{"id":1314,"date":"2013-05-01T16:23:30","date_gmt":"2013-05-01T16:23:30","guid":{"rendered":"http:\/\/phosdev.com\/todaysveterinarypractice\/?p=1314"},"modified":"2022-02-17T19:07:51","modified_gmt":"2022-02-17T19:07:51","slug":"the-practitioners-acid-base-primer-obtaining-interpreting-blood-gases","status":"publish","type":"post","link":"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/clinical-pathology\/the-practitioners-acid-base-primer-obtaining-interpreting-blood-gases\/","title":{"rendered":"The Practitioner&#8217;s Acid\u2013Base Primer: Obtaining &amp; Interpreting Blood Gases"},"content":{"rendered":"<p class=\"p1\"><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2016\/09\/T1305F04.pdf\"><img decoding=\"async\" class=\"alignnone size-full wp-image-9886\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2011\/07\/pdf_button.png\" alt=\"pdf_button\" width=\"110\" height=\"27\" \/><\/a><\/p>\n<hr \/>\n<p class=\"p1\"><em><span class=\"s1\">Lori S. Waddell, DVM, Diplomate ACVECC<\/span><\/em><\/p>\n<p class=\"p1\"><span class=\"s1\">This article is the first in a 2-part series that addresses alterations in acid\u2013base and respiratory function, which are common in both emergency patients as well as hospitalized, critically ill patients. Familiarity with obtaining and interpreting blood gases can be essential in the management of these patients.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\"><strong>Part 1<\/strong> will cover obtaining and interpreting blood gases, while <strong>Part 2<\/strong> will discuss differential diagnoses and therapeutic options associated with acid\u2013base abnormalities.<\/span><\/p>\n<h2 class=\"p3\"><span class=\"s1\"><b>ACID\u2013BASE OVERVIEW<\/b><\/span><\/h2>\n<p class=\"p1\"><span class=\"s1\">Metabolic acid\u2013base alterations can lead to: <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Altered cardiovascular, neurologic, and respiratory function<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Altered response to various drug therapies.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">Signs of acid\u2013base disturbances are usually vague and cannot be differentiated from clinical signs associated with the underlying disease, making blood gas analysis essential. Both arterial and venous blood gas samples can be used to interpret metabolic derangements.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">Metabolic acid\u2013base alterations can often be corrected via appropriate IV fluid therapy, other pharmacologic interventions and, ultimately, by addressing the underlying disease.<\/span><\/p>\n<div class=\"orange-box\">\n<h2 class=\"p3\"><span class=\"s1\"><b>REVIEW OF DEFINITIONS<\/b><\/span><\/h2>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">An <\/span><span class=\"s2\"><b>ACID<\/b><\/span><span class=\"s1\"> is a molecule that donates a hydrogen ion (H<sup>+<\/sup>) when a base molecule accepts one.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">A <\/span><span class=\"s2\"><b>BUFFER<\/b><\/span><span class=\"s1\"> is a weak acid or base, which helps protect against large changes in pH. <\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><span class=\"s1\">The <strong>primary extracellular buffer<\/strong> is bicarbonate. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">The <strong>intracellular buffers<\/strong> are phosphate, proteins, and hemoglobin.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Bone also acts as a buffer. <\/span><\/li>\n<\/ul>\n<\/li>\n<li class=\"li1\"><b><\/b><span class=\"s2\"><b>pH<\/b><\/span><span class=\"s1\"> is the measure of acidity\/alkalinity, and equal to the negative logarithm of H<sup>+<\/sup> concentration. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s2\"><b>ACIDEMIA<\/b><\/span><span class=\"s1\"> is a blood pH &lt; 7.35; <\/span><span class=\"s2\"><b>ALKALEMIA<\/b><\/span><span class=\"s1\"> is a blood pH &gt; 7.45.