Blood Gas Analysis

Carrie George, MDPediatric Critical Care MedicineAdapted 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 22-26 -2 to +2

Blood Gas Analysis

1. Determine if pH is acidotic or alkalotic

2. Determine cause:1. Respiratory2. Metabolic3. 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

• Arterial pH = 7.40• Venous pH = 7.35

6.9 7.0 7.4 7.5

Acidosis Neutral pH Alkalosis

Acid vs. Alkaline Blood pH

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

1. Check pH2. Check pCO23. 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 #2Every 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)

= 12.97If 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