Term
| Which chemoreceptors are predominantly stimulated by increased [H+] and which by hypoxemia? |
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Definition
central (brainstem) chemoreceptors are predominantly stimulated by increased [H+)
peripheral (carotid body, aortic arch) are predominantly stimulated by hypoxemia (but also by increased [H+]) |
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Term
| What four processes must be functional for there to be adequate oxygenation of blood? |
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Definition
| Inspired air must have adequate PiO2 (e.g. altitude, anesthesia) and must be delivered to the alveoli (e.g. patent airway). Alveoli must be perfused with blood and inspired O2 must quickly diffuse from alveolus into blood. |
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Term
| How can PO2 and PCO2 be used to assess the patient? |
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Definition
| these are both measured of dissolved gas in the blood, PO2 can be used to assess pulmonary gas exchange and PCO2 can be used to assess alveolar ventilation |
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Term
| Why are the values obtained by blood gas analyzers not accurate reflections of in vivo values in veterinary patients? |
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Definition
| The analyzers were designed for use in human samples and measurements are made at 37C rather than at temperatures of typical veterinary patients |
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Term
| What analytes are measured by blood gas instruments? |
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Definition
pH (i.e. H+), PCO2 and PO2
**HCO3 can be calculated from PCO2 and H+ with henderson-hasselbach equation (H = 24 x PCO2/HCO3) |
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Term
| What factor affects Hgb oxygen affinity? |
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Definition
[2,3-DPG] (higher 2,3- DPG = lower Hgb O2 affinity)
** 2,3-DPG production depends on action of phosphofructokinase, which is most functional in an alkaline environment (e.g. arterial blood) |
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Term
| At what pH is phosphofructokinase most active? How does this affect Hgb O2 affinity? |
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Definition
| phosphofructokinase is most active in an alkaline environment, increased phosphofructokinase activity results in increased production of 2,3-DPG, which results in decreased Hgb O2 affinity |
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Term
| How is the [HCO3] standard different from the [HCO3] actual? Which is a better estimate of the buffer base in blood? |
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Definition
Both are calculated values.
[HCO3] standard is the estimated HCO3 when the plasma is equilibrated at a temp of 37C and a PCO2 of 40 mmHg.
[HCO3] actual is the estimated HCO3 calculated using the Henderson-Hasselbach equation and measured H+ and PCO2 of a sample.
The [HCO3] standard is theoretically a better estimate of buffer base in blood because it is independent of changes in PCO2 |
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Term
| How should a positive BE(ecf) be interpreted? |
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Definition
| positive BE(ecf) indicates an excess of plasma buffer (e.g. HCO3, proteins) and a metabolic alkalosis |
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Term
| How should a negative BE(ecf) be interpreted? |
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Definition
| A negative BE(ecf) indicates a depletion of plasma buffer (e.g. HCO3, proteins) and thus a metabolic acidosis |
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Term
| What is the difference between BE (ecf) and BE (blood)? |
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Definition
| BE (blood) takes into consideration the contribution of Hgb as a buffer in addition to other plasma buffers (HCO3, proteins) |
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Term
| According to O2 dissociation curves, what is the paO2 necessary to keep hemoglobin oxygen saturations (SO2) >90%? |
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Definition
| PaO2 must be > 60 mmHg to maintain SO2 > 90% |
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Term
| What factors shift the O2 dissociation curve to the right (i.e. lower SO2 for the same PaO2)? |
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Definition
increased [H+] (@ highest [H+] (e.g. tissue blood) RBC have highest [2,3- DPG]
increased 2,3- DPG (lowest Hgb O2 affinity)
increased temperature
increased PCO2 |
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Term
| How do pulse oximeters determine aterial O2 saturation (spO2)? |
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Definition
Spectrophotometrically
At a high wavelength (e.g. 940 nm) oxyhemoglobin absorbs more light than deoxyhemoglobin. At a lower wavelength (e.g. 660 nm) oxyhemoglobin absorbs less light than deoxyhemoglobin. SpO2 is calculated from a ratio of these absorbance values. |
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Term
| What is the ideal sample for blood gas analysis? |
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Definition
heparinized arterial blood, the sample must be immediately sealed and no air should be present in the syringe during or after sample collection
**venous blood can be used to assess metabolic disorders but will not be an accurate reflection of pulmonary function. |
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Term
| What is the effect of exposure to room air on blood gas results? |
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Definition
increased PO2 (PO2 in sample and air quickly equilibrate, the PO2 in air is greater than that in blood)
decreased PCO2 (PCO2 in sample and air quickly equilibrate, the PCO2 is air is less than in the blood)
decreased HCO3 (due to loss of CO2)
increased pH (due to loss of CO2) |
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Term
| What is the effect of delayed sample analysis/failure to chill sample appropriately (but no air exposure) on blood gas results? |
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Definition
decrease PO2 (aerobic metabolism of WBC and PLT utilize O2)
decreased pH (anaerobic metabolism (i.e. glycolysis) of RBC, WBC, and PLT result in increased H+ production) |
