Arterial Blood Gases

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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


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