1 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Chapter 8 Interpretation of Blood Gases

2 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives After reading this chapter you will be able to:  Describe why arterial blood rather than venous blood is useful in determining a patient’s respiratory status  Define the importance of reviewing the laboratory data that reflect a patient’s clotting ability before performing an arterial puncture

3 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives (cont’d)  List the common sites for arterial puncture  Identify the test used to determine collateral circulation of the radial artery, how to perform this procedure, and how to interpret its results

4 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives (cont’d)  Describe how the following factors affect blood gas analysis:  Air bubbles in the syringe  Failing to put the sample on ice  Identify the normal duration of arterial puncture site compression  Identify normal values for these blood gas parameters at sea level, breathing room air:  pH; Pa O 2 ; Pa CO 2 ; HCO 3 – ; Sa O 2 ; P(a – a) O 2 ; Cao 2 ; base excess; Pv O 2

5 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives (cont’d)  Describe the clinical value of measuring the following indices of oxygenation:  Pa O 2 ; P(a – a) O 2 ; Sao 2 ; Ca O 2 ; Pv O 2 ; C(a – v) O 2 ; HbCO  Define hypoxia and hypoxemia  Identify the classifications of hypoxemia  Describe physiologic causes, mechanisms, and common physiologic cause of hypoxemia

6 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives (cont’d)  Explain how increases and decreases in PaCO 2, body temperature, and blood pH affect the oxyhemoglobin-dissociation curve and related SaO 2 measurements and oxyhemoglobin affinity  Explain how shifts in the oxyhemoglobin dissociation curve affect oxygen transport at the tissues and lungs  Describe the significance of and the factors that affect the following acid-base parameters:  pH; PaCO 2 ; plasma HCO 3 ; standard HCO 3 ;  Base excess

7 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives (cont’d)  Describe the Henderson-Hasselbalch equation and the ratio of HCO 3 to Pa CO 2 needed to maintain a pH of 7.40  Define simple and mixed acid-base abnormalities  Describe common causes and expected compensation for each of the following simple acid-base disorders:  Respiratory acidosis; respiratory alkalosis; metabolic acidosis; metabolic alkalosis

8 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Learning Objectives (cont’d)  List the common causes of the following mixed acid-base disorders:  Metabolic and respiratory alkalosis  Metabolic and respiratory acidosis  Describe the significance of the 95% confidence limit bands as used to assess acid-base status  Given the results of an arterial blood gas, interpret the acid-base and oxygenation status of the patient

9 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Introduction  Arterial blood O 2 and CO 2 levels reflect lung function  Analysis is helpful to guide treatment  Mixed venous blood reflects tissue conditions  Peripheral venous samples are of no value  Pulse oximetry reduces the need for ABGs but does not reflect CO 2 levels or acid- base status

10 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Arterial Blood Sampling  Chart review prior to arterial puncture  Check the blood clotting ability Low platelets or increased bleeding time indicate longer postpuncture pressure needs be applied Normal compression 3 to 5 minutes

11 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Arterial Blood Sampling (cont’d)  Radial artery preferred site  This is due to accessibility, ease of stabilization  Collateral circulation provided by ulnar artery  Other adult puncture sites include brachial, femoral, or dorsalis pedis arteries

12 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Arterial Blood Sampling (cont’d)  Modified Allen’s test  Assesses collateral circulation provided by ulnar artery  Steps to perform test Have patient form a fist Compress both the radial and ulnar arteries Have patient relax first, revealing a blanched palm Release pressure on ulnar artery Observe time required for hand to “pink up” Collateral flow adequate if this occurs in sec

13 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

14 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Arterial Blood Sampling (cont’d)  Handling of the arterial sample  Air bubbles in the sample must be removed CO 2 and O 2 may equilibrate between blood and bubbles, providing inaccurate values  Samples not analyzed within 15 min must be iced, but still must be analyzed within 1 hr This limits the effects of cellular metabolism Metabolism lowers O 2 and elevates CO 2