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Term
| What disorders are associated with decreased renal excretion of H+, and therefore metabolic acidosis? |
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Definition
| renal failure uroperitoneum distal renal tubular acidosis (decreased excretion of H+ by type A intercalated cells) Hypoaldosteronism (e.g. Addisons), aldosteron induced proteins contribute to the H+ATPase pump in type A intercalated cells) |
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Term
| What conditions are associated with excessive HCO3 loss, and therefor metabolic acidosis? |
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Definition
gastrointestinal loss (diarrhea or saliva in ruminants)
renal losses (proximal rental tubular acidosis, i.e. impaired reabsorption of HCO3- in proximal tubular epithelial cell |
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Term
| What is the hallmark of secretory acidosis? |
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Definition
Hyperchloremia (b/c Chloride moves in the opposite direction of HCO3 to maintain electroneutrality)
**secretory acidosis is loss of HCO3- from the body (e.g. diarrhea, saliva (ruminants), proximal tubular acidosis) or not being produced (distal tubular acidosis) |
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Term
| What disease process results in an inorganic acidosis? |
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Definition
renal failure (major inorganic anion is PO4)
but is also accompanied by an organic acidosis (e.g. other uremic acids) |
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Term
| What four major categories of disease are associated with respiratory acidosis? |
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Definition
Inhibition/dysfunction of central (medulla/brainstem) respiratory center (e.g. drugs, brain stem disease)
Inhibition/dysfunction of respiratory muscles (e.g. diaphragm, chest wall)
Upper airway dysfunction (e.g. FB, vomit)
impaired gas exchange at pulmonary capillaries (e.g. pulmonary disease and restrictive disease (pleural effusion, pneumothorax, diaphragmatic hernia etc.) |
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Term
| How long does it take for metabolic compensation (e.g. renal secretion of H+) to raise the blood pH during chronic hypercapnia (respiratory acidosis)? |
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Definition
2-5 days
**respiratory compensatory responses are much quicker than metabolic |
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Term
| How does hypokalemia contribute to a metabolic alkalosis? |
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Definition
1. increased renal loss of H+: When there is hypokalemia, the K+-H+ATPase exchanger in type A intercalated cells (collecting duct) promotes reabsorption of K+ and secretion of H+)
2. intracellular translocation of H+: when there is hypokalemia K+ moves from ICF to ECF and H+ moves from ECF to ICF to maintain electroneutrality |
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Term
| What is the compensatory response to metabolic alkalosis? |
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Definition
respiratory acidosis
**alkalemia inhibits central respiratory chemoreceptors and thus causes hypoventilation --> decreased exhalation of CO2 --> increased H+ |
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Term
| What conditions are necessary for paradoxical acuduria to occur? Describe the pathogenesis of paradoxical aciduria |
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Definition
alkalosis, hypochloremia, hypovolemia
**hypovolemia stimulates renal reabsorption of Na and Cl (via actions of aldosterone and angiotensin II) BUT because of Cl depletion Na reabsorption is NOT always accompanied by Cl- resulting in an electrochemical gradient that promotes excretion of H+ (thus aciduria). K+ secretion may also occur further contributing to the hypokalemia. Secretion of H+ increases the generation of HCO3- (proximal tubule epithelial cells) |
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Term
| What is the physiologic response to respiratory alkalosis (hyperventilation)? |
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Definition
renal retention of H+ (decreased conservation of HCO3- by proximal tubular epithelial cells) and renal secretion of HCO3- (type B intercalated cells)
**takes about 3-5 days to raise pH |
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Term
| When does hypoxemic stimulation of chemoreceptors occur? |
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Definition
| hypoxemic stimulation of chemoreceptors occurs when PaO2 is <60-70 mmHg |
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Term
| How does blood gas data in neonates differ from those of adult animals (esp. foals and calves)? |
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Definition
lower PaO2 (>40mmHg) at birth, quickly normalizes to adult values by 1 day
higher PaCO2, slowly normalizes to adult values by 4-7 days
higher [HCO3-], corresponding to higher PaCO2
**this occurs due to underdeveloped lungs |
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Term
| What blood gas result confirms alveolar hypoxia (decreased O2 uptake due to decreased alveolar function)? |
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Definition
decreased arterial alveolar O2 tension ratio (a/A)
increased arterial alveolar O2 tension gradient (AaDO2) |
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Term
| T/F PaO2 is not depended on [Hgb] |
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Definition
True, this is why PaO2 will only be decreased in the following hypoxic disorders:
atmospheric (decreased O2 content in inhaled air)
tidal (decreased O2 uptake due to impaired respiratory exchange e.g. obstruction, hypoventilation)
alveolar (decreased O2 uptake due to alveolar disease e.g. pneumonia, pleural effusion, pulmonary edema, fibrosis, left to right shunts, ventilation-perfusion mismatch) |
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Term
| What two conditions can result in hemoglobic hypoxia and how does the SO2 differ in these two conditions? |
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Definition
Anemia (decreased Hgb results in decreased O2 content in the blood, but the SO2 measures O2 saturation of Hgb so it will be normal)
