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87 Cards in this Set
- Front
- Back
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How can you calculate PaCO2? |
PaCO2 = production of CO2/elimination of CO2 by the lung
which can also be written as... PaCO2= alveolar ventilation/(minute ventilation-alveolar dead space) |
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What are the three determinants of PaCO2 and thus adequacy of ventilation?
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1. CO2 production,
2. minute ventilation, and 3. dead space |
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What are causes of high CO2 production(VCO2)?
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1. fever,
2. thyrotoxicosis, 3. CNS trauma, 4. overfeeding(TPN) |
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What are causes of low CO2 production(VCO2)?
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1. hypothermia,
2. hypothyroidism, 3. drugs(barbs) |
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What are causes of decreased minute ventilation(high PaCO2)?
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1. drugs,
2. CNS disease, 3. metabolic alkalosis, 4. muscle weakness, 5. sleep apnea, 6. hypothyroidism, 7. COPD |
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What are causes of increased minute ventilation(low PaCO2)?
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1. anxiety,
2. head trauma, 3, metabolic acidosis, 4. pregnancy, 5. asthma, 6. CHF |
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What are causes of increased alveolar dead space(increased PaCO2)?
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1. pulmonary vascular disease,
2. PE, 3. vasculitis, 4. COPD, 5. ARDS, 6. pulmonary fibrosis, 7. shock |
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What is physiologic dead space? How do you quantify physiologic dead space?
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The sum of anatomic and alveolar dead space. The dead space to tidal volume fraction Vd/Vt.
Vd/Vt = (PaCO2-PeCO2)/PaCO2 Normal = 0.3 |
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What is the relationship between alveolar ventilation and PaCO2 in a mechanically ventilated patient when displayed graphically?
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curvilinear (rectangular hyperbola)
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What is the most efficient way to increase oxygen content in the blood?
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increase hemoglobin
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What is the haldane effect?
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Describes how oxygen concentration determines the affinity of hemoglobin for CO2. Holds that deoxygenation of blood increases its ability to carry CO2, whereas increased oxygenation of blood decreases it's ability to carry CO2.
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What are the normal values of mixed venous oxygen saturation and tension?
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mixed venous O2 sat: 65-75%
mixed venous O2 tension: 35-45mm Hg |
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What factors determine mixed venous oxygen tension(PvO2)?
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COALS:
1. CO, 2. Oxygen consumption, 3. Amount of hemoglobin, 4. Loading of hemoglobin(saturation of Hb) 5. Saturation of hemoglobin |
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What is the equation for SvO2 (mixed venous oxygen saturation)?
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SvO2 = SaO2 - (VO2 / (CO x Hb x 13))
SaO2 = saturation of arterial O2 VO2 = oxygen consumption |
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What is the normal V/Q ratio?
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0.8 (nml ventilation is 4L/min, nml perfusion is 5L/min
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How do you calculate shunt fraction?
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Qs/Qt = (CcO2-CaO2)/(CcO2-CvO2);
Qs/Qt = shunt fraction CcO2 = content pulmonary capillary blood, CaO2 = content arterial blood CvO2 = content mixed venous blood |
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what is the normal shunt fraction? What does it represent?
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Qs/Qt=0.1
A guide as to the efficiency of the lung in facilitating the movement of O2 molecules from the alveolar space to the capillaries |
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What is the normal A-a gradient?
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10-20mmHg
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What is an approximate A-a gradient based on age?
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1/4 x the patients age
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Why do patient's under general anesthesia have a widened A-a gradient?
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increased V/Q mismatch due to altered lung and chest wall compliance
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What are causes of increased A-a gradient?
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1. V/Q mismatch,
2. diffusion impairment, 3. intracardiac(right to left) shunt |
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How do low FiO2 and hypoventilation affect the A-a gradient?
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normal A-a gradient
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How does hypercarbia affect PaO2 at a particular FiO2?
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limits PaO2 according to the alveolar gas equation
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What kind of syringes should be used for blood gas and why?
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glass syringes if possible because CO2 and O2 don't dissolve into the wall of the syringe and minimizes the risk of air bubbles in the syringe
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Why should heparin be used as the anticoagulant in ABG syringes?
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EDTA, citrates and oxalates alter the pH
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What variables are necessary for proper interpretation of blood gas?
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FiO2, temp, source, ventilator settings
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What electrode does the pH, PCO2, PO2 require for interpretation of ABG?
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pH-sanz electode
pCO2-Severinghaus electrode pO2-clark electrode |
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What factors influence blood gas values?
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age, sex, barometric pressure
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How do the kidneys and the lungs compensate for pH abnormalities?
