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

Blood Gas Analysis Acid-base status Oxygenation

Anatomy of a Blood Gas pH/pCO2/pO2/HCO3 Base: metabolic Oxygenation: lungs/ECMO Acid: lungs/ECMO The sum total of the acid/base balance, on a log scale (pH=-log[H+])

Blood Gas Norms pH pCO2 pO2 HCO3 BE Arterial 7.35-7.45 35-45 80-100 22-26 -2 to +2 Venous 7.30-7.40 43-50 ~45

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

Acid-Base Regulation Three mechanisms to maintain pH Respiratory (CO2) Buffer (in the blood: carbonic acid/bicarbonate, phosphate buffers, Hgb) Renal (HCO3-)

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

Acid vs. Alkaline Blood pH Arterial pH = 7.40 Venous pH = 7.35 6.9 7.0 7.4 7.5 Acidosis Neutral pH Alkalosis

Etiology Respiratory Metabolic Mixed

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

Respiratory Acidosis Hypercarbia from hypoventilation Findings: pCO2 increased therefore… pH decreases Example: ABG : 7.32/50/ /25

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

Metabolic Changes Remember normal HCO3- is 22-26

Rule #2 Every change in HCO3- of 10 mEq/L causes pH to change by 0.15 Increased HCO3- causes an increase in pH Decreased HCO3- causes a decrease in pH

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

Metabolic Alkalosis Loss of acid – e.g. vomiting (low Cl and kidney retains HCO3-) Gain of base – e.g. contraction alkalosis (lasix) Example: ABG – 7.55/40/ /35

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

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

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

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

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

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

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

Mixed Venous Saturation SvO2: What is it? -In simple terms, it is the O2 saturation of the blood returning to the right side of the heart - This reflects the amount of O2 left after the tissues remove what they need SvO2 = O2 delivered to tissues – O2 consumption

Oxygen Delivery O2 transport to the tissues equals arterial O2 content x cardiac output -DO2 = CaO2 x CO - Normal DO2 = 1000 ml/min

Arterial Oxygen Content CaO2 = (1.34 x Hgb x SaO2) + (PaO2 x 0.0031) Normal CaO2 = 14 +/- 1 ml/ dl Example: CaO2 = (1.34 x 10 x 95)+(78 x 0.0031) = 12.97 If Hgb is 12, CaO2 = 15.52 If PaO22 is 150, CaO2 = 13.20

Mixed Venous Oxygen Content CvO2 = (1.34 x Hgb x SvO2) + (PvO2 x 0.0031) Normal CvO2 = 14 +/- 1 ml/dl

Oxygen Consumption VO2 = (CaO2 – CvO2) x CO  Fick equation Normal VO2 = 131 +/- 2 ml/min

Mixed Venous Saturation SvO2 = O2 delivered to tissues – O2 consumption How do we know what it is? - Calculate it - Direct blood gas analysis, e.g. from a pulmonary catheter - Oximetry

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

Oxygen Saturation and pO2 An O2 saturation of 75% correlates with a PaO2 of about 45 mmHg This is on the step portion of the oxygen dissociation curve

Oxygen Dissociation Curve

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

Decreased MVO2 Oxygen delivery is not high enough to meet tissue needs. Poor saturation Anemia Poor CO Increased tissue extraction

Increased MVO2 Wedged PA catheter Improvement in previous poor situation Shunting -Tissues no longer extracting oxygen -How can you tell?

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

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

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

Cerebral/Peri-Renal NIRS Monitoring

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

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

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