Acid Base PhysiologyandArterial Blood Gas Interpretation By: Dr.behzad barekatain,MD Assistant professor of pediatrics neonatalogist
Selected Acid-Base Web Siteshttp://www.acid-base.com/http://www.qldanaesthesia.com/AcidBaseBook/http://www.virtual-anaesthesia-textbook.com /vat/acidbase.html#acidbasehttp://ajrccm.atsjournals.org/cgi/content/full/162/6/2246http://www.osa.suite.dk/OsaTextbook.htmhttp://www.postgradmed.com/issues/2000/03_00/fall.htmhttp://medicine.ucsf.edu/housestaff/handbook/HospH2002_C5.htm
Indications for ABG • (1) Severe respiratory or metabolic disorders • (2) Clinical features of hypoxia or hypercarbia • (3) Shock • (4) Sepsis • (5) Decreased cardiac output • (6) Renal failure • (7) Ideally any baby on oxygen therapy • (8) Inborn errors of metabolism • (9) ventilated infant • ………………….
INTRODUCTION An arterial blood gas (ABG) is a test that measures the oxygen tension (PaO2), carbon dioxide tension (PaCO2), acidity (pH), oxyhemoglobin saturation (SaO2), and bicarbonate (HCO3) concentration in arterial blood. Some blood gas analyzers also measure the methemoglobin, carboxyhemoglobin, and hemoglobin levels. Such information is vital when caring for patients with critical illness or respiratory disease. As a result, the ABG is one of the most common tests performed on patients in intensive care units (ICUs).
The sites, techniques, and complications of arterial sampling are reviewed here, as well as the transport and analysis of the arterial blood. Normal ABG values are provided and a common clinical situation in which the ABG results may be misleading is also described. Interpretation of abnormal ABG values and venous blood gases are also discussed
ARTERIAL SAMPLING Arterial blood is required for an ABG. It can be obtained by percutaneous needle puncture or from an indwelling arterial catheter. • A) Needle puncture — Percutaneous needle puncture refers to the withdrawal of arterial blood via a needle stick. It needs to be repeated every time an ABG is performed, since an indwelling catheter is not inserted.
Site selection The initial step in percutaneous needle puncture is locating a palpable artery. Common sites include the radial, femoral, brachial, dorsalis pedis, or axillary artery. There is no evidence that any site is superior to the others. However, the radial artery is used most often because it is accessible, easily positioned, and more comfortable for the patient than the alternative sites.
1.The radial artery is best palpated between the distal radius and the tendon of the flexor carpi radialis when the wrist is extended. To get the wrist into this position, the arm should be positioned on an arm-board with the palm facing upward and a large roll of gauze should be placed between the wrist and the arm-board in a position that extends the wrist. Taping the forearm and palm to the arm-board helps maintain the position.
2.The brachial artery is best palpated medial to the biceps tendon in the antecubital fossa, when the arm is extended and the palm is facing up. The needle should be inserted just above the elbow crease.
3.The femoral artery is best palpated just below the midpoint of the inguinal ligament, when the lower extremity is extended. The needle should be inserted at a 90 degree angle just below the inguinal ligament.
4.The dorsalispedis artery is best palpated lateral to the extensor hallucislongus tendon. It receives collateral flow from the lateral plantar artery through an arch similar to that in the hand.
5.The axillary artery is best palpated in the axilla, when the arm is abducted and externally rotated. There is good collateral flow to the arm through the thyrocervical trunk and subscapular artery; thus, the risk of ischemic complications to the arm is low. The needle should be inserted as high into the axilla as possible
Collateral circulation We believe that patients undergoing radial or dorsalis pedis artery puncture should have the collateral flow to those vessels evaluated prior to puncture, even though studies have found variable accuracy associated with such evaluations. Our belief is based upon the notion that the evaluation can be performed quickly at the bedside at no cost and with little risk for harm, but has substantial potential for benefit (ie, to identify patients who have impaired collateral circulation and, therefore, may be at increased risk of an ischemic complication).
The Allen test or modified Allen test can be performed in patients undergoing radial artery puncture. These are bedside tests that demonstrate collateral flow through the superficial palmar arch. To perform the modified Allen test, the patient's hand is initially held high with the fist clenched and both the radial and ulnar arteries compressed.This allows the blood to drain from the hand. The hand is then lowered, the fist is opened, and pressure is released from the ulnar artery. Color should return to the hand within six seconds, indicating that the ulnar artery is patent and the superficial palmar arch is intact. The test is considered abnormal if ten seconds or more elapses before color returns to the hand.
