The challenge 7.4 ACIDOSIS 7.8 7.0 Volatile ACID (CO2) & Fixed acids Defense of normal alkalinity
Types of Acids Volatile acids Easily move from liquid to gas state within the body Lung can remove H2CO3 + renal enzyme H2O + CO2 (both of which are exhaled) Carbon dioxide is therefore considered an acid
Cannot be changed to gas state within the body Examples Keto acids Lactic acids
The challenge Sources of acids: Volatile acid CO2 + H2O H2CO3 H+ + HCO3 Fixed acids Organic and inorganic source Lactic acid, ketones, Sulfuric and phosphoric acid Kidney plays an important role handling fixed acids.
DEFENCE AGAINST pH CHANGE Acute (minutes to hours) Ventilation Buffering
DEFENCE AGAINST pH CHANGE Acute (minutes to hours) Long term Renal excretion Hepatic metabolism
Chemical Buffers The body uses pH buffers in the blood to guard against sudden changes in acidity A pH buffer works chemically to minimize changes in the pH of a solution H+ OH- H+ Buffer OH- OH- H+
Biological systems and Buffering: The power of a buffer depends on: Concentration of the buffer. Whether the pK is close to the pH of the system.
Bicarbonate buffer systems: CO2 + H2O H2CO3 H+ + HCO3- Maintains a ratio of 20 parts bicarbonate to 1 part carbonic acid
Bicarbonate Buffer System If strong acid is added: HCl + NaHCO3 = H2CO3 + NaCl Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid) The pH of the solution decreases only slightly
BICARBONATE BUFFER SYSTEM H+ H2CO3H+ + HCO3- Hydrogen ions generated by metabolism or by ingestion react with bicarbonate base to form more carbonic acid H2CO3 HCO3- 20
BICARBONATE BUFFER SYSTEM H+ Equilibrium shifts toward the formation of acid Hydrogen ions that are lost (vomiting) causes carbonic acid to dissociate yielding replacement H+ and bicarbonate H2CO3 HCO3-
Bicarbonate Buffer System If strong base is added: NaOH + H2CO3 = NaHCO3 + H2O It reacts with the carbonic acid to form sodium bicarbonate (a weak base) The pH of the solution rises only slightly This system is the only important ECF buffer
Protein buffers: A. Amino acid residues of proteins take up H+ (pK=7.0) are most important NH2 NH3- B. Hemoglobin is important due to high concentrationand its increased buffering capacity when deoxygenated.
Metabolic Acidosis Deficit in HCO3- and decreased pH Causes: Increased production of nonvolatile acids. Decreased H+ secretion in the kidney Increased HCO3- loss in kidney Increased Cl- reabsorption by the kidney.
Metabolic Acidosis Body response is increased ventilation to blow off excess CO2
Determining the predicted “Respiratory pH” Acute 10 mmHg increase in PCO2 results in pH decrease of approximately 0.05 units Acute 10 mmHg decrease in PCO2 results in pH increase of approximately 0.10 units
Determining the predicted “Respiratory pH” First determine the difference between the measured PaCO2 and 40 mmHg and move the decimal point two places left. 60 - 40 = 20 X 1/2 0.10 40 – 30 = 10 0.10
Determining the predicted “Respiratory pH” If the PaCO2 is greater than 40 subtract half of the difference from 7.40 ? If this Pt has pH = 7.2 ? If this Pt has pH = 7.33 60 - 40 = 20 X ½ =10 = 0.10 pH = 7.40 – 0.10 = 7.30
Determining the predicted “Respiratory pH” If the PaCO2 is less than 40 add the difference to 7.40 40 - 30 = 10 0.10 pH = 7.40 + 0.10 = 7.50
Determining the predicted “Respiratory pH” pH 7.04 PCO2 76 76 - 40 = 36 X ½ = 18 0.18 7.40 - 0.18 = 7.22
Determining the predicted “Respiratory pH” pH 7.21 PCO2 90 90 - 40 = 50 X ½ = 25 0.25 7.40 – 0.25 = 7.15
Determining the Metabolic component RULE 10 mmol/L variance from the normal buffer base represents a pH change of approximately 0.15 units.
pH 7.21 PCO2 90 90 - 40 = 50 X ½ = 0.25 7.40 – 0.25 = 7.15 Determining the Metabolic component 7.21 -7.15 = 0.06 X 2/3 = 0.04 = 4 mmol/L base excess
pH 7.04 PCO2 76 76 - 40 = 36 X ½ = 0.18 7.40 - 0.18 =7.22 Determining the Metabolic component 7.22 -7.04 = 0.18 X 2/3 =12 mmol/L base deficit
Determining the Metabolic component pH 7.47 PCO2 18 40 – 18 = 22 = 0.22 7.40 + 0.22 = 7.62 Determining the Metabolic component 7.62-7.47 = 0.15 X 2/3 =10 mmol/L base deficit
Diagnosis of acid base disturbance Examine arterial pH: Is acidemia or alkalemia present? Examine PaCO2: Is the change in PaCO2 consistent with a respiratory component? If the change in PaCO2 does not explain the change in arterial pH, does the change in [HCO3–] indicate a metabolic component? Make a tentative diagnosis (see Table).
Diagnosis of acid base disturbance Compare the change in [HCO3–] with the change in PaCO2. Does a compensatory response exist (Table)? If the compensatory response is more or less than expected, by definition a mixed acid–base disorder exists. Calculate the plasma anion gap in the case of metabolic acidosis. Measure urinary chloride concentration in the case of metabolic alkalosis.
Base Excess/ Deficit The degree of deviation from normal total body buffer base can be calculated independent of compensatory PCO2 changes The amount of acid of base that must be added to return the blood pH to 7.4 and PCO2 to 40 at full O2 saturation and 370 C