues ** Typically as O2-Hgb complex * Oxygen is released from Hgb into the tissues - Dependent on: * Hgb * CO * Acid Base Balance - Summarized by the: * O2-Hgb Curve * Oxygen Content Equation * Oxygen Delivery Equation * Henderson-Hasselbalch Equation Ventilation The process of ventilation can be generically described as: " The exchange of air between the lungs and the atmosphere so that oxygen can be exchanged for carbon dioxide in the alveoli." - This process is summarized by the PaCO2 equation - The PaCO2 equation puts into physiological perspective one of the most common of all clinical observations: * A patient's respiratory rate and breathing effort. PaCO2 Equation PaCO2 = (VCO2 x 0.863)/(VA) Where: VCO2 = Rate of CO2 production from respiration * ml of CO2/min 0.863 = Constant * This constant is necessary to equate dissimilar units for VCO2 and VA VA = RR(Vt-Vd) *aka Alveolar Ventilation Equation - Where: *RR = Respiratory Rate ** Breathes/min *Vt = Tidal Volume ** L *Vd = Dead Space Volume ** L Ventilation, Respiratory Input Centers These receptors provide information to the Medullary Respiratory Center in the Brainstem, which controls RR and Vt. - These centers are divided into 2 divisions 1) Central Input - 4 types 1a) Central Chemoreceptors 1b) Cerebral Cortex 1c) Pneumotaxic and Apneustic Centers 1d) Progesterone Receptors 2) Peripheral Input - 4 types 2a) Peripheral Chemoreceptors 2b) J-Receptors 2c) Stretch Receptors 2d) Irritant Receptors Ventilation, Respiratory Input Centers, Central Inputs Location and/or Function 1) Central Chemoreceptors - Located in the Medullary - These receptors detect changes in blood pH 2) Cerebral Cortex 3) Pneumotaxic and Apneustic Centers - Located in the Pons of the Brain Stem - These receptors function via Bi-Directional Communication 4) Progesterone Receptors - These receptors increase ventilation Ventilation, Respiratory Input Centers, Peripheral Inputs Location and/or Function 1) Peripheral Chemoreceptors - Located in the Carotid and Aortic Sinus Bodies - These receptors detect changes in blood PaCO2, pH, and PaO2 * PaO2 only at 75 mmHg * Carotid Receptors are more active in Peds * Aortic Receptors are more active in Adults 2) J-Receptors - Located in the Walls of the Alveoli - These receptors are activated during events of Pulmonary Edema and increased interstitial pressure 3) Stretch Receptors - Located in the Smooth muscle of the Airway 4) Irritant Receptors - Located in the Epithelium of the Airway Tissue Ventilation, Respiratory Input Centers, Effects Stimulation of Respiratory Inputs can cause the increase or decrease in respiratory drive. Common causes for stimulation include: - Common causes for each include: 1) Increased Respiratory Drive Stimuli * High PaCO2 * Low pH * Low PaO2 * Pain * Anxiety * Pulmonary Edema * Histamine Release * Inhalation of Noxious Chemicals * High levels of Progesterone 2) Decreased Respiratory Drive Stimuli * Low PaCO2 * High pH - Etiologies of Increased PaCO2 * Low RR * Low Vt Oxygen Diffusion Oxygen diffusion is by far the most important aspect of a patient's respiratory ability. Oxygen diffusion describes the ability of oxygen to move from within the alveoli into the blood stream of the patient - This variable can be generally indicate a patient's efficiency in Respiration. - Dysfunctions in oxygen diffusion can be physiological or pathological - The movement of oxygen across the Alveolar Capillary membrane can be fickle at times, but is usually governed by the Equations of Oxygenation 1) Alveolar Gas Equation 2) A-a Gradient