15 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements  Arterial and mixed venous samples are useful in the evaluation of:  Oxygen status by examination of: Pa O 2, Sa O 2, Ca O 2, Pv O 2  Acid-base balance by examination of: pH, Pa CO 2, HCO 3 –, BE  Adequacy of ventilation by examination of: Pa CO 2

16 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Normal arterial blood gas values (humans)  Pa O 2 80 to 100 mm Hg  Sa O 2 >95%  Ca O 2 16 to 20 ml/dl blood  pH 7.35 to 7.45  Pa CO 2 35 to 45 mm Hg  HCO 3 – 22 to 26 mEq/L  BE 0 ± 2  P(A-a) O 2 10 to 15 mm Hg on room air  Mixed venous oxygen (PvO 2 ) 38 to 42 mm Hg

17 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Assessment of oxygenation  Pa O 2 is the pressure exerted in blood by dissolved O 2 Reflects the lungs’ ability to transfer O 2 Diminishes slowly with age  Pa O 2 below range is called hypoxemia Generally hypoxemia is classified as:  Mild PaO 2 60 to 80 mm Hg  Moderate PaO 2 40 to 59 mm Hg  SeverePaO 2 <40 mm Hg

18 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Hypoxemia occurs secondary to:  V/Q mismatch (most common cause)  Shunt  Diffusion defect  True hypoventilation  Breathing a reduced partial pressure of O 2  Hypoxia: inadequate tissue oxygen

19 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Assessment of oxygenation  SaO 2 reflects actual amount of O 2 bound to Hb compared with total capacity Clinically >90% is usually considered adequate  CaO 2 is total amount of oxygen carried in blood Reflects dissolved (PaO 2 ) and that bound to Hb 99% of oxygen is carried bound to Hb

20 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Pv O 2 sample obtained from the pulmonary artery reflects tissue oxygenation  Pv O 2 reflects balance between oxygen delivery and oxygen consumption  Pv O 2 <35 mm Hg is strong evidence of poor tissue oxygenation  Sudden drop in Pv O 2 is most often caused by impaired circulation

21 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Carboxyhemoglobin (HbCO)  Amount of CO bound to Hb (normally 0% to 1%)  Measured by co-oximetry  CO competes with O 2 for Hb binding sites CO has 200 to 250 times’ greater affinity for Hb than O 2 Decreases Hb ability to bind with oxygen Shifts oxyhemoglobin dissociation curve to the left  For any SaO 2 will release less O 2 at the tissues

22 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

23 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Clinical assessment of oxygen  Goal: provide adequate tissue oxygenation  Evaluate ability of: Lungs to oxygenate blood  ABG analysis  Pulse oximetry Cardiovascular system to distribute blood  Physical assessment (see Chapter 5)

24 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Clinical assessment of oxygen  Hypoxemia noted by low Pa O 2, Sa O 2, Ca O 2  Differentiate between causes of hypoxemia by adding Pa O 2 + Pa CO 2 while breathing 0.21 F IO 2 : If total is between 110 and 130 mm Hg then simple hypoventilation exists If total <110 mm Hg then usually lung dysfunction, i.e., shunt, V/Q mismatch, diffusion defect

25 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Assessment of acid-base balance  pH reflects balance between blood acids and bases pH <7.35 considered acidotic or acidemia pH >7.45 considered alkalotic or alkalemia

26 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Assessment of acid-base balance  Pa CO 2 is the respiratory component Most reliable indicator of effectiveness of ventilation Hyperventilation: Pa CO 2 <35 mm Hg  Causes respiratory alkalemia Hypoventilation: Pa CO 2 >45 mm Hg  Causes respiratory acidemia

27 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Assessment of acid-base balance  Plasma HCO 3 – is the metabolic component Reflects ability of renal system to deal with acids HCO 3 – <22 mEq/L causes metabolic acidemia HCO 3 – >26 mEq/L causes metabolic alkalemia HCO 3 – may rise or fall to compensate for primary respiratory dysfunction  This generally takes 12 to 24 hours Plasma HCO 3 – is affected by directly by Pa CO 2