The Allen test (from which the modified Allen test evolved) is performed identically, except these steps are executed twice: once with release of pressure from the ulnar artery and once with release of pressure from the radial artery.
For patients undergoing dorsalispedis artery puncture, the dorsalispedis artery can be occluded, followed by compression of the nailbed of the great toe and assessment of the rapidity with which color returns to the nailbed after pressure is released from the great toe
Technique Once a palpable artery has been located, blood is withdrawn using the following steps. 1.The planned puncture site should be sterilely prepped. 2.Local analgesia prior to arterial puncture should be considered, since it appears to prevent pain without adversely impacting the success of the procedure. This was illustrated by a trial that randomly assigned 101 patients undergoing arterial puncture to receive 2 percent lidocaine, normal saline, or no agent prior to the procedure.The lidocaine decreased pain without increasing the difficulty of the procedure (ie, the number of attempts), compared to the other groups.
3.The seal of a heparinized syringe should be broken by pulling its plunger. The plunger can then be pushed back into the syringe, leaving a small empty volume (eg, less than 1 mL) in the syringe. A small needle (eg, 22 to 25 gauge) should then be attached to the syringe. Arterial blood gas kits are available, which contain a heparinized plastic syringe with the plunger already pulled back to allow for the collection of 2 mL of blood without the need to break the seal. 4.Using one hand to gently palpate the artery and the other to manipulate the syringe and needle, the artery should be punctured with the needle at a 30 to 45 degree angle relative to the skin. The syringe will fill on its own (ie, pulling the plunger is unnecessary). Approximately 2 to 3 mL of blood should be removed.
5.To prevent coagulation, the syringe should be rolled between the hands for a few seconds to allow blood to mix with the heparin. 6.After withdrawing a sufficient volume of blood, the needle should be removed and pressure applied to the puncture site for five to ten minutes to achieve hemostasis.
Complications Complications due to percutaneous needle puncture are rare. They include: *persistent bleeding *Bruising *injury to the blood vessel *Circulation distal to the puncture site may also be impaired following percutaneous needle puncture, presumably due to thrombosis at the puncture site.
. B) Indwelling catheters Arterial blood can also be obtained via an indwelling arterial catheter. Indwelling catheters provide continuous access to arterial blood, which is helpful when frequent blood gases are needed (eg, respiratory failure).
Venous blood gases and other alternatives to arterial blood gases An arterial blood gas (ABG) is the traditional method of estimating the systemic carbon dioxide tension and pH, usually for the purpose of assessing ventilation and/or acid-base status. However, the necessary sample of arterial blood can be difficult to obtain due to diminished pulses or patient movement. Diminished pulses may reflect poor peripheral circulation or low blood pressure, while patient movement is frequently caused by the pain associated with arterial puncture.
A venous blood gas (VBG) is an alternative method of estimating systemic carbon dioxide and pH that does not require arterial blood sampling. Performing a VBG rather than an ABG is particularly convenient in the intensive care unit, since most patients have a central venous catheter from which venous blood can be quickly and easily obtained.
Sampling sites A VBG can be performed using a peripheral venous sample (obtained by venipuncture), a central venous sample (obtained from a central venous catheter), or a mixed venous sample (obtained from the distal port of a pulmonary artery catheter). Central venous blood gases are preferred because their correlation with arterial blood gases is the most well-established by research and clinical experience.
Peripheral venous blood gases are an alternative for patients who do not have central venous access.If a tourniquet is used to facilitate venipuncture, it should be released about one minute before the sample is drawn to avoid changes induced by local ischemia.
Mixed venous blood gases are a reasonable alternative for patients whose venous access is a pulmonary artery catheter; however, a pulmonary artery catheter should not be inserted for the sole purpose of venous blood sampling
Measurements A VBG measures the venous oxygen tension (PvO2), carbon dioxide tension (PvCO2), acidity (pH), oxyhemoglobin saturation (SvO2), and serum bicarbonate (HCO3) concentration: • PvCO2, venous pH, and venous serum HCO3 concentration are used to assess ventilation and/or acid-base status • SvO2 is used to guide resuscitation during severe sepsis or septic shock, a process called early goal-directed therapy • PvO2 has no practical value at this time. It is not useful in assessing oxygenation because oxygen has already been extracted by the tissues by the time the blood reaches the venous circulation. The inability of a VBG to measure oxygenation is the major drawback compared with an ABG. To overcome this limitation, VBGs are often considered in combination with pulse oximetry.