<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s2\"><b>ACIDOSIS<\/b><\/span><span class=\"s1\"> and <\/span><span class=\"s2\"><b>ALKALOSIS<\/b><\/span><span class=\"s1\"> refer to the process causing a pH disturbance. Four basic types of acid\u2013base disturbances have been classified by the traditional Henderson\u2013Hasselbach approach: <\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Metabolic acidosis:<\/b> A primary gain in acid or loss of base<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Metabolic alkalosis:<\/b> A primary gain in base or loss of acid<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Respiratory acidosis:<\/b> Retention of CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> due to CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> production outpacing alveolar ventilation <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Respiratory alkalosis:<\/b> Removal of CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> (by ventilation) outpacing CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> production<\/span><\/li>\n<\/ul>\n<\/li>\n<li class=\"li1\"><b><\/b><span class=\"s2\"><b>PaO<\/b><\/span><span class=\"s5\"><b><sub>2<\/sub><\/b><\/span><span class=\"s1\"> is the partial pressure of oxygen dissolved in arterial blood. It is a measure of oxygenation, not ventilation. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s2\"><b>PaCO<\/b><\/span><span class=\"s5\"><b><sub>2<\/sub><\/b><\/span><span class=\"s1\"> is the partial pressure of carbon dioxide dissolved in arterial blood. It provides the best measure of a patient&#8217;s ability to ventilate and determines whether respiratory acidosis or alkalosis is present. Remember that CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> is approximately 20\u00d7 more diffusible than O<sub>2<\/sub>, making it easier for a patient to maintain normal CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> concentrations in the presence of lung disease. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s2\"><b>PvCO<\/b><\/span><span class=\"s5\"><b><sub>2<\/sub><\/b><\/span><span class=\"s1\"> is the partial pressure of carbon dioxide dissolved in venous blood. When the sample has been obtained properly, it measures a patient&#8217;s ability to ventilate, similar to PaCO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">.<\/span><\/li>\n<li class=\"li4\"><b><\/b><span class=\"s1\"><b>BASE EXCESS\/DEFICIT (BE):<\/b><\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><span class=\"s1\">Reflects the metabolic portion of the acid\u2013base balance, which takes into account all of the body&#8217;s buffer systems<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Estimates how much base needs to be added or subtracted to achieve a normal pH at normal temperature <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Evaluates for metabolic acidosis or alkalosis.<\/span><\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p class=\"p3\"><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.54.45-PM.png\"><img fetchpriority=\"high\" decoding=\"async\" class=\"alignnone wp-image-4291 size-medium\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.54.45-PM-e1456760049221-300x295.png\" alt=\"Screen Shot 2015-06-04 at 2.54.45 PM\" width=\"300\" height=\"295\" \/><\/a><\/p>\n<\/div>\n<h2 class=\"p3\"><span class=\"s1\"><b>RESPIRATORY FUNCTION OVERVIEW<\/b><\/span><\/h2>\n<p><span class=\"s1\">Respiratory function, more specifically the patient&#8217;s ability to oxygenate and ventilate, can be evaluated with <strong>arterial blood gases<\/strong>. However, in most cases, <strong>venous blood gases<\/strong> can also be used to assess ventilation, as venous CO<span class=\"s4\"><sub><span style=\"font-size: small\">2<\/span><\/sub><\/span> is typically about 5 mm Hg higher than arterial CO<span class=\"s4\"><sub><span style=\"font-size: small\">2<\/span><\/sub><\/span><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">Other options for respiratory function evaluation include physical examination findings and pulse oximetry to detect hypoxemia, although arterial blood gases remain the gold standard.<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Patients with hypoxemia may become cyanotic, but this sign will not present until hypoxia is severe, and may only become evident just prior to death, making it an unreliable indicator of adequate oxygenation. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Respiratory rate and effort can be difficult to interpret as they are affected by many other factors, such as pain, excitement, fear, and metabolic derangements, and can be masked by sedation or anesthesia.