31 – Intro to Blood Gas Analysis Flashcards
Acidosis
- Decrease excitability
o Alteration in cardiac contractions
o Decreased vascular response to catecholamines
o Can lead to loss of consciousness
Alkalosis
- Increased excitability
o Impaired neurological function
o Impaired muscular function
o Tingling sensations, nervousness, muscle twitches
What is metabolic acidosis/alkalosis caused by?
- Imbalance in production and excretion of acids or bases by KIDNEYS
Metabolic acidosis: pH and HCO3 levels
- pH<7.35
- HCO3 <22mmol/L
Too much acid build up (metabolic acidosis)
- Shock
- DKA
- Renal failure
- Diarrhea
- Diuretics
- Lactic acidosis
- Ethylene glycol poisoning
Clinical signs of metabolic acidosis
- Headache
- Lethargy
- Nausea
- Anorexia
- Vomiting
- Diarrhea
- Coma
- Death
Metabolic alkalosis: pH and HCO3 levels
- pH>7.45
- HCO3 >26mmol/L
Excess loss of acid in blood (metabolic alkalosis)
- Excessive vomiting
- GI obstruction
- *not as common)
Clinical signs of metabolic alkalosis
- *chronic vomiting dog
- Dizziness
- Lethargy
- Weakness
- Muscle twitching
- Cramps
- Tetany
- Coma
- Death
What is respiratory acidosis/alkalosis caused by?
- Primarily by LUNGS or breathing abnormalities
Respiratory acidosis: pH and PaCO2 levels
- pH<7.35
- PaCO2 >45mmHg
What is respiratory acidosis caused by
- Hypoventilation
o Obstruction of gas exchange
o Respiratory depression
What are the clinical signs of respiratory acidosis
- Dyspnea
- Respiratory distress
- Shallow respirations
- Tachycardia
- Dysrhythmias
- Headache
- Restlessness
- Confusion
Respiratory alkalosis: pH and PaCO2 levels
- pH > 7.45
- PaCO2 <35mmHg
What is respiratory alkalosis caused by?
- Hyperventilation
o Pain
o Fear
o Anxiety fever
What are the clinical signs of respiratory alkalosis?
- Dyspnea
- Nausea
- Vomiting
- Headaches
- Restlessness
- Lethargy
- Coma
What are some chemical buffers?
- Bicarbonate, phosphate, sodium
- Potassium/hydrogen ion exchange
- Bones
- Proteins (albumin, Hg, plasma globulin)
- *IMMEDIATE RESPONSE TO CHANGES IN ACID/BASE BALANCE
Acidosis: compensatory response of chemical buffering
- H+ moves into cell, K+ moves out of cell
- RESULTS in HYPERKALEMIA
Alkalosis: compensatory response of chemical buffering
- H+ move out of cell, K+ moves into cell
- RESULTS IN HYPOKALEMIA
Respiratory buffering
- Normal biproduct of cell metabolism=CO2
- Excessive CO2 combines with water: H2CO3
o Blood pH change according to how much carbonic acid is present - *lungs can increase OR decrease RR depending on pH
*RESPONDS IN SECONDS TO MINUTES (slight delay, but pretty immediate)
What in blood stimulates the RR?
- CO2
- pH
- O2
- Central and peripheral
Central chemoreceptors
- Ventral surface of medulla
- Respond to changes in pH in CSF
Peripheral chemoreceptors
- Carotid and aortic arches
When pH decreases (renal buffering)
- Retain HCO3-
When pH increases (renal buffering)
- Excrete HCO3-
Renal buffering
- May take HOURS TO DAYS
Conversion of % to partial pressure
- O2 20.95%/100 multiple by 760mmHg=159.22mmHg
Oxygen transport
- Transported attached to Hg (4 molecules)
- Taken up in lungs and used within body tissues
- Inhale 21% oxygen in atmosphere
- Exhale: 15%
CO2 transport
- Transported as bicarbonate ion in plasma
- Byproduct of metabolism with H+ ions
- Exhaled through lungs (5-6%)
- Negligible amounts are inhaled form atmosphere
FiO2
- Fraction of inhaled gas that is O2
- *must know to interpret arterial blood sample
Partial pressure O2
- Amount O2 dissolved in plasma
- *depends on lung function
- Driving force to get O2 onto the Hb molecule
- *if lung function is normal: PaO2=5xFiO2=100mmHg
Saturation of Hb with O2
- *know graph
- If shifted left=bind O2 more tightly at tissues
- If shifted right=off load more O2 at tissues
What causes the Hb O2 saturation curve to shift left?
- Increase pH
- Decrease DPG
- Decrease temperature
What causes the Hb O2 saturation curve to shift right?
