arterial blood gas analysis by mohamed hamdy assistant lecturer of anesthesia ain-shams un iversity...

Arterial Blood Gas Analysis By Mohamed hamdy Assistant lecturer of Anesthesia Ain-Shams Un iversity Egyptian Resuscitation Council (ERC)Instructor

Interpretation of ABG Analyses Systematic Approch ABG Abnormalities

Why we do Arterial Blood Gas Analysis? Oxygenation Represented by PaO2 Ventilation Represented by Pa Co2 Acid Base Status Represented by pH, HCO3 and base deficit. Hb, Hct, oxygen saturation Electrolyte e.g. Na +, K +.

Because H+ react highly with cellular proteins resulting in alteration in their function therefore avoiding acidemia and alkalemia by tightly regulation H + which is essential for normal cellular function.

Calculation of Alveolar Gas Equation and A-a Gradient: PAO 2 =FiO 2 (Bp-pH 2 O)-PaCO 2 /R. = 21(760-47)-40/0.8 =100 mmHg. A-a Gradient is alvealo-arterial O 2 gradient. A-a Gradient = PAO 2 -PaO 2 It is normally = Age/4+4. Its Value: concise D.D of hypoxemia. e.g.: Decrease FiO 2 Hypoventilation normal A-a Gradient Ventilation perfusion mismatch Rt to Lt shunting Diffusion abnormality increase A-a Gradient

7 Oxygen Content in Alveolus Gas ) (measured during exhalation) Oxygen Content in arterial blood (equivalent to that leaving lungs) What is an A - a gradient ? : The DIFFERENCE between: In a healthy person, what would you expect the A - a to be? No difference, greater than 0, or less than 0 Normal: A a, up to ~ 10 mm Hg, varies with age

8 Factors contributing to A - a Gradient 1.Blood Shunts 2.Matching 1.Blood Shunts 2.Matching

9 Alveolar SPACE arterial vessel SIMPLE CONCEPT OF A SHUNT BLOOD FLOW CO 2 O2O2O2O2 No Gas Exchange = SHUNT AIR FLOW Blood Mixing Lowered O 2 /l00 ml

10 Total Perfusion, Q Total Ventilation Matching What? Blood to Air Flow Exchange Oxygen If the volumes used for exchange are aligned We might consider the system to be ideally matched

11 Arterial Perfusion (Q c ) Slide or Misalign the distribution volumes Alveolar Ventilation (V A ) ExchangeOxygen Dead Air Space (Airways) Shunt (Q s ) (Bronchial Artery) Some Volumes are wasted, Matching Ratio = V A /Q c = 0.8 Normal Case; Small Shunt, low volume Dead Space

12 Alveolar Ventilation V A Arterial Perfusion Q Exchange Oxygen Dead Air Space Shunt = Lung Disease with a Large A a gradient

Approach To Hypoxemia PaO2 A-a Do2pCO2 N FIO2 N Alv. Hypo. 100% O2 Corrects V/Q Mis. No CorrectionShunt Diffusion

1) Arterial/alveolar ratio(a/A) P aO 2/P AO 2 Normal value for the a/A ratio is 0.8, meaning that 80% of the alveolar oxygen is reaching the arterial system 2) PaO2/ FIO2 ratio Normal ratio is 550 (a person breathing FIO2 of 1.0 at sea level should have a PaO2 of 550 to 600 mmHg) 3) A-a gradient (on 100% oxygen) PAO2 - PaO2 Where PAO2 is calculated by the alveolar air equation previously presented

16 PO 2 and PCO 2 in Blood

4) Arterial-alveolar PCO2 Gradient (a-A PCO2) Arterial PCO2 - Alveolar PCO2 Where Alveolar PCO2 is measured by means of endtidal PCO2 Normal gradient is an alveolar PCO2 2 mmHg less than arterial, Acute increase reflects increase in physiologic dead space

Oxygen Transport Whole blood oxygen content based on: hemoglobin content and, dissolved O 2 Described by the equation: CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2)

Oxygen Content Assuming 15 g/100ml Hb concentration O2 sat of 99% Hb O2 = 1.34 x 15 x 0.99 = 19.9 ml/dL For a PaO2 of 100 Dissolved O2 = 0.003 x 100 = 0.3 ml/dL

Oxygen Content Thus, most of blood O2 content is contained in the Hb PO2 is only important if there is an accompanying change in O2 sat. Therefore O2 sat more reliable than PO2 for assessment of arterial oxygenation

Oxygen Delivery O 2 delivery = DO 2 = CO x CaO 2 Usually = 520-570 ml/min/m 2

Oxygen Uptake A function of: Cardiac output Difference in oxygen content b/w arterial and venous blood VO 2 = CO x 1.34 x Hb (SaO 2 SvO 2 ) 10

Oxygen Extraction Ratio VO 2 /DO 2 x 100 Ratio of oxygen uptake to delivery Usually 20-30% Uptake is kept constant by increasing extraction when delivery drops.

