Arterial Blood Gas Analysis

Transcription

1 Arterial Blood Gas Analysis L Lester Bones, Brains & Blood Vessels

2 Drawn from radial or femoral arteries. Invasive procedure Caution must be taken with patient on anticoagulants

3 ph: PaO 2 : 10-13kPa (75-100mmHg) PaCO 2 : 4.7-6kPa (35-45mmHg) HCO 3 : 22-26mmol/l SaO 2 : 95-99% Base excess (+2)-(-2)

4 Indication of oxygenation status Determination of acid-base state Assess ventilation

5 Respiratory Failure = PaO2 <8kPa Type I PaCO 2 <6.5kPa Type II PaCO 2 >6.5kPa

6 PaO 2 is the partial pressure of O 2 Measures O 2 dissolved in the plasma. 3% of arterial oxygen content O 2 Saturation measures the amount of Hb saturated with oxygen. 97% of arterial oxygen content.

7 Shows relationship between PaO 2 and O 2 saturation SaO2% These two values trend in the same direction Curve allows us to estimate PaO 2 based on non-invasive peripheral O 2 saturation. PaO2

8 SaO2 % Oxyhaemoglobin Dissociation Curve PO2 PaO 2

9 Potential for delay in detection of acute hypoxaemia as a large decrease in PO2 will not produce a significant fall in SaO2 until the steeper portion of O 2 Hb curve is encountered at a PO2 of approx 8kPa Pulse oximeter may not detect hypoxaemic event until well after it occurred Unable to detect hyperoxia eg. Neonatal O 2 toxicity Does not assess ventilation ie supplemental O 2 may prevent hypoxaemia despite severe hypoventilation

10 Right shift ( O 2 affinity for Hb) Acidosis Hyperthermia Hypercarbia Left shift ( O 2 affinity for Hb) Alkalosis Hypothermia Hypocarbia Decreased 2,3-DPG O2 release to cells and tissue hypoxia even when sufficient O2 in blood Increased 2,3 DPG

11 SaO2 % Oxyhaemoglobin Dissociation Curve PO2 PaO 2

12 This relationship is critical for homeostasis Significant deviations from normal PH ranges are poorly tolerated and may be life threatening Achieved by respiratory and renal systems

13 Respirator y Metabolic H 2 O + CO 2 D H 2 CO 3 D HCO 3 + H +

14 PH= log ([HCO3] [0.03xPCO2]) The Henderson-Hasselbalch equation shows that PH is determined by the ratio between HCO 3 concentration PCO 2,

15 Rapid regulation (short-term) Lungs Chronic control (long-term) Kidneys Gut

16 A process that tends to lower the extracellular fluid ph (which is equivalent to raising the hydrogen concentration). From the Henderson-Hasselbalch equation, this can be induced by a fall in the extracellular (or plasma ) bicarbonate concentration or by an elevation in the PCO2.

17 A process that tends to raise the extracellular fluid ph (which is equivalent to lowering the hydrogen concentration). From the Henderson-Hasselbalch equation, this can be induced by an elevation in the extracellular (or plasma ) bicarbonate concentration or by a fall in the PCO2.

18 CO2 is an acid HCO 3 is a base Regulated by the kidneys Regulated by the lungs Too much causes alkalosis Too much causes acidosis

19 Respiratory Acidosis **********too much CO 2 Respiratory Alkalosis **********too little CO 2

20 Metabolic Acidosis *********too little HCO3 Metabolic Alkalosis *********too much HCO3

21 ph 7.30 ( ) PaCO 2 5.3kPa (4.7-6kPa) HCO 3 15mmol/l (22-26mmol/l)

22 Failure of kidney function Blood HCO 3 which results in $ availability of renal tubular HCO 3 for H + excretion ph < 7.35

23 Fixed acid accumulation and low serum bicarbonate Renal failure Renal/GI losses Lactic Salicylate eg. diarrhoea Ketones Methanol Phosphate Ethylene glycol HCl

24 Renal/Hepatic failure Excessive diarrhoea Diabetic ketoacidosis Lactic acidosis Methanol, aspirin ingestion Cardiac arrest

25 Symptoms and signs Depend on rapidity of onset and severity Fatigue, abdominal pain, dyspnoea Kussmaul respirations Effects Negative cardiac contractility Peripheral venous constriction

26 ph 7.50 ( ) PCO kPa (4.7-6kPa) HCO 3 30mmol/l (22-26mmol/l)

27 # plasma HCO 3 > 26mmol/l ph > 7.45

28 # loss of acid from 1. stomach (vomiting eg. Pyloric stenosis/ nasogastric suction) 2. kidney (mineralocorticoid excess, loop or thiazide diuretics)

29 ph 7.50 ( ) PaCO 2 4kPa (4.7-6kPa) HCO 3 22mmol/l (22-26mmol/l)

30 ph > 7.45 PCO 2 < 4.7kPA (35mmHg) Too much CO 2 exhaled (hyperventilation)

31 Pulmonary causes Pneumonia. Pulmonary edema. Non-pulmonary causes Sepsis. Pain. Liver disease. Salicylate intoxication. Brain lesions. Pulmonary embolus. Interstitial fibrosis. Asthma.

