Arterial blood gas By Mrs. Amala Rajan Reader Medical Nursing Dept
“Life is a struggle, not against sin, not against the Money Power, not against malicious animal magnetism, but against hydrogen ions." H.L. MENCKEN
What is an ABG Arterial Blood Gas Drawn from artery- radial, brachial, femoral It is an invasive procedure. Caution must be taken with patient on anticoagulants. Arterial blood gas analysis is an essential part of diagnosing and managing the patient’s oxygenation status, ventilation failure and acid base balance.
What Is An ABG? pH [H+] PCO2 Partial pressure CO2 PO2 Partial pressure O2 HCO3 Bicarbonate BE Base excess SaO2 Oxygen Saturation
Acid/Base Balance • The pH is a measurement of the acidity or alkalinity of the blood. • It is inversely proportional to the no. of (H+) in the blood. • The normal pH range is 7.35-7.45. • Changes in body system functions that occur in an acidic state decreases the force of cardiac contractions, decreases the vascular response to catecholamines, and a diminished response to the effects and actions of certain medications. • An alkalotic state interferes with tissue oxygenation and normal neurological and muscular functioning. • Significant changes in the blood pH above 7.8 or below 6.8 will interfere with cellular functioning, and if uncorrected, will lead to death.
Acid/Base Relationship H2O + CO2 H2CO3 HCO3 + H+
Buffers • There are two buffers that work in pairs • H2CO3 NaHCO3Carbonic acid base bicarbonate • These buffers are linked to the respiratory and renal compensatory system
The Respiratory buffer response • The blood pH will change acc.to the level of H2CO3 present. • This triggers the lungs to either increase or decrease the rate and depth of ventilation • Activation of the lungs to compensate for an imbalance starts to occur within 1-3 minutes
The Renal Buffer Response • The kidneys excrete or retain bicarbonate(HCO3-). • If blood pH decreases, the kidneys will compensate by retaining HCO3 • Renal system may take from hours to days to correct the imbalance.
ACID BASE DISORDER Res. Acidosis • is defined as a pH less than 7.35 with a paco2 greater than 45 mmHg. • Acidosis –accumulation of co2, combines with water in the body to produce carbonic acid, thus lowering the pH of the blood. • Any condition that results in hypoventilation can cause respiratory acidosis.
Causes • Central nervous system depression r/t medications such as narcotics, sedatives, or anesthesia. • Impaired muscle function r/t spinal cord injury, neuromuscular diseases, or neuromuscular blocking drugs. • Pulmonary disorders such as atelectasis, pneumonia, pneumothorax, pulmonary edema or bronchial obstruction • Massive pulmonary embolus • Hypoventilation due to pain chest wall injury, or abdominal pain.
Signs & symptoms of Respiratory Acidosis • Respiratory : Dyspnoea, respiratory distress and/or shallow respiration. • Nervous: Headache, restlessness and confusion. If co2 level extremely high drowsiness and unresponsiveness may be noted. • CVS: Tacycardia and dysrhythmias
Management • Increase the ventilation. • Causes can be treated rapidly include pneumothorax, pain and CNS depression r/t medication. • If the cause can not be readily resolved, mechanical ventilation.
Respiratory alkalosis • Psychological responses, anxiety or fear. • Pain • Increased metabolic demands such as fever, sepsis, pregnancy or thyrotoxicosis. • Medications such as respiratory stimulants. • Central nervous system lesions
Signs & symptoms • CNS: Light Headedness, numbness, tingling, confusion, inability to concentrate and blurred vision. • Dysrhythmias and palpitations • Dry mouth, diaphoresis and tetanic spasms of the arms and legs.