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">If a patient is in respiratory distress, supplemental oxygen should be provided prior to obtaining samples for blood gas analysis.<\/span><\/p>\n<div class=\"orange-box\">\n<h3 class=\"p3\"><span class=\"s1\"><b>Blood Gas Analyzers<\/b><\/span><\/h3>\n<div id=\"attachment_9248\" style=\"width: 234px\" class=\"wp-caption alignnone\"><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/050613-acid-base-fig-1.png\"><img decoding=\"async\" aria-describedby=\"caption-attachment-9248\" class=\"wp-image-9248 size-medium\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/050613-acid-base-fig-1-224x300.png\" alt=\"050613 acid base fig 1\" width=\"224\" height=\"300\" srcset=\"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/050613-acid-base-fig-1-224x300.png 224w, https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/050613-acid-base-fig-1.png 404w\" sizes=\"(max-width: 224px) 100vw, 224px\" \/><\/a><p id=\"caption-attachment-9248\" class=\"wp-caption-text\">Figure 1. Arterial blood gas sample being entered into a blood gas analyzer (heska.com)<\/p><\/div>\n<p class=\"p1\"><span style=\"line-height: 1.5\">There are a variety of different blood gas analyzers on the market, ranging from small point-of-care analyzers to larger laboratory equipment (<\/span><b style=\"line-height: 1.5\">Figure 1<\/b><span style=\"line-height: 1.5\">). Smaller machines are less expensive, relatively good quality, and easy to run and maintain. Some of the larger machines require high maintenance but also provide the best quality control and are more economical when large numbers of samples are anticipated. The type of practice and volume of blood gases the practice evaluates help determine which type of analyzer is best.<\/span><\/p>\n<\/div>\n<h2 class=\"p3\"><b>OBTAINING &amp; HANDLING BLOOD GAS SAMPLES<\/b><\/h2>\n<p>It is essential that blood gas samples be properly obtained and handled, particularly venous samples. Sample error can be introduced in a number of ways (see <strong>Potential Sample Errors<\/strong>).<\/p>\n<div class=\"orange-box\">\n<h2 class=\"p3\"><b style=\"line-height: 1.5\">POTENTIAL SAMPLE ERRORS<\/b><\/h2>\n<ul>\n<li><span class=\"s1\">I<b>f a sample is obtained from a peripheral vein<\/b> in patients with poor perfusion, the sample may reflect the acid\u2013base status of that limb alone rather than that of the whole body. <\/span><\/li>\n<li><b><\/b><span class=\"s1\"><b>If a limb&#8217;s vein is occluded for several minutes<\/b> to obtain a sample, the sample may reflect lactic acidosis specific only to that limb. <\/span><\/li>\n<li><b><\/b><span class=\"s1\"><b>If the sample is not immediately evaluated or placed on ice until evaluation<\/b>, ongoing cellular metabolism by red blood cells will continue to use O<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">, produce CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">, decrease pH, and eventually, once O<sub>2<\/sub> has been depleted, increase lactate concentration, which further reduces pH as well as HCO<\/span><span class=\"s4\"><sub>3<\/sub><sup>&#8211;<\/sup><\/span><span class=\"s1\"> and BE. <\/span><\/li>\n<li><b><\/b><span class=\"s1\"><b>If the sample is exposed to air<\/b> (ie, a blood tube with air in it or an uncapped syringe), the oxygen from the atmosphere will diffuse into the sample while CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> diffuses out, which directly affects PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">, PaCO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">, and pH, rendering the calculated values for HCO<\/span><span class=\"s4\"><sub>3<\/sub><sup>&#8211;<\/sup><\/span><span class=\"s1\"> and BE inaccurate and making the sample worthless for acid\u2013base interpretation.<\/span><\/li>\n<\/ul>\n<\/div>\n<h3 class=\"p5\"><span class=\"s1\"><b>Venous Samples<\/b><\/span><\/h3>\n<ol>\n<li class=\"p5\"><span class=\"s1\">Ideally, a venous sample should be taken from a central catheter (in the cranial or caudal vena cava) or by direct jugular venipuncture in order to obtain the best representation of the global acid\u2013base and respiratory status.