- Decrease pH (ex. exercise)
- Increase DPG
- Increase temperature
Oxygen content (O2Ct)
- Total number of mLs O2 per liter of blood
- *includes both O2 Hb is carry and dissolved O2
- Ex. arterial: 200mL
Why is there a different in mammals arterial blood O2 and atmospheric pO2?
- Some alveoli are not well ventilated (DEAD SPACE)
- Some blood moves through lung w/o picking up oxygen (SHUNT)
What is the equation of O2 delivery?
- O2 delivery = O2 content x CO
What are some factors affecting O2 delivery?
- PaO2
- *Hb concentration
- CO
- Diffusion from capillaries to mitochondria
- Oxygen affinity of Hb
- Local blood flow
What are the 5 causes of hypoxemia?
- Decreased FiO2
- Hypoventilation
- Shunt
- V/Q mismatch
- Diffusion impairment
When will hypoxic drive take over?
- If PaO2 is less than 60mmHg
- *may result in hypocapnia
- Built in safety mechanism
If you want to assess LUNG function, what kind of blood gas sample will you take?
- Arterial
- Arterial blood has left lungs and NOT passed through tissues
If you want to assess BODY pH (acidosis-alkalosis) what kind of blood gas sample will you take?
- Can take a MIXED VENOUS SAMPLE
- Must have mixed blood from all body regions
- *take from pulmonary artery, but NOT easy
- **CLINICALLY ACCEPTABLE to use jugular vein
o Head and neck receives 30% CO
What are some sampling errors?
- Taking sample from wrong site
- Air bubble
- Blood clots
- Arterial and venous mixed samples
- *run samples immediately or put on ice (not longer than 2 hours)
Taking sample from wrong site
- Peripheral venous sites do NOT give idea of whole body
- Venous sites do NOT assess pulmonary function
Air bubble (sampling error)
- Remove air from sample
- Results will reflect values found in air (150mmHg O2 and low CO2)
What do the electrodes of a blood gas analyzer measure?
- pH
- pO2
- pCO2
- do NOT measure N or NO
What does the co-oximeter measure? (blood gas analyzer)
- Hg content
- Saturation of Hg molecule with O2
What is the function of a blood gas analyzer?
- Calculate bicarbonate and base excess
- Re-calculates pO2 and pCO2 for body temperature of patient
- Electrolytes; glucose; lactate; anion gap
Typical blood gas print out: what you need to do/add
- But in arterial or venous sample
- Input Inspired O2
- Operator ID
- Patient ID
- Temperature
Where do you start when evaluating a blood gas?
- Look at pH
- Does pCO2 explain change in pH?
o What other conditions will produce H+? - Define respiratory or metabolic acidosis
- Look at pO2, sO2, O2Ct
- What concentration of O2 is patient breathing?
- Patient on hypoxic drive?
- Enough Hb?
- How is O2 delivery to the tissue maintained?
Venous sample difference compared to arterial sample on a blood gas printout
- PCO2 slightly increased
- PO2 decreased (O2 has been used)
- *everything else is pretty much the same
What will you see if there is pulmonary dysfunction with good CO?
- Pulse oximeter (saturation): low
- Arterial gas: low PaO2
- Content: low
- *not getting as much O2 across membranes
What will you see if there is anemia with good CO?
- Pulse oximeter (saturation): normal
- Arterial gas: normal PaO2
- Content: low
- *even though only 1 blood cell going by the probe, it looks good!
What will you see on a blood gas printout of a hypoxemic dog?
- Low pO2
- Low sO2
- Low O2Ct
- Hyperventilating dog on hypoxemic drive
- Exhales more CO2
o Lower pCO2
o Higher pH - *O2 therapy will help to a point, but need to treat what is going on with the lung
What will you see on a blood gas printout for a hypoxemic and anemic dog?
- Low pO2
- Low sO2
- Very low O2Ct
- Low Hct
- Low Hb concentration
- *give a blood transfusion! (need more Hb)
What will you see on a blood gas printout for an anesthetised horse?
- *respiratory acidosis
- Low pH
- High pCO2
- High HCO3-
- Normal PO2 (or is it?)
- *can’t give more O2 as it is already on 100% O2
- *can VENTILATE IT: give adequate tidal volume
o Ventilation/perfusion mismatch
Horses: ventilation/perfusion mismatch
- Not designed for dorsal recumbency
- Lungs are compressed: atelectasis
- Anesthetic drugs depress brain and ventilation
What will you see on a blood gas printout (VENOUS SAMPLE) for a dog with renal failure?
- Normal PvO2 and PvCO2
- Low pH
- Low HCO3-
- *metabolic acidosis
