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Arterial and Venous Blood Gases: Indications, Interpretations, and Clinical Applications

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Arterial and Venous Blood Gases: Indications, Interpretations, and Clinical Applications 3 CE CREDITS CE Article Arterial and Venous Blood Gases: Indications, Interpretations, and Clinical Applications ❯❯ Ricardo Irizarry, DVM Abstract: Blood gas analysis is frequently requested as part of the point-of-care testing for emer- Adam Reiss, DVM, gency or critical care patients presenting with metabolic or respiratory abnormalities. With the ad- DACVECC vent of portable units, information regarding a patient’s acid–base, ventilation, and oxygenation sta- Southern Oregon Veterinary Specialty Center tus can be rapidly obtained. This article provides essential information on arterial and venous blood Medford, Oregon gas analysis with the goal of helping clinicians integrate such data in their case management. etabolic derangements and respi- the difference can be greater in severely ratory distress are common pre- hypoperfused patients.5 senting problems in emergency Arterial samples are particularly use- M 1 medicine. A focused physical examina- ful in assessing the patient’s oxygenation tion and emergency intervention should and ventilation status. For example, the precede any diagnostic testing if the clini- oxygenation status can be evaluated by At a Glance cal condition of the patient dictates such measuring the arterial partial pressure Indications urgent care. After the patient is stabilized, of oxygen (PaO2) and using this value in Page E1 a history should be taken and the patient’s additional calculations, as described in Analytes hydration, ventilation, and oxygenation step 5 below.6,a Arterial samples are usu- Page E1 status assessed. The patient’s electrolyte ally collected from the dorsal pedal artery, Step-By-Step levels and acid–base status (pH) should femoral artery, or, in anesthetized patients, Approach to Arterial also be determined. sublingual artery. Step-by-step instructions and Venous Blood for sample collection techniques can be 7,8 Gas Analysis Indications found elsewhere. Page E2 Blood gas analysis can help assess under- The Four Primary Acid– lying disease processes and the severity of Analytes Base Disorders and Their illness and can guide emergency interven- Point-of-care blood gas analyzers directly Compensatory Changes tions (e.g., IV fluid administration, oxygen measure the pH, partial pressure of oxy- Page E3 therapy, electrolyte supplementation, pos- gen (PO2), and partial pressure of CO2 itive-pressure ventilation).2 (PCO ). These measured values are then Sample Arterial Blood Gas 2 Arterial blood gases primarily pro- used to derive the percentage of hemo- Report From a Patient With vide information regarding oxygenation globin saturated with oxygen (SO2), bicar- Acute Respiratory Failure – Page E3 (i.e., oxygen loading from the lungs into bonate (HCO3 ) concentration, total CO2 the blood), ventilation (i.e., carbon diox- (TCO2) concentration, and base excess of Summary of Compensatory ide (CO ) off-loading from the blood into the extracellular fluid (BEecf). The SO is Responses in Dogs With 2 2 the lungs), and acid–base status. Venous usually determined by the PO from the Metabolic and Respiratory 2 blood gases can provide information oxygen dissociation curve. The HCO – Acid–Base Disorders 3 Page E3 on acid–base status and ventilation (i.e., 3,4 aMore information about calculating additional venous partial pressure of CO2 (PvCO 2)). oxygenation parameters is available in the com- In adequately perfused patients, the PvCO 2 panion article, “Beyond Blood Gases: Making is normally 4 to 6 mm Hg higher than the Use of Additional Oxygenation Parameters and arterial partial pressure of CO2 (PaCO 2); Plasma Electrolytes in the Emergency Room.” CompendiumVet.com | October 2009 | Compendium: Continuing Education for Veterinarians® E1 ©Copyright 2009 Veterinary Learning Systems. This document is for internal purposes only. Reprinting or posting on an external website without written permission from VLS is a violation of copyright laws. FREE CE Blood Gases: Interpretations and Clinical Applications BOX 1 A good rule is that pH generally changes in Typical Analytes the same direction as the primary disorder.12 Provided by Point-of-Care Step 2: Evaluate the Respiratory Blood Gas Analyzers Component PCO 2 provides information regarding ventila- Venous Arterial tion, or the respiratory component of acid– ❯ pH ❯ pH base balance.13 Alveolar ventilation is defined ❯ PCO 2 ❯ PCO 2 as the volume of gas per unit time that reaches a ❯ TCO2 ❯ PO2 the alveoli, where gas exchange with pulmo- –a a 14 ❯ HCO3 ❯ TCO2 nary blood occurs. a –a ❯ BEecf ❯ HCO3 Hypoventilation is characterized by increases a ❯ BEecf in PCO 2 (>45 mm Hg) as CO2 is retained in a ❯ SO2 the blood. CO2 is a volatile