pH• Normal pH is maintained by balancing the H2CO3 (carbonic acid) and HCO3- (bicarb)• Normal blood pH = 7.35-7.45• pH < 7.35 = acidosis• Ph > 7.45 = alkalosis
PaCO2• CO2 has several forms in the blood.• Like oxygen, some is dissolved directly into the plasma. The PaCO2 is the measurement of the partial pressure of carbon dioxide dissolved in the plasma. It is measured in mm Hg (millimeters of mercury).• The rest is found in the red blood cells on a hemoglobin molecule.
Buffer System Carbonic Acid - Bicarbonate Buffer System CO2 + H2O <--> H2CO3 <--> (HCO3-) + (H+)carbon dioxide + water <--> carbonic acid <--> bicarbonate + hydrogen ion NOTE THAT “CO2” on Lyte panel is HCO3• Note: The two headed arrows indicate that the process is reversible
Acid Base Balance• Understanding the cause of an acid-base imbalance is the key to treating it.• The Respiratory component of acid base balance affects the pH within minutes.• The Metabolic component of acid base balance can take days to affect pH.
A. Metabolic Acidosis• Results from renal failure, Lactic acidosis, DKA B. Respiratory Acidosis Hypoventilation• Results from respiratory failure, instrinsic or extrinsic.
C. Metabolic Alkalosis• Results from Gastric Suction, vomiting, D. Respiratory Alkalosis Hyperventilation• Results from Head Trauma, Fever, Emotions, Salicylate Ingestion, Shock at an early stage
PaO2• About 3% of the bodys oxygen is dissolved in the plasma.• PaO2 is a measurement of the partial pressure of oxygen dissolved in the plasma only. It is measured in mm Hg.• The PaO2 does not tell us about the bodys total oxygen content, but it does indicate how much oxygen was available in the alveoli to dissolve in the blood.
Oxygen and Hemoglobin• CO2s affinity for hemoglobin is much less than O2s affinity for hemoglobin.• When CO2 and O2 are both available, hemoglobin will accept oxygen rather than CO2.• In the oxygen rich environment of the alveoli, hemoglobin carries oxygen.• Oxygenated blood then travels through the body.
SaO2• The remainder of the bodys oxygen is carried attached to hemoglobin molecules.• SaO2, or oxygen saturation, measures the degree to which oxygen is bound to hemoglobin.• Sa02 is expressed as a percentage.
OxyHemoglobin Dissociation Curve• This curve describes the relationship between available oxygen and amount of oxygen carried by hemoglobin.• Oxygen-Hemoglobin affinity changes with: • variation in pH *CO2 *temperature *2,3,-DPG• Once the PaO2 reaches 60 mm Hg the curve indicates that there is little change in saturation above this point. – So, PaO2 of 60 or more is usually considered adequate. – At PaO2 of less than 60 even small changes will greatly reduce the SaO2.
Steps to ABG Interpretation1) Determine Acidosis or Alkalosis.2) Evaluate the Respiratory Mechanism3) Evaluate the Metabolic Mechanism
STEP 1Step 1. Use pH to determine ph Acidosis or Alkalosis. < 7.35 7.35-7.45 > 7.45 Normal or Acidosis Alkalosis Compensated
STEP # 2Step 2. PaCO2 Use PaCO2 to look at the < 35 35 -45 > 45 Respiratory Mechanism • Tends toward • Tends toward alkalosis Normal acidosis • Causes high or • Causes low pH Compensated pH • Neutralizes • Neutralizes low pH high pH
STEP 3Step 3. HCO3Use HCO3 to look at the < 22 22-26 > 26 Metabolic Mechanism • Tends toward • Tends toward acidosis Normal alkalosis • Causes low or Causes high pH Compensated pH • Neutralizes • Neutralizes high pH low pH
Interpretation High pH Low pH Alkalosis AcidosisHigh HCO3 Low PaCO2 High PaCO2 Low HCO3Metabolic Respiratory Respiratory Metabolic
Compensation• “Compensation" is the bodys normal response to normalize pH – By neutralizing the opposite acid base mechanism.• Example: If the pH is high because of respiratory alkalosis (low CO2): – Alkaline HCO3- will decrease to neutralize the pH. – In this case, the abnormal bicarb is not a metabolic problem; it is a metabolic solution to a respiratory problem.• It is important to determine which is the cause and which is the effect.• If you treat the compensatory abnormality, you make the pH even more abnormal.
Combined Disturbances• A “Combined Disturbance” occurs when the PaCO2 and HCO3- both alter the pH in the same direction.• A high PaCO2 and low HCO3- (acidosis).• Low PaCO2 and high HCO3- (alkalosis).• RARE
Question• If the pH is 7.30, the PaCO2 is 50, and the HCO3 is 24 what is the likely diagnosis?
Question 1• If the pH is 7.30, the PaCO2 is 50, and the HCO3 is 24 what is the likely diagnosis? Acute uncompensated RESPIRATORY ACIDOSIS
QuestionIf the pH is 7.49, the PaCO2 is25, and the HCO3 is 22 what is thelikely diagnosis?
Question #2• If the pH is 7.49, the PaCO2 is 25, and the HCO3 is 22 what is the likely diagnosis? ACUTE RESPIRATORY ALKALOSIS,
Question• If the the pH is 7.56, the PaCO2 is 39, and the HCO3 is 38, what is the likely diagnosis?
Question # 3• If the the pH is 7.56, the PaCO2 is 39, and the HCO3 is 38, what is the likely diagnosis? METABOLIC ALKALOSIS
Question• If the pH is 7.35, the PaCO2 is 25, and the HCO3 is 9, what is the likely diagnosis?
Question # 4• If the pH is 7.35, the PaCO2 is 25, and the HCO3 is 9, what is the likely diagnosis? COMPENSTATED METABOLIC ACIDOSIS
Question• If the pH is 7.30, the PaCO2 is 25, and the HCO3 is 9, what is the likely diagnosis?
Question # 5• If the pH is 7.30, the PaCO2 is 25, and the HCO3 is 9, what is the likely diagnosis? PARTIALLY COMPENSTATED METABOLIC ACIDOSIS