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lungs: hyper or hypoventilation
kidneys: vary the reabsorption of filtered bicarb and add new bicarb to the plasma flowing through the kidneys |
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What is the henderson hasselbalch equation?
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pH=6.1+log(base/acid)=6.1+log(bicarb/(0.03 x PaCO2))
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How is the pH affected by increases in arterial PaCO2?
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for every 10 mm Hg increase in PaCO2 the pH decreases by 0.08-0.1
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What are some causes of respiratory acidosis?
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1. reduced minute ventilation,
2. alveolar hypoventilation, 3. increased alveolar dead space, 4. increased CO2 production, 5. increased mechanical dead space |
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what are causes of nonanion gap acidosis?
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BADR:
bicarb loss such as GI tract, acid loads, dilution of bicarb with saline, renal defects: poor bicarb reabsorption and acid secretion OR HARDUP: Hyperalimentation Acetazolamide, acid loadRTA Diarrhea, dilution of bicarb Ureteroenteric fistula Pancreaticoduodenal fistula |
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How can you determine if the reflex compensation response of the lungs in a metabolic acidosis is appropriate?
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the PCO2 should be equal to the last two digits of the pH
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What is the major problem with giving NaBicarb for lactic acidosis?
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bicarb reacts with hydrogen ions to form carbonic acid which then dissociates to CO2 and water and the CO2 partial pressure increases which can worsen the acidosis. When CO2 cannot be eliminated, the pH of the system is only minimally changed or in fact worsened.
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If you are considering giving bicarb to a patient with a combined metabolic and respiratory acidosis what should you do first?
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treat the respiratory acidosis first
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When should you treat a metabolic acidosis with bicarb? How do you determine the dose of bicarb to give?
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Generally treat if the pH is < 7,2 and a respiratory acidosis does not exist. (if It does tx the respiratory acidosis first)
kg x deviation of bicarb from 24 x 0.2 (0.4 if infant) - 0.2 = ECF volume as a fraction of body wgt |
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Besides worsening a respiratory acidosis what other problems are associated with the use of bicarb?
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1. intraventricular hemorrhage
2. hypernatremia 3. hyperosmolarity 4. left shift of oxyhemoglobin dissociation curve due to rebound alkalosis |
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How is the solubility of O2 and CO2 affected by temperature?
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at lower temperature the solubility of oxygen and CO2 in solution is higher, there are less molecules in the gas phase and the partial pressure of both are decreased
-dec partial pressure CO2 -> pH higher at lower temps |
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What is the alpha stat strategy of interpreting a blood gas?
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uncorrected arterial blood gas values, no attempt is made to correct for partial pressure of O2 and CO2 for changes in temperature
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What is the pH stat strategy for interpreting a blood gas?
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relies upon temperature corrected values and involves administering CO2 systemically to the patient to correct for lower partial CO2 pressure secondary to its increased solubility in solution
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what are criteria for intubation?
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Mechanical
1. RR>35/min, 2. VC<15cc/kg, 3. MIF<20cm H20, Oxygenation 1. PaO2<70mmHg on FiO2 40%, 2. A-a gradient >350 torr with FiO2 100%, Ventilation 1. PaCO2 > 55 unless chronically hypercarbic, 2. Vd/Vt>0.6 |
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What are criteria for extubation?
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1. RR<30,
2. stable BP and pulse, 3. no inotropic support, 4. patient afebrile, 5. ABG good on 40% with PaO2>70 and PaCO2<55, 6. MIF more negative than a negative 20cmH20 7. VC>15cc/kg |
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What is the normal pH at birth in the umbilical vein, artery, and also what is this value at 60 min?
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umbilical vein: 7.35
umbilical artery: 7.28 60min: 7.30-7.35 |
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What is the normal pCO2 at birth in the umbilical vein, artery, also at 60 min and 24 hrs?
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umbilical vein: 40
umbilical artery: 50 60min: 30 24hrs: 30 |
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What is the normal pO2 at birth in the umbilical vein, artery, also at 60 min and 24 hrs?
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umbilical vein: 30
umbilical artery: 20 60min: 60 24hrs: 70 |
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How are amines affected by the solution becoming more acidic or basic?
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RNH3+ <-> RNH2 + H+
-As system becomes more acidic they are more charged and less lipophilic; -As the system becomes more basic they are less charged and more lipophilic |
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How are carboxylic acids affect by acidic or basic solutions?
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R-COOH <--> RCOO- + H+;
-As the system becomes more acidic they are less charged and more lipophilic -As the system becomes more basic, they are more charged and less lipophilic |
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what is the pH and pKa of narcotics, local anesthetics, and thiopental?
