Intro to ABGs PDF

Intro to ABGs Homeostasis Balancing to Normalize pH 1  ABG Interpretation Classification Calculations Confirmation 2  Steps to ABG Interpretation Classification – Primary Problem Step 1 – Check pH – Acidosis or Alkalosis –...

Intro to ABGs Homeostasis Balancing to Normalize pH 1  ABG Interpretation Classification Calculations Confirmation 2  Steps to ABG Interpretation Classification – Primary Problem Step 1 – Check pH – Acidosis or Alkalosis – Primary Cause Step 2 – Check PaCO2 – Is respiratory the cause? Step 3 – Check HCO3 – Is metabolism the cause? Step 4 – Check PaO2 – Is the patient hypoxemic? – Compensation Step 5 – Is the body compensating? 3  Steps to ABG Interpretation Initial Classification Step 6 – Technical Classification / Functional Classification Calculation Step 7 – Determine if compensation is appropriate or are there other primary causes. Step 8 – Determine Anion Gap and Bicarbonate Gap 4  Steps to ABG Interpretation Confirmation Step 9 – Assess patient Step 10 – Check for accuracy (errors) Step 11 – Final interpretation 5  Normal Values Candidly, if you don’t know the normal values you will be hopelessly lost… pH: 7.35 to 7.45 PaCO2: 35 to 45 torr (mmHg) HCO3: 22 to 26 mEq/L PaO2: 80 to 100 torr (mmHg)  7.40 pH – Normal 7.40; Normal Range 7.35 – 7.45 7  Acid-Base Imbalances Acidosis- a decline in blood pH ↓ – Metabolic acidosis: due to a decrease in bicarbonate (HCO3). ↓ – Respiratory acidosis: due to an increase in carbonic acid (PaCO2). ↑ Alkalosis- a rise in blood pH ↑ – Metabolic alkalosis: due to an increase in bicarbonate (HCO3). ↑ – Respiratory alkalosis : due to a decrease in carbonic acid (PaCO2). ↓ 8  Acid-Base Imbalances pH< 7.35: acidosis pH > 7.45: alkalosis The body’s response to acid-base imbalance is called compensation – The body gears up its homeostatic mechanism and makes every attempt to restore the pH to normal level. – May be complete if pH brought back within normal limits – Partial compensation if pH is still outside norms. 9  Rates of correction Respiratory mechanisms: take several minutes to hours Renal mechanisms: may take several days 10  Compensation If underlying problem is metabolic, hyperventilation or hypoventilation can help: this is respiratory compensation. If problem is respiratory, renal mechanisms can bring about metabolic compensation. 11  Respiratory Acidosis Carbonic acid excess caused by blood levels of CO2 above 45 mm Hg. Hypercapnia – high levels of CO2 in blood Causes: – Depression of respiratory center in brain that controls breathing rate; e.g. drugs or head trauma – Paralysis of respiratory or chest muscles – Respiratory pathogensis 12  Metabolic Acidosis Bicarbonate deficit (↓) - blood concentrations of bicarb drop below 22mEq/L (milliequivalents / liter) Causes: – Loss of bicarbonate through diarrhea or renal dysfunction – Accumulation of acids (lactic acid or ketones) – Failure of kidneys to excrete H+ Commonly seen in severe uncontrolled DM (ketoacidosis). 13  Compensation for Respiratory Acidosis Kidneys eliminate hydrogen ion and retain bicarbonate ion 14  Respiratory Alkalosis PaCO2 less than 35 mm Hg (hypocapnea) Most common acid-base imbalance Primary cause is hyperventilation – Hysteria, Hypoxia, Pain, Excessive artificial ventilation and the action of certain drugs (salicylate) that stimulate respiratory center. 15  Metabolic Alkalosis Bicarbonate excess↑ - concentration in blood is greater than 26 mEq/L Causes: – Excess vomiting = loss of stomach acid – Excessive use of alkaline drugs – Certain diuretics – Dehydration – Nasal Gastric Tubes 16  Compensation for Metabolic Alkalosis Hypoventilation to retain CO2 (hence H2CO3↑) Renal excretes more HCO3-, retain H+. 17  Acid-Base Uncompensated: Parameters: pH PaCO2 HCO3- Metabolic Alkalosis Normal Metabolic Acidosis Normal Respiratory Alkalosis Normal Respiratory Acidosis Normal  Compensation of Respiratory Alkalosis Kidneys conserve hydrogen ion Excrete bicarbonate ion 19  Partially Compensated Imbalances Occur when compensation mechanisms are activated, but have not had sufficient time to normalize the blood pH NOTE: Some people say that there is no such thing as “partially” compensated – it is kind of like being “a little pregnant” – but it is indicative of a part of the process called compensation  Partially Compensated Imbalances pH is abnormal Both PaCO2 and HCO3- are abnormal in the same direction (increased or decreased from normal) – If PaCO2 is high (↑ acid), HCO3- will also be high (↑ alkaline) to neutralize the environment – If PaCO2 is low (↓ acid), HCO3- will also be low (↓ alkaline) to neutralize the environment Acid-Base Partially Compensated: Parameters: pH PaCO2 HCO3- Metabolic hypoventilate Alkalosis > 7.45 Compensation started Metabolic hyperventilate Acidosis < 7.35 Compensation started Respiratory Alkalosis > 7.45 Compensation started Respiratory Acidosis < 7.35 Compensation started  Compensated Imbalances Occur when compensatory mechanisms have been able to fully normalize blood pH  Acid-Base Fully Compensated: Parameters: pH PaCO2 HCO3- Metabolic hypoventilate Alkalosis 7.44 Compensated Metabolic hyperventilate Acidosis 7.36 Compensated Respiratory Alkalosis 7.44 Compensated Respiratory Acidosis 7.36 Compensated