Critical Oxygen Delivery Maximal extraction ~ 0.5-0.6 Once this is reached a decrease in delivery = decrease in uptake Known as critical oxygen delivery O 2 uptake and aerobic energy production is now supply dependent = dysoxia

Tissue Oxygenation In order for tissues to engage in aerobic metabolism they need oxygen. Allows conversion of glucose to ATP Get 36 moles ATP per mole glucose

Tissue Oxygenation If not enough oxygen, have anaerobic metabolism Get 2 moles ATP per mole glucose and production of lactate Can follow VO2 or lactate levels

Oxygen Transport O 2 is transported by the blood either, Combined with haemoglobin (Hb) in the red blood cells (>98%) or, Dissolved in the blood plasma (95% 60%-85%60%-80% HCO 3 22-26 22-28 Base difference (deficit excess) -2 to 2

What is PH? PH is ve log of H + concentration. Henderson-Hasselbalch Equation pH = pKa + log ([HCO 3 ] / 0.03 x pCO 2 ) Relationship between pH & [H + ] pH [H + ] (nanomoles/l) 6.8158 6.9125 7.0100 7.179 7.263 7.350 7.440 7.531 7.625 7.720 7.815 or more simply: The Henderson equation: [H + ] = 24 x ( pCO2 / [HCO3] )

STEPS for interpretation of ABG STEP 1: Determine if numbers fit: H + = H + = (7.8-PH)100. The Rt side of the equation should be within 10% of the Lt Side. If not so: Another ABG Chemistry panel for HCO 3 should be done.

STEP 2: Determine if: Acidemia (PH 7.44) is present. STEP 3: Identify primary disturbance: PH Increase Decrease Alkalosis Acidosis Look at PCO 2 IncreasedDecreaseIncreasedDecreased Metabolic Alkalosis Respiratory Alkalosis Respiratory acidosis Metabolic acidosis

STEP 4: Look at the direction of the change of HCO 3 /PCO 2 : If it is in the same direction it is either simple or mixed change. But if it is in the opposite direction so it is mixed change. STEP 5: Calculate rate of change of Hco 3 and co 2 (Expected compensation) Disturbance ResponseExpected change Metabolic acidosis Paco 2 1.2(24-HCO 3 measured) Metabolic alkalosis Paco 2 0.7(HCO 3 -24) Acute respiratory acidosis Hco 3 0.1(PaCO 2 -40) Chronic respiratory acidosis Hco 3 0.4(PaCO 2 -40) Acute respiratory alkalosis Hco 3 0.2(40-PaCO 2 ) Chronic respiratory alkalosis Hco 3 0.4(40-PaCO 2 )

Determine the Anion Gap The Anion Gap [(Na + ) + (K + )] [(Cl - ) + (HCO 3 - )] The normal A.G is 12meq 4. Normal A.G (Hyperchloremic Acidosis) GIT loss of HCO 3 as in: diarrhea, high output fistula renal HCO 3 loss as in: RTA(I, II) TPN. CL containing acids. Wide Anion Gap Acidosis ( endogenou non-volatile acid) Keto Acidosis Uremia Lactic Acidosis Salicylism Toxins : Methanol,Paraldehyde,Ethylene glycol

All anions and cations Na + + UC = (Cl - ) + (HCO 3 - ) + UA Na + (Cl - ) + (HCO 3 - ) = UA UC

Very low or even Ve A.G

Calculation of AG in urine: Urine AG = ( Na + + K + ) CL - In a patient with a hyperchloraemic metabolic acidosis: A negative UAG suggests GIT loss of bicarbonate (eg diarrhoea) A positive UAG suggests impaired renal distal acidification (ie renal tubular acidosis).

STEP 6: If there is metabolic acidosis calculate the A.G Anion Gap = Na + - (Cl - + HCO 3 - ) = 12meq 4. Corrected A.G = observed A.G + 2.5 (normal albumin - measured albumin). If the anion gap proceed to step 7.

STEP 7: If the anion gap metabolic acidosis is present we should evaluate for additional metabolic disorder because the elevation of anion gap above normal AG = (AG-12) should be buffered by HCO 3. Adding AG to current HCO 3 will yield the corrected Hco3 which should be normal value 24 meq/l unless there is another disorder present. Corrected HCO 3 = current HCO 3 (measured) +A.G (Normal value 24 meq/l) If corrected HCO 3 >24 metabolic alkalosis is also present If corrected HCO 3

Delta ratio = (Increase in anion gap / Decrease in bicarbonate ) Delta RatioAssessment Guideline < 0.4Hyperchloraemic normal anion gap acidosis 0.4 - 0.8Consider combined high AG & normal AG acidosis BUT note that the ratio is often 2Suggests a pre-existing elevated HCO 3 level: consider a concurrent metabolic alkalosis or a pre-existing compensated respiratory acidosis.

Final step: Be sure that the interpretation of blood gas is consistent and correlated with the clinical picture of the patient.

