Arterial Blood Gases (ABG's) PDF

Arterial Blood Gases (ABG’s) Chapter 8  Chapter Overview ABG provides precise measurement of acid- base balance, and lung’s ability to oxygenate blood and remove carbon dioxide. Analysis of venous blood reflects local metabolic rates and is of little value. ABG analysis...

Arterial Blood Gases (ABG’s) Chapter 8  Chapter Overview ABG provides precise measurement of acid- base balance, and lung’s ability to oxygenate blood and remove carbon dioxide. Analysis of venous blood reflects local metabolic rates and is of little value. ABG analysis must be performed only when indicated. Useful when evaluating effectiveness of therapy.  Arterial Blood Sampling Always review chart for: – Physicians order – Patient’s condition Modified Allen’s Test – Clotting ability Sample obtained by insertion of needle into a major artery (arteriotomy). – Radial, dorsalis pedis, brachial, femoral.  ARTERIAL vs VENOUS BLOOD Arterial blood provides more information in regards to oxygenation and acid-base balance Arterial blood provides information on lung function and the adequacy of CO2 removal ABG Sampling Sites  Modified Allen’s Test Assessment of collateral circulation before radial artery sampling. Patient clenches fist while examiner obstructs radial and ulnar arteries. Patient gently opens hand while pressure is maintained over both arteries. Pressure over ulnar artery is released, and changes in color of patient’s palm are noted. Modified Allen’s Test Collateral blood flow: within 10-15 sec Positive Allen’s Test Negative test-collateral circulation is Poor, evaluate other hand or brachial site.  Arterial Blood Sampling Remove air bubbles before analysis. Store in ice water bath and send to lab within 30 minutes (especially if WBC elevated). Point of Care- analysis devices provide immediate results, computer downloaded to pt records. Apply pressure to UNTIL BLEEDING STOPS. – Longer if on heparin, low platelet count – Can apply cotton swab with bandaid if normal ABG errors – Air bubbles-O2 and CO2 may equilibrate with air O2 and CO2 – Heparin  Complications of ABG Thrombosis - formation of an abnormal clot within the vessel which can lead to cessation of blood flow Hemorrhage - patients on anticoagulation medications, or low platelet count Hematoma -leakage of blood into the tissues (common in the elderly population who may lack sufficient elastic tissue to seal the puncture site) Aerteriospasm- secondary reflex to pain  Complications continue Pain Infection Peripheral nerve damage Sometimes vasovagal responses occur Distress, anxiety, feeling of impending death, nausea, and respiratory difficulty ABG sampling  ABG Arterial Line Placement Indwelling Arterial Catheter when frequent sampling anticipated Used during acute hypotensive crisis because: Allows for continuous BP measurements Allows for easy access to blood for multiple ABGs Ensure the availability and accuracy of monitoring information Allows for continuous monitoring which is preferable to periodic, intermittent measurements Complications: Infection thrombosis Interpretation of Blood Gas Blood gas analysis evaluates: – Oxygenation status (PaO2, SaO2, CaO2, PvO2). – Acid-base balance (pH, PaCO2, HCO3-, BE). – Adequacy of ventilation (PaCO2).  