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thiopental:pH 10.5, pKa 7.6
narcotics: pH 2.6-6.0, pKa 6.1 local anesthetics: pH 5-7, pKa 8-9 |
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2 forms oxygen is present in the blood
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1. bound to hgb
2. Dissolved in plasma |
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What is the P50?
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The partial pressure of oxygen at which hemoglobin is 50% saturated.
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What is the arterial oxygen content equation?
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CaO2 = (1.34 x Hb x O2 sat) + (.003 x PaO2)
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What does a right shift in oxy-hb dissociation curve signify?
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Results in increased unloading of oxygen at the tissue level. Caused by:
1. Inc H+ ions ( acidosis) 2. Inc CO2 3. Inc temp 4. Inc 2,3-DPG |
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What does a left shift in oxy-hb dissociation curve signify?
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Results in decreased unloading at the tissue level. Caused by:
1. Dec H+ ions (alkalosis) 2. Dec temp 3. Hgb variants (methemoglobinemia) |
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How is the Haldane effect clinically significant?
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When deoxygenated blood returns to the lungs and is exposed to high levels of oxygen, hgb loses it's affinity to carbon dioxide and carbon dioxide is more easily released. At the level of the muscle however, where o2 concentration is low, the affinity of carbon dioxide is increased which is what is desirable.
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What is the Bohr effect?
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Relates oxygen binding to hgb to H+ ion concentration. The more acidic the environment, hgb will bind less avidly I. E. Facilitates oxygen dumping to tissues that need it most
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Besides arterial oxygen content, what is the second way to assess tissue oxygen delivery?
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By evaluating the mixed venous oxygen level
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Accurate sampling of true mixed venous oxygen saturation requires sampling from what location?
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Pulmonary artery
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What happens to ventilation and perfusion as you go up the lung?
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Both decrease but perfusion decreases more, causing V/Q mismatch at the top
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Explain a V/Q ratio of infinity vs 0.
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- if there is ventilation but no perfusion, ratio is infinity (large) = dead space
- if there is perfusion but no ventilation, ratio is zero (small) = shunt |
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Alveolar gas equation
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PAO2 = ((Pb - Ph20) x FiO2) - (PaCO2/0.8)
Ie PAO2 = PIO2 - (PaCO2/resp quotient) PIO2 = partial pressure of inhaled oxygen |
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What are the 5 causes of hypoxemia and their effect on the A-a gradient?
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1. Hypo ventilation - normal
2. Low FiO2 - normal 3. V/Q mismatch - increased 4. Diffusion impairment - increased 5. Right to left intracranial shunt - increased |
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What is the respiratory quotient?
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Typically equal to 0.8 Bc 200 mL of CO2 are produced for every 250 mL of O2 consumed (200/250= 0.8)
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Symptoms of high altitude sickness.
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1. HA
2. Lassitude 3. Anorexia 4. Nausea 5. Difficulty in concentrating 6. Sleep disturbance Occurs within hours of descending >10,000 feet |
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Tx of high altitude sickness.
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1. Descend to lower altitude - pts with high altitude pulmonary edema need to descend slowly with assistance Bc increased blood flow can worsen problem.
2. Supplemental O2 if available 3. Acetazolamide - works by increasing the amount of alkali (bicarbonate) excreted in the urine making the blood acidic which drives ventilation; 125mg BID starting the evening before and continuing for 3 days once highest altitude reached 4. Dexamethasone - improves symptoms but doesn't txt the problem, can help pts descend with assistance |
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4 major buffer systems in the body. Which is most important?
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1. H2CO3/HCO3 - carbonic acid/bicarbonate (most important)
2. H2PO4/HPO4 - phosphate 3. HPr/Pr- - serum protein 4. Hgb - serum hgb |
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What are causes of respiratory alkalosis?
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Virtually all cases result from hyperventilation caused by:
1. hypoxic conditions 2. CNS disorders 3. anxiety 4. ventilatory support that is too aggressive |
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What is the anion gap?
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The sum of all + charges in the body must be counterbalanced by the sum of all - charges in the body.
-Since all of the anions are not measured, there is an inequality and this is the anion gap. -Normally 12 +/- 4 (8-16) -Anion gap = Na + (Cl - HCO3) |
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What does an increased anion gap imply?
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The presence of an unmeasured anion and its conjugate H+ producing a metabolic acidosis.
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Causes of Increased anion gap metabolic acidosis
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MUDPILES
Methanol (paint thinner) Uremia Diabetic ketoacidosis, starvation, alcoholic Paraldehyde (anticonvulsant) Isoniazide Lactic acidosis Ethylene glycol (anti-freeze) Salicylates |
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What is the primary abnormality in metabolic alkalosis?