<\/span><\/li>\n<li><span class=\"s1\">Samples should be capped off to prevent exposure to air; then processed immediately. If the sample cannot be processed immediately, it should be placed on ice until evaluation can take place. <\/span><\/li>\n<li><span class=\"s1\">Samples can either be: <\/span>\n<ul>\n<li><span class=\"s1\">Processed immediately without anticoagulant <\/span><\/li>\n<li><span class=\"s1\">Drawn into a syringe that has been coated in heparin until they can be processed. <\/span><\/li>\n<\/ul>\n<\/li>\n<li><span class=\"s1\">To prepare a sample with heparin, coat a 3-mL syringe with a small amount of liquid heparin; then draw air up to the syringe&#8217;s 3-mL mark and forcibly expel the heparin several times. While most of the heparin will be removed from the syringe, enough heparin will remain to affect the measured ionized calcium, which will be unreliable.<sup>3<\/sup><\/span><\/li>\n<\/ol>\n<h3 class=\"p5\"><span class=\"s1\"><b>Arterial Samples<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">For additional information, see <b>Step by Step: Obtaining Arterial Blood Gas Samples<\/b>.<\/span><\/p>\n<ol class=\"ol1\">\n<li class=\"li1\"><span class=\"s1\">Use of a local anesthetic will make the procedure more comfortable for awake patients.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Based on the patient, there are numerous sites from which an arterial sample can be taken: <\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Awake dogs:<\/b> A metatarsal branch of the dorsal pedal artery is preferred. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Anesthetized patients:<\/b> Coccygeal, auricular, and radial arteries may also be used; sample collection from these arteries is not well tolerated in awake patients.<sup>3<\/sup><\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Small patients:<\/b> The femoral artery is typically used; however, if there is excessive bleeding after sampling, this bleeding is much harder to manage with a pressure bandage compared to other sites. Only use this site if sampling from other sites is not possible.<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Cats:<\/b> Arterial puncture in cats is particularly difficult due to their smaller arteries and the fact that they are hard to restrain. Therefore, cats should be sedated or under general anesthesia. The dorsal pedal, femoral, and coccygeal arteries are the most common sites used in anesthetized cats.<\/span><\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<p class=\"p1\"><span class=\"s1\">Patients in respiratory distress may not tolerate the positioning and restraint needed to obtain arterial samples. A venous blood gas and pulse oximetry reading may be preferable in these patients.<\/span><\/p>\n<div class=\"orange-box\">\n<h2 class=\"p3\"><span class=\"s1\"><b>STEP BY STEP: OBTAINING ARTERIAL BLOOD GAS SAMPLES<\/b><\/span><\/h2>\n<h3 class=\"p4\"><span class=\"s1\"><b>What You Will Need<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">\u2022 Lithium heparin arterial blood gas syringe with needle (usually 25 gauge) <em><strong>or<\/strong> <\/em><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">\u2022 25-gauge needle and 3-mL syringe coated with liquid heparin (as described for venous sample collection)<sup>3<\/sup><\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Step by Step: Obtaining an Arterial Sample <\/b><\/span><\/h3>\n<ol class=\"ol1\">\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Clip and aseptically prepare the site<\/b> chosen for arterial puncture; a metatarsal branch of the dorsal pedal artery is the most common site used. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Place the patient in lateral recumbency<\/b>, using the recumbent limb.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">With the nondominant hand, <b>palpate the pulse<\/b> between the second and third metatarsals. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">With the dominant hand, <b>slowly insert the needle<\/b> at a 30<sup>o<\/sup> to 40<sup>o<\/sup> angle. Continue very slowly advancing the needle, watching for a flash of blood in the needle&#8217;s hub. <\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><span class=\"s1\">If no flash is seen, slowly back the needle out, watching for a flash (it is possible that the needle was inserted through the vessel and a sample can be obtained as the needle is backed out). <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Otherwise, once the needle is very superficial, redirect it if a sample has not yet been obtained.