Overview (Cameron's) Flashcards
(42 cards)
What are the indications for an arterial blood gas?
- To assess gas exchange and ventilation
- To monitor gas exchange and ventilation in response to interventions/therapy
- Identification of acid-base disorders
- Monitoring of acid-base disorders in response to interventions/therapy
- Identification of dyshaemoglobinaemias
What are complications of arterial cannulation?
Haematoma, local infection, bleeding, sepsis, pseudoaneurysm formation
Describe the normal reference ranges for pH, paO2, pCO2, HCO3- and base excess at an fiO2 of .21
pH 7.35-7.45, paO2 80-100, paCO2 35-45, HCO3- 22-26, base excess -2 to +2
What equation describes the oxygen content in blood?
- Arterial oxygen content = Hb x oxygen saturation x 1.39 (paO2 x 0.0031)
Is paO2 or oxygen saturation more important when it comes to oxygen delivery to the tissues?
- Oxygen saturation
* Arterial oxygen content = Hb x oxygen saturation x 1.39 (paO2 x 0.0031)
How is hypoxaemic respiratory failure defined?
A clinically significant decreased in PaO2 usually to be less than 60 mmHg
Why is a PaO2 value of 60 important with regards to the oxygen-Hb dissociation curve?
For any drop in PaO2 below 60 the oxygen saturation falls precipitously with any further decrease in PaO2
What are four factors that cause the oxygen dissociation curve to move left and right?
Temperature, acidosis/alkalosis, PaCO2 and 2, 3-DPG
What changes to the four factors that shift the oxygen dissociation curve move the curve left?
Decrease in PaCO2, fall in temperature, alkalosis and fall in 2,3-DPG concentration
What changes to the four factors that shift the oxygen dissociation curve move the curve right? What is the result?
- Increase in PaCO2, temperature rise, acidosis and increases in 2, 3-DPG concentration
- Facilitates more effective delivery of oxygen to peripheral tissues which is beneficial in the presence of hypoxia
What is the PaO2 when approx 50% of haemoglobin is saturated with oxygen?
- 26.6 mmHg
How can you evaluate a patient’s oxygenation from an ABG?
- PAO2 = FiO2 (Patm - PH2O) - (PaCO2)/R
* where Patm = 760, pH2O = 47 and R = 0.8
What is a normal range for the P(A-a)O2? What increases it?
- P(A-a)O2 = Age/4 + 4
* Increases in cigarette smoking, increasing fiO2 and advanced age
What are the 5 possible causes for hypoxaemia?
- Decreased inspired fractional oxygen (high altitudes)
- Impaired diffusion - interstitial fibrosis thickens the membrane (rare, because diffusion usually is done in 1/3 of the circulatory time available)
- Shunting
- Ventilation-perfusion (V/Q) mismatch
- Hypoventilation
What are the different types of shunts?
- Extrapulmonary - acquired or congenital cardiac abnormalities - septal defects
- Intrapulmonary - severe pneumonia, atelectasis, pulmonary AV malformation, hepatopulmonary syndrome
Under normal physiology what is the V/Q ratio normally? Why?
V/Q ratio is normally 0.8. This is because perfusion is more pronounced at the lung bases and ventilation is more pronounced at the lung apices
What are non-perfused alveolar units referred to as?
Physiological dead space
Give an example where the V/Q ratio is 0 and one where the V/Q ratio is infinity.
V/Q ratio of 0 occurs in shunt (alveoli are not ventilated but are perfused)
V/Q ratio of infinity occurs in fully ventilated but unperfused alveoli (massive PE)
How can you pick up a V/Q mismatch clinically?
Increased alveolar-arterial PO2 gradient
What does the alveolar gas equation tell us about the relationship between oxygen and carbon dioxide partial pressures?
- Significant alveolar hypoventilation can result in proportional decrease in alveolar oxygen
- Thus, an increase in PaCO2 will be associated with a decrease in PAO2
How can you determine if hypoxaemia is secondary to hypercapnoea/hypoventilation or due to a V/Q mismatch/shunt?
- If the (A-a)PO2 is normal - secondary to hypercapnoea
* If increased - secondary to shunt or V/Q mismatch
What is a recommended approach to the evaluation of respiratory failure using ABGs?
- Confirm hypoxaemia - PaO2 <60 +/- SaO2 <90%
- Establish if there is an increased alveolar-arterial oxygen tension gradient (A-a)O2 (PAO2 = fiO2 (713) - PaCO2/0.8)
- Is there evidence of hypoventilation? PaCO2 is <50 there is alveolar hypoventilation
- Hypoxic altitude if hypoxaemia, normal (A-a)PO2 and the PaCO2 is not elevated
- Is the hypoxaemia entirely accounted for by hypoventilation (CNS depression, respi mm failure)? –> the (A-a)PO2 gradient is normal (<15) in this case. If the (A-a)O2 gradient is elevated (>15) then pneumonia/ARDS are likely causative
- If PaCO2 is normal, hypoxaemia is present and there is a raised (A-a)O2 gradient then response to oxygen discriminates between shunt and V/Q mismatch (shunt - does not improve, V/Q mismatch - does improve)
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Fill in the primary acid-base disorder table:
- Respiratory acidosis: ↓ pH, ↑ CO2, ↑ bicarbonate compensation
- Respiratory alkalosis: ↑ pH, ↓ CO2, ↓ bicarbonate compensation
- Metabolic acidosis: ↓ pH, ↓ HCO3, ↓ CO2 compensation
- Metabolic alkalsosis: ↑ pH, ↑ HCO3, ↑ CO2 compensation
Describe the recommended approach to acid-base disorders:
- Determine if there is alkalaemia or acidaemia present
- Determine the primary disturbance - is it respiratory or metabolic. Established by assessing the relationship between the pH and pCO2 direction of change
- Assess whether the primary disturbance has been compensated (the 6 formulae…). If the anticipated compensation is not present, it is likely that a mixed acid-base disorder exists
- Calculate the anion gap in the presence of metabolic acidosis (high or normal)
- If there is an increased anion gap present then determine the delta ratio to establish if there is a mixed acid-base disorder
- Identify the ‘initiating factor’ if the gas reflects a metabolic alkalosis + a reason for the kidney not to be excreting bicarb appropriately
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