test 1 part 2 Flashcards
What affects the rate of oxygen consumption?
Metabolic rate
Amount of oxygen available (i.e. delivered to tissue)
Rate of oxygen usage
As long as intracellular PO2 is ≥ 1 mmHg oxygen usage depends on [ADP] not PO2
- pO2 >1 and ADP = 1.5 normal => can produce more ATP so rate of O2 usage increases
ADP = 0.5 normal and pO2 >1
- Limit amount of O2 that can be utilized because of limited ATP so rate of O2 usage decreases
- Adenine base used up
Tissue Oxygen Gradients (Average) : Capillary to interstitial fluid
95 mmHg to 40 mmHg = 55 mmHg gradient
- gradient stays relatively constant
Tissue Oxygen Gradients (Average) : Interstitial fluid to intracellular fluid
40 mmHg to 23 mmHg = 17 mmHg gradient
- gradient stays relatively constant
What happens if arterial PO2 drops to 70 mmHg?
70 mmHg – 55 mmHg = 15 mmHg Interstitial
15 mmHg – 17 mmHg = -2 mmHg => no O2 gets inside of the cell
importance of arterial pO2
- it is important to keep an adequate intracellular pO2 so we keep pO2 high
Manage Patient’s Flow
- as flow rate increases, level of O2 consumption increases
Fixed target values based on weight or body surface area (BSA)
OR use oxygen consumption plateau
Once flow is established, what patient parameters tell us if the target blood flow is acceptable
Pressure / SvO2 / Acid-base status
- SvO2 doesn’t always tell you if all of the tissues are being perfused correctly
Manage Patient’s Hematocrit/Hemoglobin
Keep the patient’s hematocrit (HCT) above a specified value: 24 to 25%
How to keep our hematocrit from dropping below our target
Minimize prime RAP / VAP Hemoconcentrate Enhance urine output Give packed red blood cells - Tends to be treated as if it were independent of all other physiologic parameters.
Problems With Current Model
Monitor “summary” parameters that do not tell us what is happening at the organ and/or tissue level
Concepts of Goal Directed Perfusion
Concentrate on OXYGEN DELIVERY nadir (lowest level) rather than hematocrit nadir
Use CO2 derived variables as an indication of actual tissue perfusion
Make treatment decisions based on the concepts and using evidence based values
what is king
- Oxygen Delivery
control components for oxygen delivery
Pump flow and HCT management
i.e. guarantee an adequate oxygen supply to the tissues
With constant pump flow, oxygen delivery is directly related to the HCT
Maintaining absolute values no longer the goal
What is the optimal amount of oxygen delivery?
>280 ml O2/minute/meter2
at 34 degrees
CO2 Derived Variables – Tissue Perfusion
CO2 derived variables are better than O2 derived variables at predicting “lactic shock”
Respiratory Quotient
- VCO2/VO2 (volume of CO2 being produced / volume of O2 being consumed) provide an indication of the ANAEROBIC THRESHOLD
Ratio 0.8 to 1.1 normal
Ratio >1.1 indicates CO2 production higher than expected based on oxygen consumption
Lactate and CO2 are products of anaerobic metabolism
DO2/VCO2
- delivery of O2 / volume of CO2 production
- ratio gives an indication of quality of perfusion
Keep ratio >5 (Oxygen delivery should be 5x greater than CO2 production)
Treat if ratio falls below 5 (increase pump flow, increase hemoglobin content, decrease temperature, check anesthesia level)
If DO2/VCO2 ratio too low
- increase anything that increases delivery of O2
- increase hemoglobin, increase BF, decrease surface area
Background Information on Krogh’s Capillary Cylinder Model (Oxygen Pressure Field Theory)
- increase in metabolism => capillary dilation and flow increases
August Krogh – Published 1918
Rate of oxygen delivery at capillaries depends on number and distribution of capillaries
Demonstrated change in tissue flow with change in tissue metabolism
With increased metabolism more capillaries opened increasing the density of capillaries in a given area and thus increasing oxygen delivery
Each capillary supplies specific volume of tissue
Postulated the movement of oxygen out of the capillary
radius of capillary and radius of surrounding tissue cylinder and ratios
- about 5 microns
- about 10 microns
Ratio = capillary X-sectional area / cylinder X-sectional area = 1/4 (under normal resting conditions) - area of cylinder is 4x larger than the area of the capillary
PaO2 from arterial to venous inside capillary
80 mmHg to 40 mmHg
PaO2 from arterial to venous of the interstitial fluid
20 mmHg to 10 mmHg (averages)
- the pO2 close to the capillary will be higher than the pO2 farther away - greatest rate of diffusion is where there is the greatest gradient (arterial side where the gradient is equal to 80 - 20 = 60 mmHg) - rate of diffusion decreases as you go from arterial to venous end because the gradient decreases