<\/span><\/li>\n<\/ul>\n<\/li>\n<li class=\"li1\"><span class=\"s1\">Once a flash is seen:<\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><span class=\"s1\">If an arterial blood gas syringe is being used, <b>allow the syringe to automatically fill<\/b>.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">If a 3-mL syringe is being used, <b>gently aspirate the plunger<\/b> to withdraw blood. <\/span><\/li>\n<\/ul>\n<\/li>\n<li class=\"li1\"><span class=\"s1\">After the quantity of blood needed has been obtained, <b>remove the needle from the artery and apply pressure to the site<\/b>, followed by application of a pressure bandage for 30 to 60 minutes.<sup>3<\/sup><\/span><\/li>\n<\/ol>\n<div id=\"attachment_3149\" style=\"width: 310px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3149\" class=\"wp-image-3149 size-medium\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Figure-2-arterial-blood-gas-stick-300x225.jpg\" alt=\"Figure 2 arterial blood gas stick\" width=\"300\" height=\"225\" srcset=\"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Figure-2-arterial-blood-gas-stick-300x225.jpg 300w, https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Figure-2-arterial-blood-gas-stick.jpg 400w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/><p id=\"caption-attachment-3149\" class=\"wp-caption-text\">Figure 2. Obtaining an arterial blood gas sample from the dorsal pedal artery of a 2-year-old neutered male pitbull that was anesthetized for exploratory laparotomy and foreign body removal (from the pylorus of stomach) after presenting for vomiting<\/p><\/div>\n<h3><b>Step by Step: Placing an Arterial Catheter<\/b><\/h3>\n<p class=\"p1\"><span class=\"s1\">Arterial catheters are extremely useful in patients that require repeat arterial blood gas sampling, such as those on mechanical ventilation.<\/span><\/p>\n<ol class=\"ol1\">\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Use of sedation or a local anesthetic<\/b>, such as lidocaine, is recommended. Placement during general anesthesia is ideal.<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Clip and aseptically prepare the site<\/b> chosen for arterial puncture; a metatarsal branch of the dorsal pedal artery is the most common site used. The radial, coccygeal, femoral, and auricular arteries may also be catheterized; however, catheters at these sites are less well tolerated in awake patients and, therefore, more commonly used during anesthesia.<sup>3,4<\/sup><\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Place the patient in lateral recumbency<\/b>, using the recumbent limb.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Using an over-the-needle short catheter (usually 22-gauge), <b>penetrate the skin<\/b> between the second and third metatarsals and approximately 1\/3 distally from the hock to the metatarsophalangeal joint. <i>If the skin is very thick, make a small nick in the skin with the bevel of a 20-gauge needle prior to inserting the catheter, which prevents burring.<\/i><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">The artery travels dorsolaterally (<b>Figure 3<\/b>), at an approximately 30<sup>o<\/sup> angle to a perpendicular line drawn between these 2 joints; <b>angle the catheter appropriately.<\/b><\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Once arterial blood has flashed into the catheter, <b>advance it into the artery and remove the needle stylet. <\/b><\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Attach a t-piece to the catheter, inject a small quantity (1\u20131.5 mL) of heparinized saline, and tape the catheter in place<\/b> as would be done for a standard IV catheter. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Label the catheter as <i>arterial<\/i><\/b> to avoid inadvertent administration of injections through the catheter.<\/span><\/li>\n<\/ol>\n<p><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/050613-acid-base-fig-3.