32 ph 7.30 ( ) PaCO 2 8kPa (4.7-6kPa) HCO 3 26mmol/l (22-26mmol/l)

33 Too much CO2 ph < 7.35 PCO2 > 6kPa (45mmHg)

34 Any cause of hypoventilation: Neuromuscular disease Head injury Sedatives, narcotics Atelectasis Obstructed airway

35 ph 7.21, PaCO 2 8.4kPa, Pa kPa, HC0 3 27mmol/L 1. Evaluate the ph. ph is not in normal range, but is decreased (acidosis). 2. Evaluate the PaC0 2.

36 Evaluate the ph Normal Below 7.35= acidaemia Above 7.45= alkalosis If a patient has a mixed acid-base disorder, the ph identifies the process in control

37 Evaluate ventilation Normal PaCO kPa (35-45mmHg) greater than 6kPa= ventilatory failure and respiratory acidosis less than 4.7kPa = alveolar hyperventilation and respiratory

38 Evaluate metabolic process Normal HCO mmol/l HCO 3 < 22 = metabolic acidosis HCO 3 > 26 = metabolic alkalosis

39 Determine primary and compensating disorder When both PCO 2 and HCO 3 are abnormal, one reflects the primary acid-base disorder and the other reflects the compensating disorder To decide which is which, check the ph

40 Three states of compensation possible: 1. Noncompensation- alteration of only pco 2 or HCO 3 2. Partial compensation- when both pco 2 and HCO 3 are abnormal and because compensation is incomplete, the ph is also abnormal;

41 Evaluate oxygenation Normal PO kPA (75-100mmHg) PO kPa (60-75mmHg) = mild hypoxemia PO kPa (40-60mmHg)= moderate hypoxemia

42 Interpret Final analysis should include Degree of compensation The primary disorder

43 Respiratory acidosis D metabolic alkalosis Respiratory alkalosis D metabolic acidosis In respiratory conditions, therefore, the kidneys will attempt to compensate and visa versa

44 Extra base chemicals in patient s blood Includes bicarbonate, sulfates, phosphates Amount of acid needed to bring ph to 7.4 Negative value is a base deficit or acidosis

45 Anion Gap Na + K + = Cl - Albumin Other Cations Na + - (Cl - +HCO 3- ) = Anion Gap HCO 3- Other Anions

46 Anion Gap= Na+ - (Cl - +HCO 3- ) Measure of fixed or organic acids eg. Phosphates, ketones, lactate, drugs: salicylates, biguanides, toxins: ethylene glycol Helps determine presence and cause of metabolic acidosis.

47 Normal= 12 +/-2 <12= Diarrhoea, RTA, CaCl or other acids > 15 = Methanol Uraemia Diabetic ketoacidosis Paraldehyde Alcoholic ketoacidosis

48 In chronic respiratory acidosis (COPD) the kidneys increase the elimination of H+ and absorb more HCO3. The ABG will show normal ph, #CO2 and #HCO3 Buffers kick in within minutes. Respiratory compensation is rapid and starts within minutes and complete within 24 hours. Kidney compensation takes hours and up to 5 days

49 PARAMETER COMPENSATION Metabolic Acidosis PCO 2 1kPa per 6 mmol/l Δ HCO 3 Metabolic Alkalosis PCO 2 1kPa per 6 mmol/l Δ HCO 3 Acute Respiratory Acidosis Chronic Respiratory Acidosis HCO 3 HCO mmol/l per 1kPa Δ PCO2 3mmol/l per 1kPa Δ PCO2 Acute Respiratory Alkalosis HCO 3 1.5mmol/l per 1kPa Δ PCO2 Chronic Respiratory Alkalosis HCO 3 3 mmol/l per 1kPa Δ PCO2

50 Compare to previous ABG Look at patient s history and present situation Determine what is needed to correct situation

51 Mr. O Brien is a 60 y/o with pneumonia. He is admitted with dyspnoea, fever. ABG: ph 7.28 ( ) PCO 2 7.5kPa (4.7-6kPa) HCO 3 25mmol/l (22-26mmol/l) PO 2 9.3kPa (10-13kPa)

52 Case #1 Respiratory Acidosis with mild hypoxaemia

53 Ms. O Keeffe is a 24 year old college student. She has a history of Crohn s disease c/o 4 day history of bloody diarrhoea. An ABG is obtained: ph 7.28 ( ) PCO 2 5.7kPa (4.7-6kPa) HCO 3 20mmol/l (22-26mmol/l) PO kPa (10-13kPa)

54 Case #2 Non-compensated Metabolic Acidosis

55 You find Mr. O Regan in respiratory distress. H/O diabetes and is febrile. ABG: ph 7.00 ( ) PCO 2 7.9kPa (4.7-6kPa) HCO 3 14mmol/l (22-26mmol/l) po 2 8.5kPa (10-13kPa)

56 Case #3 Metabolic and Respiratory acidosis with moderate hypoxemia

57 Mrs Kinnane is a thin, 61y/o COPD patient. Her ABG: ph 7.37 PCO 2 8.4kPa HCO 3 35mmol/l PO 2 7.7kPa SaO 2 89%

58 Case #4 Fully compensated respiratory acidosis with moderate hypoxemia

59 Mrs. Murphy is found pulseless and not breathing. After a couple of minutes she responds with a pulse and is breathing. ABG: ph 6.89 PCO kPa HCO 3 13mmol/l

60 Case #5 Metabolic & Respiratory Acidosis With severe hypoxemia

61 Uptodate.com Googlescholar.com Kumar and Clarke s Textbook of medicine Oxford Hand-book of Medicine