ABG SAVES LIVES

http://orlandohealth.com/MediaBank/Docs/SLP/2010%20ABG%20SLP.pdf

The Normal Values:

• pH: 7.35 - 7.45
• pCO2: 35 - 45mmHg/ 4 - 6
• pO2: 80 - 100mmHg/ 10 - 13
• HCO3: 22 - 26mmHg
• BE: +/- 3mEQ

Cellular Metabolism

HCO3-  +  H+  -->  CO2  +  H2O

• The by-product of cellular mechanism is CO2 and H2O
• CO2 is carried from the blood to the lungs
• Excess CO2 will combine with H2O to produce H2CO3 (Carbonic Acid)
• Why is it important to maintain normal cellular metabolism?
• The shift of equation in to maintain the normal body pH:
• CO2 will change the pH in the opposite direction (CO2 increase, PH decrease)
• HCO3- will change the pH in the same direction (HCO3- increase, pH increase)

Interpretation of ABG:

• ABG provides information on the pt's:
• Oxygenation status
• Acid-base balance status

Acid-Base Balance

• Normal PH range : 7.35 - 7.45
• This narrow normal PH range need to be maintain for normal cellular metabolism to take place
• pH less than 6.8 and more than 7.8 is not compatible with life
• Two main organs that are involved in the acid-base balance are the kidneys and the lungs
• The body maintains the normal PH range by two buffering mechanisms/ systems:
• Lung/ Respiratory buffer response (Respiratory Compensation)
• Renal/ Metabolic buffer response (Metabolic Compensation)

Respiratory (Lungs) Buffer Response

• Respiratory compensation takes 1-3min
• The level of carbonic acid will eitrher increase/ decrease the rate and depth of ventilation until appropriate CO2 level is re-established
• How does the carbonic acid increase/ decrease the rate and depth of ventilation? What is the underlying mechanism?

Metabolic (Renal) Buffer Response

• Metabolic compensation will take hours to days
• In the effort to maintain normal pH in the blood, the kidney either excrete or retain HCO3-
• When the pH rises, the kidney will compensate by excreting HCO3- in the urine
• When the pH decreases, the kidney will compensate by retaining HCO3-
• What is the role of haemodyalisis in acid-base balance?

Oxygenation status in ABG

• The component of ABG used to evaluate the blood oxygenation is paO2
• Why is the pO2 lower then the sO2 in certain circumstances? The answer lies behind the Oxyhaemoglobin Dissociation Curve.
• Type I Respiratory Failure
• Type 2 Respiratory Failure

Respiratory Acidosis

• Low pH, High pCO2
• Differential Dx
• Management

Respiratory Alkalosis

• High pH, Low pCO2
• Differential Dx
• Management

Metabolic Acidosis

• Low pH, Low HCO3-
• Differential Dx
• Management

Metabolic Alkalosis

• High pH, High HCO3-
• Differential Dx
• Management

Compensation

• In the event of an acid-base imbalance, the body attempts to compensate via two primary buffering response system which is the lungs and the kidneys
• The aim is to restore the body normal pH for optimal cellular metabolism to take place
• Compensation can be divided into:
• Uncompensated
• either the respiratory or metabolic component is abnormal, not both
• pH is abnormal
• Partially compensated
• both the respiratory and metabolic components are abnormal
• pH is abnormal
• Fully compensated
• both the respiratory and metabolic components are abnormal
• pH is within the normal range
• pH = 4 is the normal range
• In compensation, we need to know:
• are we dealing with acidosis or alkalosis
• which system is the primary problem
• which system is compensating

Mixed Respiratory and Metabolic Acidosis

• Low pH
• High pCO2 and Low HCO3-

Mixed Respiratory and Metabolic Alkalosis

• High pH
• Low pCO2 and High HCO3-

Chronic and Acute changes in ABG

• Metabolic compensation takes hours to days
• Respiratory compensation takes 1-3mins
• Hence, a Full metabolic compensation indicates a chronic problem whereas a Full respiratory compensation indicates an acute problem

Other issues in ABG interpretation

• What is the expected pco2 calculation in metabolic and respiratory acidosis?
• What is the role of Anion Gap calculation in metabolic acidosis?
• What is the role of Base excess calculation?
• False results in ABG
• Is it important to differentiate between a venous or arterial blood sample in ABG interpretation? How to differentiate between venous and arterial blood gases? How does this effect the ABG result interpretation? - regardless of whether it is venous or arterial, if pco2 is high - need to hyperventilate the patient, don't wait to repeat the ABG.

	7.35 – 7.45
pH	7.29	ACIDOSIS
pCO2	30	ALKALOSIS
HCO3-	18	ACIDOSIS
PARTIALLY  COMPENSATED  METABOLIC  ACIDOSIS
pO2	80	LOW
pCO2	60	HIGH
TYPE 2  RESPIRATORY  FAILURE