Assessment of Oxygenation Involves evaluation of oxygen content in the blood and the patient’s tissue oxygenation. Oxygen primarily transported bound to Hgb. Smaller portion as dissolved gas in the blood. Data that reflects oxygenation: – PaO2 arterial partial pressure – SaO2 Hb saturation – CaO2 arterial content  Partial Pressures of Oxygen PaO2 PaO2-pressure exerted PaO2 depends on O2 by dissolved oxygen in concentration and the arterial blood. barometric pressure, as Reflects the lung’s well as lung ability to transfer O2 function(depends on from the inspired gas age or presence of into the circulating disease) blood. not all of the O2 in the alveoli diffuse into the pulmonary capillaries  Partial Pressure of Oxygen in Arterial Blood PaO2 Normal value: 80-100 mmHg on room air. Predicted PaO2 depends on several factors: – PB (760 mmHg at sea level, Atlanta-740) – Age (for every year over 60=drop of 1 mmHg) – FiO2 = fractionally inspired oxygen 0.21 to 1.0 – PAO2 = FiO2 (PB-PH2O) - (PaCO2 x 1.25) or PaCO2/0.8 – Either 1.25 or 0.8-factor for CO2 production  Alveolar air equation PAO2= FiO2(PB-PH2O) - (PaCO2 x 1.25) PAO2 = 0.21 (747-47) – (40 x 1.25) or (40/.8) = (0.21 x 700) – 50 = 150 -50 = 100 mmHg PAO2 > PaO2 by 10-15 mmHg Use this when evaluating if oxygen therapy is effective by comparing PAO2 with PaO2. This is useful when evaluating diffusion vs shunting problems PAO2: partial pressure of O2 in the alveoli PB: barometric pressure PaCO2: partial pressure of arterial CO2 PH2O: water vapor pressure in alveoli 1.25: factor based on the ratio of CO2 production to O2 consumption (respiratory quotient)  PaO2 If measured PaO2 is below normal, regardless of the FiO2, it is termed hypoxemia (PaO2 is less than 80 mmHg). Hypoxia is a condition of inadequate tissue oxygenation. Classification of hypoxemia (age 40%-smoke inhalation, fires – Visual impairment, myocardial disturbances, LOC, death.  SpO2 Measurement Conventional pulse oximeters use only two wavelengths of light, so they cannot distinguish between oxyhemoglobin and carboxyhemoglobin. They provide false high oxygen readings even when carboxyhemoglobin levels are as high as 70%. The new oximeter also uses eight wavelengths of light to measure oxyhemoglobin, instead of two.  Clinical Assessment of Oxygenation Tissue perfusion and oxygenation are evaluated better by using physical exam. ABG’s do not evaluate tissue oxygenation. Most important indicator of oxygenation: CaO2 - assess Hgb ( from either CBC or co-oximeter or blood gas machine). HYPOXEMIA - failure of the respiratory system to oxygenate blood adequately: seen as – Decreased PaO2, SaO2, CaO2.  Anemic Patient-Case Patient has a PaO2 95 mm Hg, SaO2 97% and Hgb of 7.5 g/dl. Does this patient have a normal blood oxygenation level? Hgb is half of normal so pt. is hypoxic. CaO2 = 1.34 x 7.5 x.97 + 0.003 x 95 CaO2 = 10.1 vol %, normal is 20 vol %. Carrying capacity half of normal. Thus a normal PaO2 should not mislead you into believing that a normal blood oxygenation is normal. Clinical Assessment of Oxygenation Increased P(A-a)O2, FiO2 and decrease PaO2. – Severe respiratory problem Hypoxemia with normal P(A-a)O2 – Low PIO2 at high altitude – Hypoventilation If hypoxemia present and: – PaCO2 + PaO2 = < 110 mmHg : lung disease (FiO2 =0.21) 50 + 50 = 100 mmHg – PaCO2 + PaO2 = 110-130 mmHg : Hypoventilation (FiO2= 0.21). 60 + 60 + 120 mmHg – PaCO2 + PaO2 = >130 mmHg: error or high FiO2. 50 + 150 mmHg = 200 mmHg  Clinical Assessment of Oxygenation Clinical signs of hypoxemia: – SOB (especially on exertion). – Tachycardia, tachypnea – Hypertension – Cyanosis (not in an anemic pt) – Worsening hypoxemia leads to bradycardia, met. Acidosis, hypotension and LOC.  Points to Remember Can a patient with a normal A-aDO2 be hypoxic? Yes. 60 mm Hg-50 mm Hg = 10 mm Hg PaO2 of 50 causes hypoxia The 10 mm Hg only tells us that diffusion is normal. No significant shunting present.  