28 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Interpretation of Blood Gas Measurements (cont’d)  Assessment of acid-base balance  Standard HCO 3 – is plasma HCO 3 – that would be present if the Pa CO 2 were 40 mm Hg Theoretically provides a pure metabolic component  Base excess quantifies metabolic component Includes the buffering ability of RBCs Negative/positive: depends on buffer deviation from normal BE varies directly with pH

29 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Respiratory and Metabolic Acid-Base Disorders  Henderson-Hasselbalch equation  Defines the effects of HCO 3 – and CO 2 on pH pH = pK + log [HCO 3 – ] (renal) [Pa CO 2 x 0.03] (lungs) pK of system = 6.1 (constant) 0.03 = solubility factor to convert mm Hg to mEq/L  Normal ratio HCO 3 – /PaCO 2 is 20:1 As this changes the pH is directly affected

30 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Simple Acid-Base Disorders  Simple respiratory acidosis  Decreased alveolar ventilation  Pa CO 2 rises causing a fall in pH  May be caused by: Pulmonary disease Decreased drive to breathe: drug overdose, paralysis, head trauma Diminished ability to breathe: NMD, trauma, obesity  Compensation = renal retention of HCO 3 –

31 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Simple Acid-Base Disorders (cont’d)  Simple respiratory alkalosis  Alveolar ventilation exceeds CO 2 production  Pa CO 2 falls, causing a rise in pH  May be caused by: Pain, moderate hypoxemia, acidosis, anxiety  Compensation occurs by renal loss of HCO 3 – Fully compensated if pH returns to normal Partial compensation: pH returns toward normal

32 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Simple Acid-Base Disorders (cont’d)  Simple metabolic acidosis  Plasma HCO 3 – or BE falls below normal  Caused by: Decreased production or excess loss of buffers Increased production of acids or decreased ability to excrete acids  Compensation occurs by hyperventilation This is occurs rapidly Lack of compensation indicates a concurrent respiratory defect or respiratory acidosis

33 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Simple Acid-Base Disorders (cont’d)  Simple metabolic alkalosis  Elevated levels of plasma HCO 3 – or BE  Caused by: Accumulation of buffers in blood/significant acid loss  Compensation occurs by hypoventilation Seldom significant compensation in alert patient Comatose patients may have a significant response with a very high Pa CO 2  May require supplemental oxygen

34 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Mixed Acid-Base Disorders  Occurs when two simple acid-base disorders occur simultaneously  When HCO 3 – and Pa CO 2 deviate in opposite directions it is more difficult  Must know extent of metabolic and respiratory compensation that should occur with each disorder  When compensation is not appropriate a mixed disorder is usually present

35 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.

36 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Mixed Acid-Base Disorders (cont’d)  Respiratory and metabolic acidosis  Easily identified because the low HCO 3 – and high Pa CO 2 generally cause severe acidosis  Occurs in variety of situations Cardiopulmonary resuscitation COPD with hypoxia Poisoning and drug overdose

37 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Mixed Acid-Base Disorders (cont’d)  Respiratory and metabolic alkalosis  Easily identified because high HCO 3 and low Pa CO 2 may result in severe alkalosis  Situations that could result in this mixed disorder Critically ill patients in ICU Ventilator-induced alkalosis in the face of chronic hypercapnia –

38 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Mixed Acid-Base Disorders (cont’d)  Metabolic acidosis and respiratory alkalosis  More difficult to identify: each abnormality usually compensates for the other  Suspect a mixed disorder whenever the degree of compensation is more than expected  Critically ill patients are most likely to this combination of acid-base disorders  Prognosis is poor

39 Mosby items and derived items © 2010 by Mosby, Inc., an affiliate of Elsevier Inc. Mixed Acid-Base Disorders (cont’d)  Metabolic alkalosis and respiratory acidosis  Suspect a mixed disorder when the degree of compensation is more than expected  pH 7.40 with significant changes in Pa CO 2 and HCO 3 – indicates mixed disorder  This form of mixed acid-base disorder seen in COPD patients who retain CO 2 Steroid and diuretic therapies often cause metabolic alkalosis