acid, so retention 15 aCalculated value. of CO2 leads to respiratory acidosis. In most instances, respiratory acidosis is caused by some aspect of ventilatory failure, whereby concentration, TCO2 concentration, and BEecf normal amounts of CO2 produced by tissue are also derived from formulas and nomo- metabolism cannot be properly excreted by QuickNotes grams. BOX 1 lists some of the analytes typi- alveolar minute ventilation.13 Common causes The most common cally reported by point-of-care analyzers. of hypoventilation include those affecting neu- – acid–base distur- The BEecf, HCO3 concentration, and TCO2 rologic control of respiration (e.g., anesthesia, bance encountered concentration all serve as measures of the sedation), breathing mechanics (e.g., diaphrag- in small animals metabolic component of the patient’s acid– matic hernia, pleural space disease), or proper base status, whereas PCO 2 evaluates ventilation flow of air through the airways (e.g., upper or is metabolic and represents the respiratory component of lower airway obstruction) or the alveoli.15 acidosis, which the acid–base status.9 Oxygenation, as calcu- Hyperventilation is characterized by de - is represented lated from the PaO2, is also part of the respira- creases in PCO 2 as the CO2 is blown off from by a lower pH, a tory component. the alveoli, which leads to respiratory alkalosis 16 negative BEecf or (PCO 2 <35 mm Hg). Causes of hyperventilation – Step-By-Step Approach to Arterial and include hypoxemia, pulmonary disease, pain, lower HCO3 concentration, and Venous Blood Gas Analysis anxiety, and overzealous manual or mechanical a compensatory Step 1: pH ventilation. Hyperventilation may also develop The blood pH represents the overall balance as a compensation for metabolic acidosis.16 decrease in the of all the acid (acidotic) and base (alkalotic) Although oxygenation may not directly PCO 2 in an attempt processes in the body.7,10 It is determined by affect the acid–base balance, it should be – to blow off excess the ratio between the metabolic (HCO3 ) and assessed in critically ill patients. acid load. respiratory (PCO 2) components of the acid– base balance.11 Step 3: Evaluate the Metabolic In general, acidemia is defined as a blood Component pH below 7.35 and alkalemia as a blood pH The metabolic contribution to the acid–base 10 – above 7.45 (7.4 is neutral). Based on the balance can be assessed with the HCO3 con- Henderson–Hasselbalch equation, the pH centration and the BEecf.12,17 Typical refer- – – can be defined by the ratio of the HCO3 ence ranges for HCO3 are 19 to 23 mEq/L in – 18 concentration ([HCO3 ]) to the dissolved CO2 dogs and 17 to 21 mEq/L in cats. Values less concentration ([αPCO 2]) in the extracellular than these ranges indicate metabolic acidosis, fluid12: whereas values greater than the ranges indi- cate metabolic alkalosis. – [HCO3 ] (metabolic) – pH ≈ As mentioned above, the HCO3 concentra- CO [αP 2] (respiratory) tion is calculated from the pH and PCO 2; thus, it is not independent of respiratory activity.19 In this equation, α is the solubility coefficient In an attempt to isolate the metabolic compo- for CO2, and it equals 0.03. nent from respiratory influences, the concept E2 Compendium: Continuing Education for Veterinarians® | October 2009 | CompendiumVet.com FREE Blood Gases: Interpretations and Clinical Applications CE of BEecf was developed.17 The BEecf takes TABLE 1 The Four Primary Acid–Base Disorders and into account all of the body’s buffer systems, a – Their Compensatory Changes including HCO3 , to predict the quantity of acid or alkali required to return the extracellu- Conditions Primary Disorder Compensation lar fluid compartment to neutrality (pH = 7.4) – ↓pH and ↓HCO3 (↓BEecf) Metabolic acidosis ↓PCO 2 10 while the PaCO 2 is held constant at 40 mm Hg. By standardizing for the effects of the respira- – ↑pH and ↑HCO3 (↑BEecf) Metabolic alkalosis ↑PCO 2 tory component, the BEecf is representative of – all the metabolic acid–base disturbances in a ↓pH and ↑PCO 2 Respiratory acidosis ↑HCO3 (↑BEecf) patient.17 Normally, the BEecf is 0 ± 4 mEq/L.12 – Lower values (BEecf <–4) indicate metabolic ↑pH and ↓PCO 2 Respiratory alkalosis ↓HCO3 (↓BEecf) acidosis, whereas higher values (BEecf >+4) aRose BD, Post TW. Introduction to simple and mixed acid–base disorders. In: Clinical Physiology of Acid–Base and Electrolyte 5th ed. New York: McGraw-Hill Book Co; 2001:541. indicate metabolic alkalosis. Disorders. Metabolic acidosis can be caused by increases in the generation of hydrogen ions (H+) from endogenous (e.g., lactate, ketones) or exogenous TABLE 2 Sample Arterial Blood Gas Report From a acids (e.g., ethylene glycol, salicylates) and by the inability of the kidneys to excrete H+ from dietary Patient With Acute Respiratory Failure protein (renal failure). These increases in H+ in the Analyte Value Reference Range
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