pulm- arterial blood gas

what is normal pH?

7.38-7.44

what is pH of blood based on?

amount of H+ ions in body

where does H+ come from?

• acids

• volatile- derived from CO2, excreted by lungs

• non-volatile- lactic acid, keto acids, metabolic products of sulfur and phosphorus containing compounds; excreted by kidneys

what is the buffer system equation?

CO2 + H2O ⇔ H2CO3 ⇔ H⁺ + HCO3^-

how do central chemoreceptors respond to an increase in pH (>7.44)?

need to inc pCO2 → hypoventilate → slow breathing or hold breath

how do central chemoreceptors respond to a decrease in pH (< 7.38)?

need to dec pCO2 → hyperventilate → inc resp rate to breathe off CO2

which chemoreceptors respond to changes in pCO2 and are located in the medulla?

central

which chemoreceptors respond to changes in arterial pO2 and are located on carotid bodies/aortic arch?

peripheral

how do peripheral chemoreceptors respond to dec in p)2 < 60mmHg?

respiratory rate inc

where does bicarb reabsorption occur?

proximal and distal renal tubule

where does H+ excretion occur?

distal renal tubule

What is the kidney response to inc pH?

dec HCO3 reabsorption and dec H+ excretion

what is the kidney response to dec pH?

inc HCO3 reabsorption and inc H+ excretion

where can you take blood for an arterial blood gas (ABG)?

• radial artery

• brachial artery

• femoral artery

• axillary artery

• dorsalis pedis artery

what are absolute contraindications to ABG?

• no pulse or poor collateral circulation

• cellulitis or local infx

• aneurysms at puncture site

• AV fistula or graft above puncture site

• active raynauds syndrome radial site

what are relative contraindications to ABG?

• chronic renal dz

• presence of arterial dz

• bleeding disorders; anticoagulant rx

what must be performed prior to obtaining an ABG?

allen test; ensure collateral circulation

how is ABG recorded in a note?

pH/CO2/O2/HCO3

what are normal values for PaCO2?

38-42 mmHg

what are normal values for PaO2?

75-100 mmHg

what are normal values for HCO3-?

23-26 mEq/L

what pH range is considered acidic?

< 7.38

what pH range is considered alkalotic?

> 7.44

what PaCO2 range is considered alkalotic?

< 38

what PaCO2 range is considered acidic?

> 42

what HCO3 range is considered acidic?

< 23

what HCO3 range is considered alkalotic?

> 26

what does the ROME mnemonic mean?

respiratory, opposite, metabolic, equal

• pH and CO2 are in opposite directions → RESPIRATORY

• pH and HCO3- are in same direction → METABOLIC

what is respiratory acidosis?

inc pCO2: >42 mmHg

dec pH: < 7.38

HCO3- normal or elevated

what is respiratory alkalosis?

inc pH: > 7.44

dec pCO2: < 38

HCO3: normal (23-26)

what is metabolic acidosis?

dec pH: < 7.38

dec HCO3: < 23

PaCO2 normal (38-42)

hyperkalemia likely to occur

what is metabolic alkalosis?

inc pH: > 7.44

inc HCO3: > 26

PaCO2: normal (38-42)

hypokalemia occurs

what are causes of respiratory acidosis?

• obstructive pulm dz

• neuromuscular disorders causing weakness of respiratory muscle

• respiratory center depression

• lung conditions- obesity/Pickwikian syndrome, flail chest/multiple rib fractures, kyphoscoliosis

what is a normal ion gap?

7-13 mEq/L

How do you calculate an anion gap?

Na⁺ - [HCO3^- + Cl^-]

what are causes of respiratory alkalosis?

• stimulation of brainstem respiratory center: emotional state, fever, pregnancy, salicylate and sepsis

• mechanical overventilation

• cardiac dz

why is there decreased HcO3 in metabolic acidosis?

• inc accumulation of nonvolatile acids

• loss of bicarb from kidney/GI tract

• diminished renal acid excretion

what are causes of high anion gap metabolic acidosis?

• Methanol ingestion

• Uremia (inc BUN)

• Diabetic ketoacidosis

• Propylene glycol (Paraldehyde)

• Iron and Isoniazid

• Ethanol and Ethylene glycol

• Salicylates and starvation

what are causes of normal anion gap metabolic acidosis?

• Hyperalimentation

• Acetazolamide

• Renal tubular acidosis

• Diarrhea

• Uretero-pelvic shunt

• Post-hypocapnia

• Spironolactone

**anion gap NOT increased due to increased reabsorption of Cl- to maintain electroneutrality

what are causes of metabolic alkalosis?

• gastric loss of HCL

  • vomiting

  • nasogastric suction

• renal loss of H+

  • some diuretics

  • inc aldosterone (Conn’s syndrome)

  • inc cortisol (Cushing’s syndrome)

what is it called when both CO2 and HCO3 are out of range, but one is acidotic and one is alkalotic?

compensation

how do the lungs compensate?

alter CO2 excretion → occurs over minutes to hours

how do the kidneys compensate?

alter HCO3 reabsorption and H+ excretion → occurs over days

what is the compensatory mechanism for respiratory acidosis?

kidneys compensate to inc HCO3 → H+ excreted in urine and HCO3 reabsorption inc

what is the compensatory mechanism for respiratory alkalosis?

kidneys compensate by dec HCO3 → renal excretion of HCO3 inc and reabsorption dec

what is the compensatory mechanism for metabolic acidosis?

lungs compensate by dec pCO2 → hyperventilate to “breath off” CO2

what is the compensatory mechanism for metabolic alkalosis?

lungs compensate by inc pCO2 → hypoventilate to “keep in” CO2

How can you tell the difference between full vs partial compensation?

full: pH normal

partial: pH abnormal

what is normal PaO2 for adults under 70 on room air?

75-100 mmHg

what is normal PaO2 for adults on room air between 70-79?

70-100 mmHg

At what PaO2 level does hypoxemia occur?

< 60

• mild: 60-75

• moderate: 40-60

• severe: < 40

At what O2 sat does confusion occur?

< 91%

What O2 sat is considered life threatening?

< 70%