Management • Resolve the underlying problem • Monitor for respiratory muscle fatigue • When the respiratory muscle become exhausted, acute respiratory failure may ensue
Metabolic Acidosis • Bicarbonate less than 22mEq/L with a pH of less than 7.35. • Renal failure • Diabetic ketoacidosis • Anaerobic metabolism • Starvation • Salicylate intoxication
Sign & symptoms • CNS: Headache, confusion and restlessness progressing to lethargy, then stupor or coma. • CVS: Dysrhythmias • Kussmaul’s respirations • Warm, flushed skin as well as nausea and vomiting
Management • Treat the cause • Hypoxia of any tissue bed will produce metabolic acids as a result of anaerobic metabolism even if the pao2 is normal • Restore tissue perfusion to the hypoxic tissues • The use of bicarbonate is indicated for known bicarbonate - responsive acidosis such as seen with renal failure
Metabolic alkalosis • Bicarbonate more than 26m Eq /L with a pH more than 7.45 • Excess of base /loss of acid can cause • Ingestion of excess antacids, excess use of bicarbonate, or use of lactate in dialysis. • Protracted vomiting, gastric suction,hypchoremia,excess use of diuretics, or high levels of aldesterone.
Signs/symptoms • CNS: Dizziness, lethargy disorientation, siezures & coma. • M/S: weakness, muscle twitching, muscle cramps and tetany. • Nausea, vomiting and respiratory depression. • It is difficult to treat.
COMPONENTS OF THE ABG pH: Measurement of acidity or alkalinity, based on the hydrogen (H+) 7.35 – 7.45 Pao2 The partial pressure oxygen that is dissolved in arterial blood. 80-100 mm Hg. PCO2: The amount of carbon dioxide dissolved in arterial blood. 35– 45 mmHg HCO3 : The calculated value of the amount of bicarbonate in the blood 22 – 26 mmol/L B.E: The base excess indicates the amount of excess or insufficient level of bicarbonate. -2 to +2mEq/L (A negative base excess indicates a base deficit in blood) SaO2:The arterial oxygen saturation. >95%
Stepwise approach to ABG • Step 1: Acidemic or Alkalemic? • Step 2: Is the primary disturbance respiratory or metabolic? • Step 3. Asses to Pa O2. A value below 80mm Hg indicates Hypoxemia. For a respiratory disturbance, determine whether it is acute or chronic. • Step 4. For a metabolic acidosis, determine whether an anion gap is present. • Step 5. Assess the normal compensation by the respiratory system for a metabolic disturbance
STEPS TO AN ABG INTERPRETATION • Step:1 • Assess the pH –acidotic/alkalotic • If above 7.5 – alkalotic • If below 7.35 – acidotic
Contd….. • Step 2: • Assess the paCO2 level. • pH decreases below 7.35, the paCO2 should rise. • If pH rises above 7.45 paCO2 should fall. • If pH and paCO2 moves in opposite direction – primary respiratory problem.
contd • Step:2 • Assess HCO3 value • If pH increases the HCO3 should also increase • If pH decreases HCO3 should also decrease • They are moving in the same direction • primary problem is metabolic
Step 3 Assess pao2 < 80 mm Hg - Hypoxemia For a resp. disturbance : acute, chronic • The differentiation between A/C & CHR.respiratory disorders is based on whether there is associated acidemia / alkalemia. • If the change in paco2 is associated with the change in pH, the disorder is acute. • In chronic process the compensatory process brings the pH to within the clinically acceptable range ( 7.30 – 7.50)
J is a 45 years old female admitted with the severe attack of asthma. She has been experiencing increasing shortness of breath since admission three hours ago. Her arterial blood gas result is as follows: • pH : 7.22 • paCO2 : 55 • HCO3 : 25 • Follow the steps • pH is low – acidosis • paCO2 is high – in the opposite direction of the pH. • Hco3 is Normal. • Respiratory Acidosis • Need to improve ventilation by oxygen therapy, mechanical ventilation, pulmonary toilet or by administering bronchodilators.
