Acid-Base Balance Disturbances

Hydrogen ion homeostasis Acids are produced continuously during normal metabolism. (provide H+ to blood) H+ ion concentration of blood varies between narrow limits pH of the extracellular fluid = 7.35 – 7.45 Constant H+ concentration within physiological limits is physiologically important to preserve the enzyme activity and metabolism

Buffering of acids (H+) in blood H+ is generated during intracellular metabolism from several sources (~ 150 000 mmol H+ is produced every day) They are continuously neutralized by buffers resulting in no gain of H+ ions = No pH change

Disorders of Acid-Base Balance   Increase in H+ concentrations results in a decrease in pH of blood (acidosis) Decrease in H+ concentrations results in an increase in pH of blood (alkalosis)  Alkalosis or Acidosis describes any abnormality in H+ balance whether : 1- Compensated Alkalosis or Acidosis No blood pH changes (pH of blood is within normal range). Buffer concentrations are abnormal Compensatory mechanisms try to restore pH to normal if pH is changed. 2- Uncompensated alkalosis or acidosis (alkalaemia or acidaemia) Abnormal pH of blood (above or below normal range)

Relation between pH & buffer Assessment of Acid-Base Balance Normal pH of blood is not an indication of acid-base balance. Accordingly, in order to assess acid-base balance (status) of blood , we should assess pH & buffer concentration of blood Relation between pH & buffer Henderson-Hasselbach Equation [HCO3-] pH = 6.1 + log --------------------------------- pCO2 + 0.225

Acid-base DIAGNOSIS IS CONFIRMED BY LABORATORY INVESTIGATIONS OF Acid-Base Balance Disturbances 1- Acidosis : - Metabolic ↓↓ HCO3- - Respiratory ↑↑ CO2 2- Alkalosis : - Metabolic ↑↑ HCO3 - Respiratory ↓↓ CO2 DIAGNOSIS IS CONFIRMED BY LABORATORY INVESTIGATIONS OF pH, pCO2 & pO2 & HCO3- Sample: Arterial Blood using Procedure: Blood gas analysis

1-Metabolic Acidosis Causes: ↓↓ HCO3-   Causes:  I- Increased production of H+  Common Causes of increased H+ (acids) in the blood:  1- Increased endogenous acid production. - Diabetic ketoacidosis (increased ketone bodies in blood) - Lactic acidosis (increased lactic acid in blood).  2-Ingestion of acids (or substance that produces an acid) - Poisons: as salicylate (aspirin) overdose - Methanol ingestion - High protein diet.  3-decreased acid (H+) excretion by the kidney: in renal failure. II- Loss of bicarbonate: e.g. in diarrhea

Compensatory mechanisms of metabolic acidosis Metabolic Acidosis cont Compensatory mechanisms of metabolic acidosis 1- Exhaustion of bicarbonate buffer with shift of reactions to CO2 production. Stimulation of the respiratory centre to eliminate excess CO2 formed (CO2 wash) 2- Increase in renal acid excretion of H+  

LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH : Low HCO3: Low PCO2 : Low: as CO2 is produced then exhaled by lungs by rapid respiration PO2: Normal

2-Respiratory Acidosis ↑↑ CO2   Causes Impaired carbon dioxide excretion and thus blood pCO2 increases. caused by any pulmonary (lung) cause resulting in hypoventilation.  1-Chronic respiratory acidosis: occurs due to chronic obstructive airway diseases. Chronic bronchitis Emphysema Bronchial asthma  2-Acute respiratory acidosis: occurs due to acute respiratory failure Cardiac arrest Neuromuscular disorders of chest wall Depression of the respiratory centre in the brain by: cerebral disease or drugs

Compensation: by kidney via ↑ HCO3- reabsorption ↑ H+ excretion LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: Low PCO2: High (due to the respiratory problem) HCO3: High (due to compensation) PO2 : Low (due to the respiratory problem)

3-Metabolic ALkalosis ↑↑ HCO3- The primary abnormality in metabolic alkalosis is the increased plasma bicarbonate level. (HCO3-). Causes: Less common 1- Intake of a large amounts of alkali as sodium bicarbonate: (if intake is more than 1000 mmol/day) More common 2- Loss of H+ (acids) from the body: 1- From the kidneys (increased excretion of acids, H+ ions): a- Mineralcorticoid (aldosterone) excess b- Severe potassium deficiency 2- From the GIT (increased loss acids, H+ ions): vomiting and gastric wash

LABORTORY INVESTIGATION: Compensation: by lungs via inhibition of respiration which leads to ↑ PCO2 LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: High HCO3: High PCO2 : High (due to compensatory respiratory depression)

4- Respiratory Alkalosis ↓↓ CO2  Causes: The PCO2 is reduced due to: Hyperventilation which may be due to: 1- Respiratory centre stimulation as in cases of: Anxiety Cerebral disease (infection, tumour) 2- pulmonary embolism. 3- Fevers 4- Hepatic failure   Mechanism The decrease in PCO2 leads to an increase in pH as follows: [HCO3-] pH = 6.1 + Log ---------------------- PCO2 X 0.225

