Lecture 21: Blood Gas Interpretation (Exam 3)

Question 1

Define homeostasis

Answer 1

The maintenance of constant condition through dynamic equilibrium of the internal envi of the body

Question 2

What regulates the body

Answer 2

• Lungs
• Kidneys
• Liver/GI

Question 3

What amount of CO2 & excess H+ do carnivores produce

Answer 3

• Produce CO2
• Excess H+ precursors

Question 4

What amount of CO2 & excess H+ do herbivores produce

Answer 4

• Produce CO2
• Excess HCO3- precursors

Question 5

What are the 3 principal mechanisms to buffer H+

Answer 5

• Chemical
• Respiratory
• Renal

Question 6

Describe the chemical mechanism to buffer H+

Answer 6

• Extracellular buffering by bicarbonate works w/in seconds
• Phosphate, hemoglobin, & proteins are intracellular buffers that work w/in 2 - 4 H

Question 7

Describe the respiratory mechanism to buffer H+

Answer 7

Chemoreceptors in the body monitor changes in [H+] & pCO2 to adjust respiratory pattern & it works w/in mins to H

Question 8

Describe the renal mechanism to buffer H+

Answer 8

Increased renal excretion of H+ takes H to days

Question 9

What does henderson-hasselbalch equation doe

Answer 9

• Relates pH to components of the bicarbonate buffer system
• Any acid base disturbance is instantly reflected in one or both of its buffer components & their ratio determines pH (ideal ratio of HCO3: pCO2 is 20:1)
• Many approaches to the dx & tx of acid base disorders are based on this equation

Question 10

Fill out the following chart

Answer 10

Question 11

What is the primary disturbance of metabolic acidosis

Answer 11

HCO3- decreasing

Question 12

What is the primary disturbance of metabolic alkalosis

Answer 12

HCO3- goes up

Question 13

What is the primary disturbance of respiratory acidosis

Answer 13

CO2 has increased

Question 14

What is the primary disturbance of Respiratory alkalosis

Answer 14

Decrease in CO2

Question 15

What is a mixed disturbances

Answer 15

• Two separate primary disorders occurring in a px @ one time
• Can have a neutralizing or additive effect on pH
• Triple disorder can occur w/ MAC, MAL, & RAC/RAL

Question 16

What will you see if there is a mixed disturbance

Answer 16

• PCO2 & HCO3- are changing in opposite directions
• Norm pH w/ abnorm PCO2 &/or HCO3
• pH changes in the opposite direction that predicted for the primary disorder

Question 17

What independent variables determine the stewart’s approach

Answer 17

• PCO2
• Strong ion difference (SID) - NA, K, Cl, Ca, Mg
• Total concentration of nonvolatile weak acids

Question 18

What causes RAC

Answer 18

• Pleural space disease, pneuomothorax, severe pulmonary disease
• Upper air way obstruction
• Neuro disease
• Ax drugs & equipment dead space
• Decreased functional residual capacity
• Malignant hyperthermia
• Cardiopulmonary arrest

Question 19

What causes RAL

Answer 19

• Pain
• Fear
• Anxiety
• Stress
• Hypotension
• Low cardiac output
• Sepsis or SIRs
• Pulmonary thromboembolism
• Overzealous IPPV
• Respiratory dx
• Hypoxemia
• Fever/hyperthermia
• Severe anemia

Question 20

What causes MAC

Answer 20

• Vomiting
• Diarrhea
• Renal loss of HCO3- or retention of H+
• IV nutrition
• Dilutional acidosis
• Ammonium chloride
• Hypomineralcorticisim

Question 21

What causes MAL

Answer 21

• Vomiting due to pyloric obstruction
• Hypochloremia & hypokalemia
• Furosemide
• Hypermineralocorticism
• Contraction alkalosis

Question 22

What are the consequences of acidosis

Answer 22

• Impairs cardia contractility & response to catecholamines (decrease CO -> decreased renal & hepatic blood flow)
• Ventricular arrhythmias or fibrillation
• Arterial vasodilation & venous constriction (centralizes blood vol & causes pulmonary congestion)
• Shifts Oxygen-hem curve to the right
• Insulin resistance that impairs uptake of glucose
• Hyperkalemia due to transcellular shift
• Increased iCa2+
• CNS depression & coma
• Osteodystrophy & hypercalciuria

Question 23

What are the consequences of alkalosis

Answer 23

• CNS sx (agitation, disorientation, stupor, & coma)
• Seizures or tetany due to hypocalcemia (rare)
• Hypokalemia due to transcellular shifting causes muscle weakness, cardiac arrhythmias, GI motility disturbances, & altered renal fxns
• Shifts oxygen-hemoglobin curve to the left which impairs oxygen release from hemoglobin initially

Question 24

What is the diff btw/ the arterial & venous blood gas

Answer 24

• Arterial: is oxygenated & is used to eval respiratory gas exchange
• Venous: useful in determining AB status; slightly lower pH & high pCO2 than arterial blood due to local tissue metabolism. Can’t comment on oxygenation status

Question 25

Describe the values seen in an arterial sample

Answer 25

* PaO2 ~ 80 - 110 mmHg on room air or ~ 500mmHg if on 100% Ox * SaO2 > 88% (pulse ox) * Bright cherry red color * Pulsatile flow if catheter is placed and arterial waveform present when attached to a pressure transducer

Question 26

Describe the values seen in an venous sample

Answer 26

* PvO2 ~ 35 - 45 mmHg regardless of FiO2 * SvO2 65 to 75% * Darker red * No pulsatile flow from catheter & no atrial wave form present when attached to pressure transducer