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-9249 size-medium\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/050613-acid-base-fig-3-e1456761915946-291x300.png\" alt=\"050613 acid base fig 3\" width=\"291\" height=\"300\" \/><\/a><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Arterial Catheters in Cats<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">Arterial catheters are difficult to place in cats due to their small size. Additionally, cats have less collateral circulation in their distal limbs and, therefore, are more predisposed to ischemic injury of the foot or tail after arterial catheterization. Arterial catheters should not be left in cats for longer than 6 to 8 hours.<sup><sub>4<\/sub><\/sup><\/span><\/p>\n<h3 class=\"p4\"><span class=\"s1\"><b>Skill Set Required<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">Obtaining arterial blood samples requires more skill than that required for obtaining venous samples; the vessel cannot be seen and only palpated by pulse (with the exception of the auricular artery, which can often be seen and felt running down the middle of the dorsal aspect of the pinna). Practice, especially on anesthetized patients, and knowledge of the anatomic location of the artery are helpful when developing this skill.<\/span><\/p>\n<\/div>\n<h2 class=\"p4\"><b style=\"line-height: 1.5\"><strong>INTERPRETING BLOOD GAS RESULTS<\/strong><\/b><\/h2>\n<p>There are 6 steps required to interpret blood gas results:<\/p>\n<h3 class=\"p7\"><span class=\"s1\"><b>1. Determine If Sample Is Venous or Arterial <\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">Either sample type can be used to evaluate overall acid\u2013base status, with the exception of severe shock and post arrest situations, which may result in large discrepancies between arterial and venous samples. Poor tissue perfusion can result in sizeable increases in CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> and secondary decreases in pH on the venous side despite low to normal CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> on the arterial side.<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">Although information can be gained about ventilation from a venous sample, only an arterial sample can truly assess oxygenation. <\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">If unable to obtain an arterial sample, use: <\/span>\n<ul class=\"ul2\">\n<li class=\"li1\"><span class=\"s1\">Pulse oximetry to measure oxygen saturation<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Venous sample to evaluate acid\u2013base status and estimate ventilation. <\/span><\/li>\n<\/ul>\n<\/li>\n<li class=\"li1\"><span class=\"s1\">If the patient is intubated, end tidal CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> can also be used to estimate ventilation, but with severe pulmonary disease, end tidal CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> can be much lower than PaCO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">.<\/span><\/li>\n<\/ul>\n<p><span class=\"s1\"><b>2. Assess Patient for Acidemia (pH &lt; 7.35) or Alkalemia (pH &gt; 7.45) <\/b><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">If pH is within normal limits, the patient&#8217;s body may have compensated for underlying disturbances or a mixed disturbance may be present. See Steps 3 and 4 to evaluate if metabolic or respiratory disturbances are present despite normal pH.<\/span><\/p>\n<p class=\"p7\"><span class=\"s1\"><b>3. Perform Additional Assessments for Acidosis<\/b><\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Respiratory acidosis<\/b> is present if PaCO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> &gt; 45 mm Hg. <\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Metabolic acidosis<\/b> is present if BE &lt; -4 mmol\/L (or HCO<\/span><span class=\"s4\"><sub>3<\/sub><sup>&#8211;<\/sup><\/span><span class=\"s1\"> &lt; 19 mmol\/L).<\/span><\/li>\n<\/ul>\n<p class=\"p7\"><span class=\"s1\"><b>4. Perform Additional Assessments for Alkalosis<\/b><\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Respiratory alkalosis<\/b> is present if PaCO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> &lt; 35 mm Hg<\/span><\/li>\n<li class=\"li1\"><b><\/b><span class=\"s1\"><b>Metabolic alkalosis<\/b> is present if BE &gt; 2 mmol\/L (or HCO<\/span><span class=\"s4\"><sub>3<\/sub><sup>&#8211;<\/sup><\/span><span class=\"s1\"> &gt; 25 mmol\/L)<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">For cats, substitute the reported normal values for PaCO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> and BE from <b>Table 1<\/b> into steps 3 and 4.