Arterial Blood Gases
Signs and symptoms of metabolic acidosis
- HA - Decr BP - Hyperkalemia - Muscle twitching - Warm, flushed skin (vasodilation) - N/V/D - Changes in LOC (confusion and incr drowsiness) - Kussmaul respirations (compensatory hyperventilation)
Signs and symptoms of respiratory acidosis
- Hypoventilation --> Hypoxia - Rapid, shallow respirations - Decreased BP - Skin/Mucosa pale to cyanotic - Headache - Hyperkalemia - Dysrhythmias (increased K+) - Drowsy, Dizzy, disorientation - Muscle weakness, hyperreflexia
What are some complications of performing ABGs?
- Local pain and paresthesia - Bruising - Local minor bleeding - Vasovagal response - Local hematoma from moderate or major bleeding - Artery vasospasm - Infection at puncture site - Arterial occlusion from local hematoma - Local nerve injury (This list is common --> rare)
What are contraindications to performing an ABG?
- No palpable pulse - Cellulitis or open infection present - Negative Allen test - Arteriovenous fistula proximal to site - Pt has severe coagulopathy - Abnormal anatomy
Signs and symptoms of metabolic alkalosis
- Restlessness, followed by lethargy - Dysrhythmias (tachycardia) - Compensatory hypoventilation - Confusion (decreased LOC, dizzy, irritable) - Nausea, vomiting, diarrhea - Tremors, muscle cramps, tingling of fingers and toes - Hypokalemia
Signs and symptoms of respiratory alkalosis
- Seizures - Deep, rapid breathing - Hyperventilation - Tachycardia - Decreased or normal BP - Hypokalemia - Numbness and tingling of extremities - Lethargy and confusion - Light headedness - Nausea/vomiting
How to complete Allen's test
1. Instruct pt to clench their fist 2. Apply occlusive pressure to both ulnar and radial aa 3. While applying occlusive pressure, have pt relax their hand and watch for blanching of palm and fingers 4. Release occlusive pressure from only ulnar a 5. Wait 15 seconds, observing the hand for flushing caused by capillary refill
What is the normal range for HCO3-?
22-30 mEq/dL
How are ABGs interpreted?
3 names (ID last name, then middle, then first) (Un)compensated Metabolic or Respiratory Acidosis or Alkalosis
What is the normal range for pCO2?
35-45 mmHg
What is a normal anion gap?
6-12 mEq/L
What is the normal pH of the body?
7.35-7.45
What is a pH of < 7.35?
Acidosis High [H+]
What does a near normal/normal HCO3- indicate in a primary respiratory disorder?
Acute process
What is a pH of > 7.45?
Alkalosis Low [H+]
What is O2 content and what is its reference range?
Amount of oxygen per 100 mL of blood Range: 15-22 vol%
What do you need to calculate if it is is a metabolic disorder?
Anion gap
What is the principle alkaline substabnce?
Bicarbonate (HCO3-)
What are chemical buffers?
Bicarbonate, phosphate, and protein First line of defense Changes in pH are very rapid (seconds)
Etiologies of respiratory alkalosis
CHAMPS C: CNS dz H: Hypoxia A: Anxiety (MC) M: Mechanical ventilation P: Pain S: Sepsis, Salicylates (early)
Etiologies of respiratory acidosis
CNS depression Airway obstruction Morbid obesity Myopathy Pneumonia Pulmonary edema
What is the disrupted substance for respiratory disorders? Compensatory condition and mechanism?
CO2 Metabolic Bicarb/proton retention or excretion
What is the principle substance affecting acid?
Carbonic acid (CO2)
What are the 2 main mechanisms that work together to keep acid/base levels normal and stable?
Chemical buffers and physiological buffers
Tic-Tac-Toe method of ABG interpretation
Column with pH tells you if its acidosis or alkalosis Position of pH, PaCO2, and HCO3 reveal origin of imbalance - If pH and pCO3 are in same column --> resp problem - If pH and HCO3- are in same column --> metabolic problem Once 3 items (including column head) are in a vertical row, you have the primary imbalance
What is pH?
Concentration of H+ ions in a solution pH = -log10 [H+]
Metabolic alkalosis: General characteristics and Compensation
Decreased H+ or increased HCO3- --> increased pH Compensation: decreased ventilation to retain CO2 Can be chloride responsive or chloride unresponsive
How does the respiratory system attempt to compensate for alkalosis?
Decreases RR and depth to retain CO2
Is there no/partial/full compensation of the pH falls in the normal column?
Full compensation
What is the disrupted substance for metabolic disorders? Compensatory condition and mechanism?
HCO3- Respiratory Hyper/hypoventilation
Respiratory alkalosis: General characteristics and Compensation
Hyperventilation Decreased CO2 --> increased pH Compensation: kidneys excrete HCO3- and conserve H+
Respiratory acidosis: General characteristics and Compensation
Hypoventilation Increased CO2 --> decreased pH Compensation: kidneys reabsorb and produce HCO3- and excrete H+
Metabolic acidosis: General characteristics and Compensation
Increased H+ or decreased HCO3- --> decreased pH Compensation: increased ventilation to exhale CO2 Can be categorized as normal anion gap or high anion gap
How does the respiratory system attempt to compensate for acidosis?