Case 1 A 75-year-old man presents to the ED after a witnessed out of hospital VF cardiac arrest. Arrived after 10 minutes, CPR had not been attempted. The paramedics had successfully restored spontaneous circulation after 6 shocks. On arrival the man is comatose with a GCS of 3 and his lungs are being ventilated with 50% oxygen via ET tube. He has a ST with rate of 120 min -1 and a blood pressure of 150/95 mmHg.

ABG Analysis reveals: F i O 2 0.5 pH7.10 PaCO 2 6.0 kPa (45 mmHg) PaO 2 7.5 kPa(56 mmHg) HCO 3 - 14 mmol l -1 BE- 10 mmol l -1

Case 2 A 65-year-old man with severe COPD has just collapsed in the respiratory high-care unit. On initial assessment he is found to be apnoeic but has an easily palpable carotid pulse at 90 min -1. A nurse is ventilating his lungs with a BVM and supplementary O2 (with reservoir )

ABG Analysis reveals: F i O 2 0.85 (estimated) pH 7.20 PaCO2 (80 mmHg) PaO2 (147 mmHg) HCO3- 40 mmol l-1 BE+ 14 mmol l-1

Case 3 A 75-year-old lady is admitted to the ED following a VF cardiac arrest, which was witnessed by the paramedics. A spontaneous circulation was restored after 4 shocks, but the patient remained comatose and apnoeic. The paramedics intubated her trachea, and on arrival in hospital her lungs are being ventilated with an automatic ventilator using a tidal volume of 900 ml and a rate of 18 breaths min -1.

ABG Analysis reveals: F i O 2 1.0 pH7.60 PaCO 2 2.65 kPa (20 mmHg) PaO 2 25.4 kPa (192 mmHg) HCO 3 - 20 mmol l -1 BE-2 mmol l -1

Case 4 An 18-year-old male insulin dependent diabetic is admitted to the ED. He has been vomiting for 48 hours and because he was unable to eat, he omitted his insulin. He has a ST at a rate of 130 min -1 and his blood pressure is 90/65 mmHg. He is breathing spontaneously with deep breaths at a rate of 35 min -1 and is receiving oxygen 6 l min -1 via O2 mask. His GCS is 12 (E3, M5, V4 ).

ABG Analysis reveals: F i O 2 0.4 pH 6.79 PaCO 2 (14 mmHg) PaO 2 (129.2 mmHg) HCO 3 - 4.7 mmol l -1 BE- 29.2 mmol l -1

Case 5 His vital signs are: Heart rate 120 min -1 sinus tachycardia warm peripheries Blood pressure70/40 mmHg Respiratory rate35 breaths min -1 SpO 2 on oxygen92% Urine output50 ml in the last 6 hours GCS13 (E3, M6, V4)

ABG Analysis reveals: F i O 2 0.4 (approx) pH7.12 PaCO 2 4.75 kPa (36 mmHg) PaO 2 8.2 kPa(62 mmHg) HCO 3 - 12 mmol l -1 BE- 15 mmol l -1

Case 6 Which patient is more hypoxemic, and why? Patient A: pH 7.48, PaCO 2 34 mm Hg, PaO 2 85 mm Hg, SaO 2 95%, Hemoglobin 7 gm% Patient B: pH 7.32, PaCO 2 74 mm Hg, PaO 2 55 mm Hg, SaO 2 85%, Hemoglobin 15 gm%

Patient A: Arterial oxygen content =.95 x 7 x 1.34 = 8.9 ml O2/dl Patient B: Arterial oxygen content =.85 x 15 x 1.34 = 17.1 ml O2/dl Patient A, with the higher PaO2 but the lower hemoglobin content, is more hypoxemic.

The PO2 in a cup of water open to the atmosphere is always higher than the arterial PO2 in a healthy person (breathing room air) who is holding the cup. Case 7 True or False

A patient is admitted to the ICU with reabeted vomiting the following BLOOD GASES pH: 7.40 PCO2: 38 HCO3: 24 PO2: 72 ELECTROLYTES, BUN & CREATININE Na: 149 K: 3.o Cl: 100 CO2: 24 BUN: 110 Creatinine: 8.7 What is(are) the acid-base disorder(s)? Case 8

The patient was both uremic (causing metabolic acidosis) and had been vomiting (metabolic alkalosis).

55 yrs old pt. who drink fifth of wesky per day has 2 wks history of diarrhea, Anion gap is 20, HCO3 = 10, PH = 7.30, PO2 =0.90 mmHg, PCO2 = 30 mmHg. Case 9 What is(are) the acid-base disorder(s)?

25 yrs pt. come to ER with fever, abd. pain, vomiting, with the history of migrane PH = 7.33, PCO2 = 8mmHg, PO2 = 80 mmHg, HCO3 = 4, Sodium = 140 mmol, K = 3 mmol, CL = 108 mmol. What is(are) the acid-base disorder(s)? Case 10

Life is a struggle, not against pharama, not against the physics, not against Anesthesia Exame, but against hydrogen ions. M. Hamdy

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arterial blood gas analysis by mohamed hamdy assistant lecturer of anesthesia ain-shams un iversity...

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