Decreased O2 bound to Hgb (e.g. methemoglobinemia, CO poisoning) results in both decreased oxygen content in the blood AND decreased SO2 **however, the pulse oximeter cannot distinguish between O2 and CO so SpO2 may actually be erroneously high in the case of CO poisoning, and to a lesser extent methemoglobinemia) |
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Term
| How does methemoglobinemia inhibit O2 delivery to tissues? |
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Definition
| with methemoglobinemia both ferrous (Fe+2) and ferric (Fe+3) heme molecules are present. O2 cannot bind to ferric (Fe+3) heme reducing O2 content of blood. The ferrous (Fe+2) heme in this state has a greater affinity for O2 resulting in less delivered to tissues and further contributing to a state of hypoxia. |
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Term
| Why does CO poisoning result in hypoxia? |
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Definition
| CO has a much greater affinity for Hgb than does O2 so it binds to Hgb displacing O2 and causing hypoxia |
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Term
| What types of hypoxic disorders will not affect blood gas results? |
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Definition
Stagnant (decreased delivery of O2 to tissue due to poor blood circulation e.g. shock, vascular occlusion, hyperviscosity syndrome)
Histotoxic (defective O2 utilization by tissue due to interference with metabolic pathways e.g. drugs)
Demand (increased O2 demand by hyperfunctioning cells (e.g. hyperthyroidism, pyrexia) |
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Term
| In general, how does pulse oximetry (spO2) correspond to oxygen saturation (SaO2)? |
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Definition
| pulse oximetry underestimates SaO2 when high, and overestimates SaO2 when low |
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Term
| What are the weak anions in the SID equation? |
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Definition
| OH, HCO3, CO3, albumin, PO4 |
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Term
| What are the weak cations in the SID equation? |
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Definition
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Term
| How are strong anions and strong cations as defined by SID? |
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Definition
| strong anions and cations are those ions that are completely dissociated in physiologic fluids |
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Term
| How do strong anions affect acid base according to SDI? |
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Definition
| strong anions (completely dissociated anions) are considered acids, because when added to ECF H+ shifts out of cells and into ECF to maintain electroneutrality |
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Term
| How do strong cations affect acid base according to SDI? |
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Definition
| Strong cations (completely dissociated) are considered bases because when added to ECF H+ moves into cells (out of ECF) to maintain electroneutrality |
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Term
| How does the SID change with an "acidosis"? |
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Definition
SID decreases
An acidosis would be an accumulation of strong anions or a deficit of strong cations, SID = strong cations - strong anions |
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Term
| How does SID change with an "alkalosis"? |
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Definition
SID increases
An alkalosis would be an accumulation of strong cations or a deficit of strong anions, SID = strong cations - strong anions |
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Term
| What is the relationship between SID3 and [HCO3-] when [Alb], [PO4], and [unmeasured anions] remain constant? |
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Definition
increased SID3 indicates increased [HCO3], alkalosis
decreased SID3 indicates decreased [HCO3], acidosis
SID3 ≈ [HCO3] + [alb+PO4] + [unmeasured anions]
**this is a relatively unredictable relationship and may not be present if there are changes in [Alb], [PO4], and [unmeasured anions] |
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Term
| How does anion gap correspond to components of the SID equations? |
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Definition
if anion gap is increased, corresponds to the concentrations of unmeasured anions
SIDtrue = [Na+K+fCa+fMg] - [Cl+unmeasured anions]
so anion gap can be used to calculate SID, unmeasured anions= AGpatient - AGnormal), but if there is a decreased HCO3 there really is no need to calculated SID |
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Term
| How do changes in Atot [albumin+PO4] affect acid base status according to SID? |
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Definition
increased Atot (secondary to hyperalbuminemia or hyperphosphatemia) results in decreased HCO3 and an acidosis
Decreased Atot = alkalosis
SIDtrue ≈ [HCO3] + [Atot]
**b/c plasma concentrations of the other weak anions (OH, CO3) and all weak cations (H, NH4) are relatively low compared to HCO3, albumin, and PO4 |
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Term
| Why does loss of free water result in an alkalosis on the basis of SID? |
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Definition
free water loss results in proportional increase in [Na] and [Cl], but [Na] increases more than [Cl] on an absolute basis --> increased SID --> alkalosis
SID = [Na+K+fCa+fMg] - [Cl + unmeasured anions]
**addition of free water (i.e. dilution hyponatremia causes an acidosis for the same reason, absolute [Na] is decreased more than absolute [Cl] with the addition of free water |
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Term
| How do changes in Cl affect acid base status according to SID? |
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Definition
increased Cl --> decreased SID --> acidosis
decreased Cl --> increased SID ---> alkalosis
SID = [Na+K+fCa+fMg] - [Cl + unmeasured anions] |
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Term
| How do changes in Na affect acid base status according to SID? |
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Definition
increased Na --> increased SID --> alkalosis
decreased Na ---> decreased SID --> acidosis
SIDtrue= [Na+K+fCa+fMg] + [Cl+unmeasured anions] |
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