Increases RR and depth to blow off excess CO2
What are physiologic buffers?
Lungs and kidneys Occurs when chemical buffers cannot resist changes alone
Etiologies of high anion gap metabolic acidosis
MUDPILES M: Methanol U: Uremia (kidney failure) D: DKA P: Propylene glycol I: Isoniazid, ifxn, iron, inborn errors of metabolism L: Lactic acidosis E: Ethylele glycol S: Salicylates, starvation ketoacidosis
What is pO2 and what is its reference level?
Measure of amount of dissolved oxygen gas in the blood; evaluates effectiveness of oxygen therapy Range: 80-100 mmHg
What is the key role of chemical buffers?
Movement of hydrogen ions
What is anion gap?
Na - (Cl + HCO3) *Must correct for low albumin (increase AG by 2.5 for every 1 decrease in albumin) Identifies type of metabolic acidosis present Uncovers anions elevated in the blood but not routinely included in our measurements
When looking at the parameter (pCO2 or HCO3-) not associated with pH, if it is in the normal column --> [no compensation/partial compensation/full compensation]
No compensation
Which ABG values influence ventilation?
PaCO2 (NOT pH, PaCO2, HCO3- or SaO2)
Which ABG values influence oxygenation?
PaO2 and SaO2 (NOT pH, PaCO2, or HCO3-)
When looking at the parameter (pCO2 or HCO3-) not associated with pH, if it is in the opposite column --> [no compensation/partial compensation/full compensation]
Partial compensation
What is O2 sat and what is its reference range?
Percentage of hemoglobin that is carrying oxygen Range: 95-100%
How to interpret results of Allen's test
Positive result: flushing takes < 15 sec Negative result: flushing takes > 15 seconds; abnormal
What happens if the kidneys are unable to completely buffer the pH of blood?
Primary metabolic condition arises (or compensation from a respiratory disorder)
What happens if the lungs are unable to exhale CO2 properly?
Primary respiratory condition arises (or a compensation to metabolic disorders)
What might abnormal ABG results indicate?
Pt not getting enough oxygen Pt not getting rid of enough CO2 Problem with pts kidney function
What is the third line of defense for acid-base regulation?
Renal Changes in pH are slow (reacts in hours to days) Key role: movement of bicarbonate, retention/excretion of acids, generation of additional buffers
What is base excess/deficit and what is its reference range?
Represents the amount of buffering anions in the blood Excess = metabolic alkalosis Deficit = metabolic acidosis Range: 0 +/- 2 mEq
What is the second line of defense for acid-base regulation?
Respiratory Changes in pH are rapid (react in minutes) Key role: eliminate or retain CO2
What are indications for ABGs?
Sx of an oxygen/carbon dioxide, or pH imbalance Known respiratory, kidney, or metabolic dz with an acute exacerbation Oxygen therapy, mechanical ventilation, or undergoing prolonged anesthesia Breathing affected by head, neck, or back trauma
Etiologies of normal/non-gap metabolic acidosis
USED CARP U: Ureterostomy S: Small bowel fistula E: Extra Cl D: Diarrhea C: Carbonic anhydrase inhibitors A: Addison's dz R: Renal tubular acidosis P: Pancreatic fistula
What is PaCO2?
Unbound CO2 in the blood High PaCO2 = resp acidosis Low PaCO2 = resp alkalosis
What is chloride responsive metabolic alkalosis and what can cause it?
Urine chloride (UCl) < 20 mEq Causes: - Antacids - Diuretic therapy - NG suctioning - Vomiting
What is chloride unresponsive metabolic alkalosis and what can cause it?
Urine chloride (UCl) > 20 mEq Causes: - Bartter's syndrome - Cushing's syndrome - Gitelman's syndrome - Glucocorticoids
What does an arterial blood gas (ABG) tell us?
Valuable info about person's lung function Detects an acid-base imbalance Monitor tx for lung dz or evaluate effectiveness of oxygen therapy Tells us about oxygenation, ventilation, acid-base status, +/1 levels of carboxyhemoglobin and methemoglobin
What is compensation?
When renal and respiratory systems make adjustments to regain acid-base balance
What is correction?
When the values for both components return to normal levels
What is partial compensation?
When there is a respiratory or metabolic response but pH remains abnormal
What is full/complete compensation?
When there is a response and the pH returns to normal
Which ABG values influence acid-base?
pH PaCO2 HCO3- (NOT PaO2 or SaO2)
What is included in an arterial blood gas (7 things)?
pH pCO2 HCO3- pO2 O2 sat O2 content Base excess/deficit
What is the Henderson-Hasselbalch equation?
pH = [HCO3-] / pCO2 Provides simplified view of how pH is regulated Shows relationship of pH to bicarb and CO2
What happens to pH as pCO2 increases?
pH decreases
What happens to pH as bicarb increases?
pH increases
