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Blood Gas Analysis Carrie George, MD Pediatric Critical Care Medicine Adapted from Dr. Lara Nelson.

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Presentation on theme: "Blood Gas Analysis Carrie George, MD Pediatric Critical Care Medicine Adapted from Dr. Lara Nelson."— Presentation transcript:

1 Blood Gas Analysis Carrie George, MD Pediatric Critical Care Medicine Adapted from Dr. Lara Nelson

2 Blood Gas Analysis Acid-base status Oxygenation

3 Anatomy of a Blood Gas pH/pCO 2 /pO 2 /HCO 3 Base: metabolic Oxygenation: lungs/ECMO Acid: lungs/ECMO The sum total of the acid/base balance, on a log scale (pH=-log[H+])

4 Blood Gas Norms pHpCO 2 pO 2 HCO 3 BE Arterial to +2 Venous ~ to +2

5 Blood Gas Analysis 1.Determine if pH is acidotic or alkalotic 2.Determine cause: 1.Respiratory 2.Metabolic 3.Mixed 3. Check oxygenation

6 Acid-Base Regulation Three mechanisms to maintain pH –Respiratory (CO2) –Buffer (in the blood: carbonic acid/bicarbonate, phosphate buffers, Hgb) –Renal (HCO 3 - )

7 Acid-Base Equation: the carbonic acid/bicarbonate CO2 + H2O H2CO3 HCO3- + H+ Respiratory componentBlood/renal component Acid Base

8 Arterial pH = 7.40 Venous pH = Acidosis Neutral pH Alkalosis Acid vs. Alkaline Blood pH

9 Etiology Respiratory Metabolic Mixed

10 Rule #1 Every change in CO2 of 10 mEq/L causes pH to change by 0.08 (or Δ1 = 0.007) Increased CO2 causes a decreases in pH Decreased CO2 causes an increase in pH

11 Respiratory Acidosis Hypercarbia from hypoventilation Findings: –pCO 2 increased therefore… pH decreases Example: ABG : 7.32/50/ /25

12 Respiratory Alkalosis Hypercarbia from hypoventilation Findings: –pCO2 decreased… therefore pH increases Example: ABG – 7.45/32/ /25

13 Metabolic Changes Remember normal HCO 3 - is 22-26

14 Rule #2 Every change in HCO 3 - of 10 mEq/L causes pH to change by 0.15 Increased HCO 3 - causes an increase in pH Decreased HCO 3 - causes a decrease in pH

15 Metabolic Acidosis Gain of acid – e.g. lactic acidosis Inability to excrete acid – e.g. renal tubular acidosis Loss of base – e.g. diarrhea Example: –ABG – 7.25/40/ /15

16 Metabolic Alkalosis Loss of acid – e.g. vomiting (low Cl and kidney retains HCO 3 - ) Gain of base – e.g. contraction alkalosis (lasix) Example: –ABG – 7.55/40/ /35

17 Mixed pH depends on the type, severity, and acuity of each disorder Over-correction of the pH does not occur

18 Practical Application 1.Check pH 2.Check pCO2 3.Remember Rule #1 Every change in CO2 of 10 mEq/L causes pH to change by 0.08

19 Practical Application cont. 4. Does this fully explain the results? 5. If not, remember Rule #2 Every change in HCO3- of 10 mEq/L causes pH to change by 0.15

20 Example #1 ABG- 7.30/48/ /22 Acidotic or Alkalotic? pCO2 High or Low? pH change = pCO2 change? Combined respiratory and metabolic acidosis

21 Example #2 ABG- 7.42/50/ /32 Acidotic or Alkalotic? pCO2 High or Low? pH change = pCO2 change? Metabolic alkalosis with respiratory compensation

22 Oxygen Supply and Demand Arterial oxygen depends on: -Lungs ability to get O2 into the blood -Ability of hemoglobin to hold enough O2

23 Bedside Questions of Oxygenation Does supply of O2 equal demand? Is O2 content optimal? Is delivery of O2 optimal?

24 Mixed Venous Saturation SvO 2 : What is it? -In simple terms, it is the O 2 saturation of the blood returning to the right side of the heart - This reflects the amount of O 2 left after the tissues remove what they need SvO 2 = O 2 delivered to tissues – O 2 consumption

25 Oxygen Delivery O 2 transport to the tissues equals arterial O 2 content x cardiac output -DO 2 = CaO 2 x CO - Normal DO 2 = 1000 ml/min

26 Arterial Oxygen Content CaO 2 = (1.34 x Hgb x SaO 2 ) + (PaO 2 x ) Normal CaO 2 = 14 +/- 1 ml/ dl Example: CaO 2 = (1.34 x 10 x 95)+(78 x ) = If Hgb is 12, CaO 2 = If PaO2 2 is 150, CaO 2 = 13.20

27 Mixed Venous Oxygen Content CvO 2 = (1.34 x Hgb x SvO 2 ) + (PvO 2 x ) Normal CvO 2 = 14 +/- 1 ml/dl

28 Oxygen Consumption VO 2 = (CaO 2 – CvO 2 ) x CO Fick equation Normal VO 2 = 131 +/- 2 ml/min

29 Mixed Venous Saturation SvO 2 = O 2 delivered to tissues – O 2 consumption How do we know what it is? - Calculate it - Direct blood gas analysis, e.g. from a pulmonary catheter - Oximetry

30 Normal Mixed Venous Saturation Normal value -68%-77% -Change from arterial saturation of 20% to 30% Values less than 50% are worrisome, or a change of 40%- 50% Values less than 30% suggest anaerobic metabolism The most useful application is to follow trends

31 Oxygen Saturation and pO 2 An O 2 saturation of 75% correlates with a PaO 2 of about 45 mmHg This is on the step portion of the oxygen dissociation curve

32 Oxygen Dissociation Curve

33 Utility of MVO2 Gives information about the adequacy of oxygen delivery Suggests information about oxygen consumption Can help determine the usefulness of clinical interventions

34 Decreased MVO 2 Oxygen delivery is not high enough to meet tissue needs. Poor saturation Anemia Poor CO Increased tissue extraction

35 Increased MVO 2 Wedged PA catheter Improvement in previous poor situation Shunting -Tissues no longer extracting oxygen -How can you tell?

36 End-Organ Perfusion Brain - Neurologic exam Kidneys -Urine output - Creatinine Lacitic acidosis

37 NIRS Near Infrared Regional Spectroscopy An alternative strategy for measuring localized perfusion

38 How the INVOS System Works rSo2 index represents the balance of site-specific O2 delivery and consumption It measures both venous (~75%) and arterial (~25%) blood Indicates adequacy of site-specific tissue perfusion in real- time Correlates positively with SvO2, but is site-specific and noninvasive rSO2 is not a simple blood gas, it measures the amount of oxyhemoglobin in the tissue

39 Cerebral/Peri-Renal NIRS Monitoring

40 Cerebral rSO2 Normal values: - 30% less than the arterial saturation - Even in cyanotic heart disease this is true Concentrating values : - A change of 20% from baseline - rSO2 < 60% As with MVO2 trends are the most helpful application

41 Peri-Renal rSO2 Normal Values: - 10%-15% less than the arterial saturation - Even in cyanotic heart disease this is true Concerning values: - A change of 20% from baseline -rSO2 < 60% As with MVO2 trends are the most helpful application

42 Why Monitor Both? More information is always better Perfusion is differentially distributed, i.e. generally cerebral blood flow is maintained at the expense of other organs


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