Question 27

What is need to obtain a sample

Answer 27

Question 28

What are good sites to get a sample from in small animals

Answer 28

* Dorsal pedal a * Auricular a * Femoral a * Caudal a * Lingual a or v

Question 29

What are good sites to get a sample from in large animals

Answer 29

* Facial a * Transverse facial a * Lateral dorsal MT a * Auricular a * Lingual a * Femoral a * Median a

Question 30

What do blood gas analyzers directly measure

Answer 30

* pH * PO2 * PCO2

Question 31

What do blood gas analyzers calculate

Answer 31

* HCO3- * BE * SaO2

Question 32

Describe pH blood gas values

Answer 32

* Reflects the overall balance of acid/base producing processes in the body & the H+ concentration in the ECF * log(1/H+) * one unit change in pH = 10 fold increase or decrease in H+ * Range of 6.8 to 7.8 is compatible w/ life

Question 33

Describe PaO2 blood gas values

Answer 33

Oxygen molecules dissolved in the plasma phase of an arterial sample (not bound to Hb) depends on FiO2 & barometric pressure

Question 34

Describe PaCO2 blood gas values

Answer 34

Reflection of the respiratory component of acid-base balance, used to determine if the px is hypocapnic, hypercapnic, or eucapnic. Inversely related to alveolar ventilation

Question 35

Describe bicarbonate (HCO3-)

Answer 35

* Part of the bicarbonate-carbonic acid buffering system & is mainly responsible for regulating the pH of body fluids * Facilitates the transport of CO2 from the body tissues to the lungs & changes in respiration rate will alter the bicarbonate-carbonic acid ratio & pH * Is an assessment of the metabolic component of acid base status

Question 36

What is total carbon dioxide (TCO2)

Answer 36

Amount of CO2 gas present in the plasma

Question 37

What is base excess (BE)

Answer 37

Amount of strong acid or alkali req to titrate 1L of blood to a pH of 7.4 @ 37 degree C while the partial pressure of CO2 is constant 40 mmHg * Is identical in venous or arterial blood same * Base excess = metabolic alkalosis * Base deficit = metabolic acidosis

Question 38

How is BE used to calculate bicarb theraby

Answer 38

* mEq to infuse = base deficit x kg of BW x 0.3 * Infuse 1/3 of calculated vol over 20 mins then reassess acid base status * Use of bicarb therapy is controversial due to potential for serious side effects

Question 39

What is SaO2

Answer 39

The % of all ava heme binding sites saturated w/ oxygen from an arterial sample is calculated value based on the position on the oxygen hemoglobin dissociation curve & PaO3 (150 PaO2 = 100% SaO2) want to stay at or above 95%

Question 40

Fill the chart out for the norm values of an arterial blood sample:

Answer 40

Question 41

What is the best way to know if a sample is venous or arterial

Answer 41

* Arterial: SaO2 > 88% * Venous/mixed sample/bad pulmonary disease: SaO2 < 88%

Question 42

Hypoventilation = what PaCO2

Answer 42

Increased

Question 43

Hyperventilation = what PaCO2

Answer 43

Decreased

Question 44

How do you get the Alveolar arterial O2 gradient (A-a)

Answer 44

* The efficiency of gas exchange * Equation is for room air

Question 45

Fill out the chart for the A-a gradients in the dog (ADS = acute respiratory distress syndrome)

Answer 45

Question 46

How do we assess how the animal is oxygenating if the the px is on ax

Answer 46

* Use PaO2: FiO2 ration

Question 47

Answer the following example

Answer 47

Question 48

Fill out the chart

Answer 48

Question 49

How do you determine the anion gap? what are the norms?

Answer 49

* Norm for dogs = 12 - 24 mEq/L * Norm for cats = 13 - 27 mEq/L * Is composed of phosphate =, sulfate, plasma proteins, & organic acid anions * Increased AG more common & useful ID the cause of metabolic. acidosis

Question 50

What effects sample accuracy

Answer 50

* Air bubbles (Increased paO2) * Excess heparin (decreased pH) * Delay in analysis (decreased PaO2 & pH) * Blood clot in the sampe * Syringe * Temp & barometric therapy

Question 51

Define hypoxemia

Answer 51

* Decreased PaO2, SaO2, or hemoglobin content * The amount of oxygen in the blood (CaO2) determines the severity * Usually PaO2 < 60 mmHg &/or SpO2 < 90%

Question 52

Define Hypoxia

Answer 52

* General term for impairment of oxygen delivery to tissue (DO2) * Takes into account cardiac output (CO) & oxygen uptake @ tissue level * Therefore, hypoxemia is on type of hypoxia

Question 53

Causes of hypoxemia in ax

Answer 53

* V/Q mismatch * Hypoventilation * Low FiO2 * Right to left shunt * Diffusion impairment * Most common is a R to L shunt

Question 54

What is V/Q mismatch

Answer 54

* Change in hemodynamics * Not getting O2 but the arterioles are coming by (decrease in Ventilation w/ norm circulation) * Change in circulation or blood flow in the px * Contributes to px oxygen levels

Question 55

What is the oxygen content (CaCO2) equation? Why use it?

Answer 55

* CaO2 directly reflects the total # of oxygen molecules in arterial blood (both bound & unbound to hemoglobin) * Hemoglobin concentration is the main contributor * Times it by cardiac output to get DO2 * DO2 is the what we care the most about (rate of oxygen delivery)