LABORTORY INVESTIGATION: Respiratory Alkalosis Compensation : by kidney by ↓ HCO3 reabsorption and ↓ H+ secretion LABORTORY INVESTIGATION: Sample: Arterial Blood Equipment: Blood Gas Analyzer pH: High HCO3: Low PCO2 : Low

Practice Problems Acid-Base Imbalances interpretation of Arterial Blood Gases (ABG) RESP

Getting an arterial blood gas sample

Blood Gas Report Acid-Base Information pH PCO2 HCO3 [calculated vs measured] Oxygenation Information PO2 [oxygen tension] SO2 [oxygen saturation]

PRIMARY AND SECONDARY ACID-BASE DERANGEMENTS [HCO3-] pH = 6.1 + log --------------------------------- PCO2 + 0.225 Acid-Base Disorder Primary Change Compensatory Change Respiratory acidosis PCO2 up HCO3 up Respiratory alkalosis PCO2 down HCO3 down Metabolic acidosis HCO3 down PCO2 down Metabolic alkalosis HCO3 up PCO2 up

Steps for interpretation of ABG Know normal values of pH, (PCO2), and (HCO3-). Look at the patient's ABG's to determine what's abnormal high or low. Correlate the abnormal values of PCO2 and HCO3- to the abnormality of pH. Name the disorder, the cause, and the source of any compensation.

1- Normal values for ABG's: 2- Evaluate the patient's ABG's: is the pH normal? Is it too high or too low? Is it acidosis or alkalosis? Is the HCO3- normal? Is it too high or too low? Will it cause acidosis or alkalosis? Will it correct acidosis or alkalosis? Is the CO2 normal? Is it too high or too low? Will it cause acidosis or alkalosis? Will it correct acidosis or alkalosis?

3-Correlate the abnormal values: If only one of the two parameters (CO2 or HCO3-) is abnormal, then its value should be consistent with the pH (for example, if the CO2 is high, since that causes a drop in pH, the pH should be low). If both of the parameter are abnormal, then usually one is CAUSING the problem, and the other is trying to CORRECT (COMPENSATE FOR) the problem. (For example, if the CO2 is high, and is causing the pH imbalance, then the pH must be low, since CO2 behaves as an acid. If HCO3- level is also abnormal, then usually it will be high, to compensate for the low pH, since it is a base.)

Respiratory acidosis (with or without renal compensation) 4- Name the disorder: Respiratory acidosis (with or without renal compensation) Respiratory alkalosis(with or without renal compensation) Metabolic acidosis (with or without respiratory compensation) Metabolic alkalosis (with or without respiratory compensation) 5- Suggest a possible cause For example, a cause of chronic respiratory acidosis is emphysema.

Practice Problem 1 ABG's: pH 7.31 PCO2 55 mm Hg HCO3- 28 mEq/L Normal values for ABG's: pH range 7.35 - 7.45 PCO2 35 - 45 mm Hg HCO3-, 22 - 26 mEq/L pH is too low - acidosis;  PCO2 is too high, would cause acidosis or correct alkalosis; HCO3- is normal, neither causing nor correcting imbalance high PCO2 is correlated with low pH, which is consistent with patient's report because PCO2 is causing the problem, this is respiratory acidosis; because bicarbonate is normal, there is no compensation

Practice Problem 2 ABG's: pH 7.34 PCO2 55 mm Hg HCO3- 35 mEq/L pH – low = acidosis PCO2 – high = respiratory acidosis HCO3 - high = renal compensation

Practice Problem 3 ABG's: pH 7.31 PCO2 35 mm Hg HCO3- 20 mEq/L pH – low = acidosis HCO3 - low = metabolic acidosis PCO2 – normal; no compensation

Practice Problem 4 ABG's: pH 7.35 PCO2 25 mm Hg HCO3- 20 mEq/L pH – low = acidosis HCO3 - low = metabolic acidosis PCO2 – low = respiratory compensation

Practice Problem 5 ABG's: pH 7.48 PCO2 25 mm Hg HCO3- 24 mEq/L pH – high = alkalosis PCO2 – low = respiratory alkalosis HCO3 - normal; no compensation

Practice Problem 6 ABG's: pH 7.44 PCO2 25 mm Hg HCO3- 20 mEq/L pH – high = alkalosis PCO2 – low = respiratory alkalosis HCO3 - low = renal compensation

Practice Problem 7 ABG's: pH 7.48 PCO2 40 mm Hg HCO3- 33 mEq/L pH – high = alkalosis HCO3 – high = metabolic alkalosis PCO2 – normal; no compensation

Practice Problem 8 ABG's: pH 7.44 PCO2 55 mm Hg HCO3- 33 mEq/L pH – high = alkalosis HCO3 – high = metabolic alkalosis PCO2 – high = respiratory compensation