<\/span><\/p>\n<p class=\"p7\"><span class=\"s1\"><b>5. Assess Oxygenation <\/b><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">Normal PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> is 90 to 100 mm Hg. If the patient is on supplemental oxygen, PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> should equal approximately 5\u00d7 the FiO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">; the FiO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> of room air is 21%.<\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">These rules apply to the normal values listed in <b>Table 1<\/b> for dogs.<\/span><\/p>\n<p class=\"p7\"><span class=\"s1\"><b>6. Determine Whether Compensatory Changes Have Occurred<\/b><\/span><\/p>\n<p class=\"p1\"><span class=\"s1\">For example, if a primary metabolic acidosis is present, a compensatory respiratory alkalosis may also exist. Remember the rules of compensation:<\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">A change in the respiratory or metabolic component of the acid\u2013base status normally induces an opposite compensatory response in an effort to normalize the pH.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">The lungs can compensate quickly by adjusting minute ventilation in a matter of minutes.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">The kidneys compensate more slowly, with compensation beginning within a few hours and maximum compensation taking 4 to 5 days.<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">The absence or presence and degree of compensation provides some information about the chronicity of the disturbance (<b>Table 2<\/b>).<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">Overcompensation does not occur.<\/span><\/li>\n<\/ul>\n<p><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.55.16-PM.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-4292 size-large\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.55.16-PM-1024x285.png\" alt=\"Screen Shot 2015-06-04 at 2.55.16 PM\" width=\"650\" height=\"181\" srcset=\"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.55.16-PM-1024x285.png 1024w, https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.55.16-PM-300x84.png 300w, https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.55.16-PM-768x214.png 768w, https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Screen-Shot-2015-06-04-at-2.55.16-PM.png 1346w\" sizes=\"(max-width: 650px) 100vw, 650px\" \/><\/a><\/p>\n<h3 class=\"p5\"><span class=\"s1\"><b>Other Useful Equations<\/b><\/span><\/h3>\n<p class=\"p1\"><span class=\"s1\">Another tool for interpreting lung function is the PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">:FiO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> ratio, which allows arterial blood gases and concentrations of inspired oxygen to be evaluated and compared. A normal PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\">:FiO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> ratio is approximately 500 (PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> = 100 mm Hg, FiO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> = 0.21). If a patient is on 100% oxygen, the expected PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> would be 500 mm Hg. This is helpful in interpreting samples from patients under general anesthesia and those on supplemental oxygen that are too unstable to obtain samples when breathing room air.<\/span><\/p>\n<h2 class=\"p3\"><span class=\"s1\"><b>IN SUMMARY<\/b><\/span><\/h2>\n<p class=\"p1\"><span class=\"s1\">Interpretation of venous and arterial blood gases can be essential to treatment of many patients. Blood gas analyzers are becoming more common in veterinary practices and this analysis can aid in diagnosis and therapy for patients, indicating: <\/span><\/p>\n<ul class=\"ul1\">\n<li class=\"li1\"><span class=\"s1\">When fluid therapy is indicated<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">What fluid types are the best choices<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">If sodium bicarbonate should be administered<\/span><\/li>\n<li class=\"li1\"><span class=\"s1\">When oxygen and mechanical ventilation are needed, including when the patient can be weaned off this support.