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An increase in bicarb usually from the loss of bicarb poor fluid. The body store of bicarb is therefore contained in a smaller volume and bicarb concentration is increases.
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Common causes of metabolic alkalosis.
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1. Vomiting
2. NGT suctioning 3. Diuretic drugs - excretion of a large volume of acidic urine (loop and thiazide diuretics) |
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Most common ionizing group in biologic systems? 2nd most common.
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-MC = amines (narcotics, local anesthetics) - charged in the protonated form
-2nd MC = carboxylic acids (thiopental) - uncharged in the protonated form |
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What must one know in assessing lipophilicity?
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The major ionizing group as well as the pKa.
-If the pKa is high enough it moves out of the range where pH adjustments are relevant. -If the pKa is 11, then at any physiological pH, the material will almost exclusively in the protonated form |
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Explain how local anesthetics and narcotics are subject to fetal ion trapping. What about thiopental?
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In the more acidic fetal environment, the dissociation is to the left - the charged form which has difficulty crossing the placental barrier and thus traps compounds.
- It is not trapped. In the more acidic fetal environment, dissociation is to the left - to the uncharged form which can more easily cross the placental barrier. |
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Effect of CO2 and bicarb on CSF.
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CO2 diffuses rapidly across the BBB into the CSF. Bc bicarb crosses the BBB much more slowly, a paradoxical CSF acidosis results despite a normal or high blood pH.
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Describe the effect of fever on ABG values.
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PO2 and PCO2 values will be artificially elevated. The rise in PCO2 will lead to a fall in pH.
-Blood gas electrodes are mainained at 37 degrees in most hospitals. -Heating results in decreased solubility in plasma and increased gas tension. -As solution temp increases, more molecules enter the gas phase and are sensed as partial pressure. -The PCO2 of blood decreases about 4.5% and the pH increases about 0.015 unit/degree centigrade with cooling. |
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If an air bubble is introduced into a blood gas sample what changes will occur in the sample? Explain.
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-The PaO2 will increase and the PaCO2 will decrease.
-O2 in room air is about 105 mmHg and in a blood gas sample (FiO2 0.21) about 75-100. Therefore, O2 will go from a higher concentration to a lower concentration and will go from bubble to the sample, raising the PaO2 in the sample. -CO2 is neglibile in the bubble and is 40 mmHg in the sample. Therefore, CO2 will go from a higher concentration in the sample to the lower concentration in the bubble -> decreasing PaCO2 in sample. |
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The determinants of venous oxygen are identified by rearranging the __ equation.
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Fick equation
SvO2 = SaO2 - (VO2/Q x Hgb x 13) |
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What factors cause low mixed venous oxygenation?
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1. low CO
2. high O2 consumption 3. anemia 4. severe right shift of the oxy-hgb dissociation curve causing increased unloading of hgb at the tissue level (ex. sicke cell disease) 5. hypoxia or any other factor causing a low hgb saturation |
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What are factors that cause a high mixed venous oxygenation?
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1. decreased O2 consumption at the tissue level (ex. SNP poisoning)
2. severe left shift of the oxy-hgb dissociation curve resulting in decreased unloading of hgb at the tissue level (carboxyhemoglobinemia) |
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By far the greatest amount of oxygen in normal arterial blood is bound to....
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hemoglobin
-Only about 1.5% of the total content of O2 in arterial blood is dissolved in plasma. |
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What is the most efficient way to increase oxygen content?
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Increase hemoglobin
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Right shift of the oxyhemoglobin dissociation curve.
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Results in increases unloading of oxygen at the tissue level.
1. Increased hydrogen ions (acidosis) 2. Increased CO2 3. Increased temp 4. Increased 2,3-DPG 5. Sickle cell disease |
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Left shift of the oxyhemoglobin dissociation curve.
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Results in decreased unloading of oxygen at the tissue level.
1. Alkalosis 2. Dec temp 3. Hemoglobin variants - methemoglobin, carboxyhemoglobin, fetal hemoglobin |
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Local anesthetics and narcotics are weak ___. The pKa's of local anesthetics range from ___. What happens as pH decreases?
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Bases
7.6-8.9 As pH decreases, more of these drugs exist in their ionic form. |
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Electrolyte abnormalities found in pyloric stenosis.
When should surgery proceed? |
Hyponatremia
Hypokalemia Hypochloremia Metabolic alkalosis with compensatory respiratory acidosis -Surgery should not proceed until Na >130, K >3, Cl >85, and UOP 1-2 mL/kg/hr |