EXAMPLE 2: Mr. D is a 55 years old admitted with recurring bowel obstruction has been experiencing intractable vomiting for the last several hours. His ABG is: • pH : 7.5 • paCO2 :42 • HCO3 : 33 • Metabolic alkalosis • Management: IV fluids, measures to reduce the excess base
BASE EXCESS • Is a calculated value estimates the metabolic component of an acid based abnormality. • It is an estimate of the amount of strong acid or base needed to correct the met. component of an acid base disorder (restore plasma pH to 7.40at a Paco2 40 mmHg)
Formula • With the base excess is -10 in a 50kg person with metabolic acidosis mM of Hco3 needed for correction is: = 0.3 X body weight X BE = 0.3 X 50 X10 = 150 mM
Anion GAP Step 4 • Calculation of AG is useful approach to analyse metabolic acidosis AG = (Na+ + K+) – (cl- + Hco3-) • * A change in the pH of 0.08 for each 10 mm Hg indicates an ACUTE condition.* A change in the pH of 0.03 for each 10 mm Hg indicates a CHRONIC condition.
REMEMBER • K etoacidosis • U remia • S epsis • S alicylate & other drugs • M ethanol • A lcohol (Ethanol) • L actic acidosis • E thylene glycol
COMPENSATION • Step 5 • A patient can be uncompensated or partially compensated or fully compensated • pH remains outside the normal range • pH has returned within normal range- fully compensated though other values may be still abnormal • Be aware that neither the system has the ability to overcompensate
ABG Interpretation Step 5 cont… Determine if there is a compensatory mechanism working to try to correct the pH. ie: if have primary respiratory acidosis will have increased PaCO2 and decreased pH. Compensation occurs when the kidneys retain HCO3.
Assess the PaCO2 • In an uncompensated state – when the pH and paCO2 moves in the same direction: the primary problem is metabolic. • The decreasing paco2 indicates that the lungs acting as a buffer response (blowing of the excess CO2) • If evidence of compensation is present but the pH has not been corrected to within the normal range, this would be described as metabolic disorder with the partial respiratory compensation.
Assess the HCO3 • The pH and the HCO3 moving in the opposite directions, we would conclude that the primary disorder is respiratory and the kidneys acting as a buffer response: are compensating by retaining HCO3 to return the pH to normal range.
Example 3 • Mrs. H is admitted, he is kidney dialysis patient who has missed his last 2 appointments at the dialysis centre his ABG results: • pH : 7.32 • paCo2 : 32 • HCO3 : 18 • Pao2 : 88 • Partially compensated metabolic Acidosis
Example 4 • Mr. K with COPD.His ABG is: • pH : 7.35 • PaCO2 : 48 • HCO3 : 28 • PaO2 : 90 • Fully compensated Respiratory Acidosis
Example 5 • Mr. S is a 53 year old man presented to ED with the following ABG. • pH : 7.51 • PaCO2 : 50 • HCO3 : 40 • Pao2 : 40 (21%O2) • He has metabolic alkalosis • Acute respiratory alkalosis (acute hyperventilation).
~ PaCO2– pH Relationship 80 7.2060 7.30407.4030 7.5020 7.60
Precautions • Excessive Heparin Decreases bicarbonate and PaCO2 • Large Air bubbles not expelled from sample PaO2 rises, PaCO2 may fall slightly. • Fever or Hypothermia, Hyperventilation or breath holding (Due to anxiety) may lead to erroneous lab results • Care must be taken to prevent bleeding
2SD NORMAL CL.ACCEPTABLE • PH 7.35 – 7.45 7.30 – 7.50 • PCO2 35 – 45 30 – 50 • PO2 97 >80 (ON 21% O2) (ON VENTILATOR) 60 – 90 • HCO3 24 - 28
Take Home Message: • Valuable information can be gained from an • ABG as to the patients physiologic condition • Remember that ABG analysis if only part of the patient • assessment. • Be systematic with your analysis, start with ABC’s as always and look for hypoxia (which you can usually treat quickly), then follow the four steps. • A quick assessment of patient oxygenation can be achieved with a pulse oximeter which measures SaO2.
It’s not magic understanding ABG’s, it just takes a little practice!