Correlation with arterial blood gases Central venous *The central venous pH is usually 0.03 to 0.05 pH units lower than the arterial pH *the PCO2 is usually 4 to 5 mmHg higher *little or no increase in serum HCO3. Mixed venous blood gives results similar to central venous blood. peripheral venous *ThepH is approximately 0.02 to 0.04 pH units lower than the arterial pH *the venous serum HCO3 concentration is approximately 1 to 2 meq/L higher *the venous PCO2 is approximately 3 to 8 mmHg higher. There are no venous to arterial conversions for the central venous, mixed venous, or peripheral venous PvO2 or SvO2.
Misleading results The correlation between arterial and venous blood gas measurements varies with the hemodynamic stability of the patient. This observation has two practical consequences: First, clinicians should be wary of VBG results and preferentially obtain an ABG in hypotensive patients. Second, periodic correlation of the venous measurements with arterial measurements should be performed whenever venous measurements are used for serial monitoring.
END-TIDAL CARBON DIOXIDE Measurement of end tidal carbon dioxide (PetCO2) is another method of non-invasively estimating the arterial carbon dioxide tension (PaCO2). This technique, called capnography, requires a closed system of gas collection, either with a tight fitting mask or a ventilator circuit. A sample of expired gas is analyzed by infrared or mass spectrometry, and then displayed as a numerical value or a graph. The PetCO2 is usually within 1 mm of the PaCO2 in healthy adults, but it is far less accurate in critically ill adults because of the dependence of CO2 production on cardiac output .Thus, routine use of PetCO2 exists primarily in newborn ICUs, operating rooms, and emergency departments, to provide early warning of endotracheal tube complications. PetCO2 is reviewed in detail separately.
TRANSCUTANEOUS CARBON DIOXIDE Systems that measure both transcutaneous carbon dioxide (ptcCO2) and pulse oximetry are an attractive option because they overcome the major limitations of both ABGs (invasive arterial sampling) and VBGs (lack of information about oxygenation). Such combination systems generally have a heating element that raises the skin temperature to 42 to 45ºC to increase local perfusion, an electrode to measure ptcCO2, and a light emitter and sensor to measure arterial oxyhemoglobin saturation.
Older studies suggested that ptcCO2 measurements are accurate in neonates, but not critically ill adults because of poor peripheral perfusion (peripheral artery disease, hypotension, vasopressors). Devices have since improved and more recent observational studies suggest that the newer systems may be more accurate in critically ill patients, although their accuracy diminishes when the arterial carbon dioxide tension (PaCO2) is greater than 56 mmHg.
Such combination systems have limitations: 1.They may be difficult to keep calibrated 2.may be difficult to mount in a way that prevents air trapping 3.may take up to an hour to sufficiently warm the skin. 4.the devices must be attached to an ear, which may be difficult in agitated patients or in those who had neurosurgery Given the limitations of noninvasive monitoring, any persistent or unexpected change in the ptcCO2 or oxyhemoglobin saturation should be confirmed with an ABG. Clinical trials are necessary before combined pulse oximetry and ptcCO2 monitoring become routine care.
Comparison of Blood Gas Analysis at different sites ArterialCapillaryVenous • PH Same ---------- Lower • PO2 Higher ---------- Lower • PCO2 Lower Higher • HCO-3Same ---------- Same • Recommendation Good Fair Bad
SPECIMEN CARE Regardless of the method used to withdraw the arterial blood, several issues should be considered prior to sending the specimen to the laboratory: 1.Gas diffusion through the plastic syringe is a potential source of error. However, it appears that the clinical significance of the error is minimal if the sample is placed on ice and analyzed within 15 minutes. Using a glass syringe will also prevent this error. 2.The heparin that is added to the syringe as an anticoagulant can decrease in the pH if acidic heparin is used. It can also dilute the PaCO2, resulting in a falsely low value.Thus, the amount of heparin solution should be minimized and at least 2 mL of blood should be obtained.
3.Air bubbles that exceed 1 to 2 percent of the blood volume can cause a falsely high PaO2 and a falsely low PaCO2. The magnitude of this error depends upon the difference in gas tensions between blood and air, the exposure surface area (which is increased by agitation), and the time from specimen collection to analysis. The clinical significance of this error can be decreased by gently removing the bubbles without agitation and analyzing the sample as soon as possible.