Points to Remember Patient is receiving an FiO2 0.60, the PaO2 is 95 mm Hg, PaCO2 30 mm Hg. Is the patient hypoxemic? No. – The PaO2 is normal and the patient is not considered to be hypoxemic. Is the PaO2 normal at an FiO2 of 0.60? No – The expected PaO2 should be 380 mm Hg. – Thus the A-aDO2 is 285 mm Hg. – Thus serious oxygen exchange is occurring from ventilation-perfusion mismatch.  Points to Remember A patient has a PaO2 of 95 mm Hg and a Hgb level of 7.5 gm/dl. Can this patient be hypoxic? Yes – Because content will be reduced from anemia. – This patient only has 10 Vol % available at the tissue level as compared to a normal of 20 Vol %.  Points to Remember A patient has a SpO2 of 90% and the Hgb is 5 gm/dl. Is this patient hypoxic? – Yes. Similar to previous case where the PaO2 is low and low Hgb. – 90% = 60 mm Hg – Low PaO2 and low Hgb produces hypoxic state. – Thus don’t let the SpO2 fool you into believing the patient is not hypoxic.  Points to Remember Can a patient with smoke inhalation with a SpO2 of 98% and Hgb of 13 gm/dl be hypoxic? Yes CO is attached to oxygen binding sites. – ABG needed to assess level of HbCO level. If SaO2 = 85% then PaO2 would be 50 mm Hg. Pulse oximeter does not distinguish oxyhemoglobin from carboxyhemoglobin. SpO2 only gives you the % of attachment.  Differential Diagnosis of Hypoxemia PaO2 A-aDO2 PaCO2 O2 Helpful Hypoventilation/ Low Normal High yes Low PAO2 Drugs, neuro, trauma Diffusion Defect Low High Normal- yes Pulmonary Edema low Shunting Low High Normal- No-use Atelectasis low CPAP /PEEP V/Q mismatch Low High Normal- Yes Pt. positioning low 59  Diffusion Defect Case A 77 year-old female patient has entered the ED with a history of left heart failure. Respiratory rate is 38/min, pulse 148/min, SpO2 90% and B/P 158/96 mm Hg. Breath sounds reveal crackles in bases. ABG’S done on room air reveals: pH 7.48, PaCO2 33 mm Hg, PaO2 60 mmHg with an A-aDO2 of 40 mmHg. The patient is placed on a nasal cannula at 3 L/min and 30 minutes later respiratory rate is 25/min, pulse is 110/min, SpO2 is 96% and BP 126/90 mm Hg. ABG’s pH 7.41, PaCO2 39 mm Hg, PaO2 82 mm Hg, A-aDO2 20 mm Hg. Based on these findings you would conclude: The patient has responded to oxygen therapy. What is the problem: Diffusion defect. Why? Diffusion Defect PaO2 - Low A-a DO2- High PaCO2- Respond to O2- Normal/low Yes Pulmonary Edema (CHF) Diffusion Case  Hypoventilation Case 76 year-old has a pH 7. 30, PaO2 65 mm Hg,, PaCO2 52 mm Hg, A-aDO2 23 mm Hg. Patient assessment includes respiratory rate 35/min, some accessory muscle use, BP 138/88, cool extremities. Breath sounds are diminished throughout both lung fields. The patient is placed on a nasal cannula at 3 L/min and 60 minutes later ABG shows pH 7.40, PaO2 87 mm Hg,, PaCO2 40 mm Hg, A-aDO2 15 mm Hg. Respiratory rate 8/min, BP 128/76, less accessory muscle use. Breath sounds are more audible throughout both lung fields. Which of the following caused this problem? Hypoventilation. Why? PaO2 A-aDO2 PaCO2 O2 Helpful Hypoventilation Low Normal High yes Drugs, neuro, trauma  Shunting Case A 65 year-old patient is admitted to ICU. ABG’s on 4 L/min nasal cannula show: pH 7.47, PaO2 45 mm Hg,, PaCO2 32 mm Hg, HCO3 23 mEq/L, A-aDO2 150 mm Hg. Patient was then changed to an FiO2 1.0. ABG’s 30 minutes later show: pH 7.42, PaO2 52 mmHg, PaCO2 43 mmHg, HCO3 25 mEq/L, A-aDO2 145 mm Hg. Patient was then placed on positive pressure ventilation (PPV). ABG’s after 1 hour show: pH 7.41, PaO2 76 mm Hg, A-aDO2 50 mm Hg, PaCO2 39 mm Hg. Which of the following caused the problem prior to PPV administration? Shunting. Why?  Shunting Case PaO2 A-aDO2 PaCO2 O2 Helpful Shunting Low High Normal-low No-use CPAP Atelectasis /PEEP