<\/span><\/li>\n<\/ul>\n<p class=\"p1\"><span class=\"s1\">BE = base excess\/deficit; CO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> = carbon dioxide; FiO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> = fractional inspired oxygen concentration; GI = gastrointestinal; HCO<\/span><span class=\"s4\"><sub>3<\/sub><sup>&#8211;<\/sup><\/span><span class=\"s1\"> = bicarbonate; PaO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> = partial pressure of oxygen in arterial blood; PaCO<\/span><span class=\"s4\"><sub>2<\/sub><\/span><span class=\"s1\"> = partial pressure of carbon dioxide in arterial blood<\/span><\/p>\n<h3 class=\"p1\">References<\/h3>\n<ol>\n<li class=\"p1\"><span class=\"s1\">DiBartola SP. Introduction to acid\u2013base disorders. In DiBartola SP(ed): <i>Fluid, Electrolyte, and Acid\u2013Base Disorders in Small Animal Practice<\/i>, 4th ed. St. Louis: Elsevier Saunders, 2012, pp 231-252.<\/span><\/li>\n<li class=\"p1\"><span class=\"s1\">Middleton DJ, Ilkiw JE, Watson ADJ. Arterial and venous blood gas tensions in clinically healthy cats. <i>Am J Vet Res<\/i> 1981; 42:1609-1611.<\/span><\/li>\n<li class=\"p1\"><span class=\"s1\">Mazzaferro EM Hauser C. Arterial puncture and catheterization. In Burkett Creedon JM, Davis H (eds): <i>Advanced Monitoring and Procedures for Small Animal Emergency and Critical Care<\/i>, Ames Iowa, Wiley-Blackwell, 2012, pp 69-81.<\/span><\/li>\n<li class=\"p1\"><span class=\"s1\">Mazzaferro EM. Arterial Catheterization. In Silverstein DC, Hopper K (eds): <i>Small Animal Critical Care Medicine<\/i>. St. Louis: Elsevier Saunders, 2009, pp 206-208.<\/span><\/li>\n<\/ol>\n<h3 class=\"p8\"><span class=\"s1\">Suggested Reading<\/span><\/h3>\n<p class=\"p8\"><span class=\"s1\">De Morias HA, Leisewitz AL. Mixed acid\u2013base disorders. In DiBartola SP (ed): <i>Fluid, Electrolyte, and Acid\u2013Base Disorders in Small Animal Practice<\/i>, 4th ed. St. Louis: Elsevier Saunders, 2012, pp 302-315.<\/span><\/p>\n<p class=\"p8\"><span class=\"s1\">DiBartola SP. Metabolic acid\u2013base disorders. In DiBartola SP (ed): <i>Fluid, Electrolyte, and Acid\u2013Base Disorders in Small Animal Practice<\/i>, 4th ed. St. Louis: Elsevier Saunders, 2012, pp 253-286.<\/span><\/p>\n<p class=\"p8\"><span class=\"s1\">Johnson RA, De Morias HA. Respiratory acid\u2013base disorders. In DiBartola SP (ed): <i>Fluid, Electrolyte, and Acid\u2013Base Disorders in Small Animal Practice<\/i>, 4th ed. St. Louis: Elsevier Saunders, 2012, pp 287-301.<\/span><\/p>\n<p class=\"p8\"><span class=\"author-bio\"><strong><a href=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Lori-S.-Waddell.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignleft wp-image-9250\" src=\"https:\/\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2013\/05\/Lori-S.-Waddell-150x150.png\" alt=\"Lori S. Waddell\" width=\"100\" height=\"114\" \/><\/a>Lori S. Waddell<\/strong>, DVM, Diplomate ACVECC, is an adjunct associate professor in critical care at the University of Pennsylvania School of Veterinary Medicine, working in the Intensive Care Unit. Her areas of interest include colloid osmotic pressure, acid\u2013base disturbances, and coagulation in critically ill patients. Dr. Waddell received her DVM from Cornell University; then completed an internship at Angell Memorial Animal Hospital in Boston, Massachusetts. After her internship, she worked as an emergency clinician in private practice until pursuing a residency in emergency medicine and critical care at University of Pennsylvania.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Lori S.<\/p>\n","protected":false},"author":1,"featured_media":2749,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"iawp_total_views":13670,"footnotes":""},"categories":[374],"tags":[13],"class_list":["post-1314","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-may-june-2013","tag-peer-reviewed","column-features","clinical_topics-clinical-pathology"],"acf":{"hide_sidebar":false,"hide_sidebar_ad":false,"hide_all_ads":false},"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v24.7 (Yoast SEO v27.3) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>The Practitioner&#039;s Acid\u2013Base Primer: Obtaining &amp; 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