4.This reduces oxygen consumption by leukocytes (ie, leukocyte larceny), which can cause a factitiously low PaO2.This effect is most pronounced in patients whose leukocytosis is profound. In addition, it reduces the likelihood that error due to gas diffusion through the plastic syringe or air bubbles will be clinically significant.
TRANSPORT The arterial blood should be placed on ice during transport to the lab and then analyzed as quickly as possible.
ANALYSIS Analysis of arterial blood is usually performed by automated blood gas analyzers, which automatically transport the specimen to electrochemical sensors to measure pH, PaCO2, and PaO2: • The PaCO2 is measured using a chemical reaction that consumes CO2 and produces a hydrogen ion, which is sensed as a change in pH • The PaO2 is measured using oxidation-reduction reactions that generate measurable electric currents • Automated blood gas analyzers rinse the system, calibrate the sensors, and report the results. Rigorous quality control by the laboratory is essential for accurate results.
Arterial blood gas measurements are effected by temperature. Specifically, pH increases and both PaO2 and PaCO2 decrease as temperature declines. The effect of temperature on blood gas measurements
Modern automated blood gas analyzers can report the pH, PaO2, and PaCO2 at either 37ºC (the temperature at which the values are measured by the blood gas analyzer) or at the patient's body temperature. Most centers report the values of pH, PCO2, and PO2 at 37ºC, even if the patient's body temperature is different. However, this practice is controversial
Terminology of ABG • Acidemia— An arterial pH below the normal range (less than 7.36). • Alkalemia— An arterial pH above the normal range (greater than 7.44). • Acidosis — A process that tends to lower the extracellular fluid pH (hydrogen ion concentration increases). This can be caused by a fall in the serum bicarbonate (HCO3) concentration and/or an elevation in PCO2. • Alkalosis — A process that tends to raise the extracellular fluid pH (hydrogen ion concentration decreases). This can be caused by an elevation in the serum HCO3 concentration and/or a fall in PCO2.
Metabolic acidosis — A disorder that causes reductions in the serum HCO3 concentration and pH. • Metabolic alkalosis — A disorder that causes elevations in the serum HCO3 concentration and pH. • Respiratory acidosis — A disorder that causes an elevation in arterial PCO2 and a reduction in pH. • Respiratory alkalosis — A disorder that causes a reduction in arterial PCO2 and an increase in pH.
Simple acid-base disorder — The presence of one of the above four disorders with the appropriate respiratory or renal compensation for that disorder. • Mixed acid-base disorder — The simultaneous presence of more than one acid-base disorder. Mixed acid-base disorders can be suspected from the patient's history, from a lesser- or greater-than-expected compensatory respiratory or renal response, and from analysis of the serum electrolytes and anion gap. As an example, a patient with severe vomiting would be expected to develop a metabolic alkalosis due to the loss of acidic gastric fluid. If, however, the patient developed hypovolemic shock from the fluid loss, the ensuing lactic acidosis would lower the elevated serum HCO3 possibly to below normal values, resulting in acidemia
INTERPRETATION ABGs provide information about oxygenation, ventilation, and acid-base balance • Acid-Base & ventilation Information • pH • PCO2 • HCO3 [calculated vs measured] • Oxygenation Information • PO2 [oxygen tension] • SO2 [oxygen saturation]
Each day approximately 15,000 mmol (considerably more with exercise) of carbon dioxide (CO2, which can generate carbonic acid as it combines with water) and 50 to 100 meq of nonvolatile acid (mostly sulfuric acid derived from the metabolism of sulfur-containing amino acids) are produced. Acid-base balance is maintained by normal pulmonary and renal excretion of carbon dioxide and nonvolatile acid, respectively.
General principles in ABG interpretation The Henderson-Hasselbalch equation shows that the pH is determined by the ratio of the serum bicarbonate (HCO3) concentration and the PCO2, not by the value of either one alone. Each of the simple acid-base disorders is associated with a compensatory respiratory or renal response that limits the change in ratio and therefore in PH. pH = 6.10 + log ([HCO3-] ÷ [0.03 x PCO2])
Hydrogen Ions H+ is produced as a by-product of metabolism. [H+] is maintained in a narrow range. Normal arterial pH is around 7.4. A pH under 7.0 or over 7.8 is compatible with life for only short periods.