Physiology of ECMO Flashcards
Our bodies depend on…
Delivering the appropriate amount of oxygen to each cell
The ability of each cell to take-up and consume the proper amount of oxygen
What ECMO will do to help
Drain venous blod
provide a pump for cardiac output
oxygenate and remove CO2 via an artificial lung
maintain temperature via a heat exchanger
returning blood to the patient via a:
vein (VV) or an artery (VA)
ECMO Physiological Goals
Improve blood O2 delivery
Remove CO2
allow normal aerobic metabolism to take place while “resting” the lungs
provide cardiac and/or respiratory support as necessary
What are the two places of gas exchange we look at ?
Pulmonary respiration
Tissue respiration
Pulmonary respiration
refers to the gas exchange between blood and inspired gas
Tissue respiration
refers to the exchange of oxygen and carbon dioxide at cellular level
Aerobic Tissue Oxygenation
In order for tissues to engage in aerobic metabolism they need oxygen
allows conversion of glucose to ATP
Get 36 moles ATP per mole glucose
Anaerbic Tissue Oxygenation (Hypoxia)
If not enough oxygen is available, we have anaerobic metabolism
Get 2 moles ATP per mole glucose and production of lactate
4 principle hemodynamic processes
- O2 content
- O2 delivery
- o2 consumption
- o2 reserve/return
Oxygen Content Equation (CaO2)
CaO2 = 1.34* HGb* SaO2 +(0.003 *paO2100)
What is 1.34 in the oxygen content equation?
the amount of oxygen (ml at 1 atm) bound per gram of hemoglobin
What is the 0.003 in the oxygen content equation?
represents the amount of oxygen dissolved in plasma
WHat is the primary way to increase oxygen content?
Hemoglobin
O2 Delivery Equation (DO2)
Available Oxygen x Delivery Rate
Arterial Content x Cardiac Output
What is normal DO2?
600 cc/min/m2
(900-1000) cc/min
How do we assess the effectiveness of oxygen transport?
Arterial and mixed venous blood gas determination (central lab)
Noninvasive pulse oximetry
Invasive mixed venous saturation monitoring
Continuous indwelling arterial blood gas monitoring
Point of care blood gas monitoring
Transcutaneous po2 monitoring
transcutaneous meausre of local tissue oxygen saturation
Continuous indwelling arterial blood gas monitoring
20 gauge fiberoptic probe that measures pH, pCO2 and PO2
conflicting results on accuracy/very expensive/ no commerical units
Transcutaneous PO2 monitirng
meausres oxygen tension of heated skin (43C) more problems than worth so not commonly used clinically
How Does the Body Compensate
Increased cardiac output
Extract more oxygen from hemoglobin at the systemic capillaries
increase amount of hemoglobin and red cell mass
Increased cardiac output
primary phsyiology response
healthy heart: 15-25 L/min
Extract more oxygen from the hemoglobin at the systemic capillaries
decrease venous saturation to 32%
below 32% anaerobic metabolism and metabolic acidosis start
Increase amount of hemoglobin and red cell mass
takes weeks to develop
Oxygen Consumption
the difference between the deliveyr and what is returned
AVO2 difference = Ca-Cv
Ca
the oxygen concentration of arterial blood (oxygenated blood)
Cv
the oxygen concentration of venous blood (deoxygenated blood)
Oxygen Consumption is a function of….
Cardiac output
difference in oxygen content b/w arterial and venous blood
VO2 = CO x 1.34 x Hb (SaO2 - SvO2) 10
Oxygen Consumption (O2 Demand)
150 cc/min/m2
200-240 cc/min
Oxygen Extraction Ratio
VO2/Do2 x 100
Ratio of oxygen uptake to delivery
Usually 20-30%
uptake is kept constant by increasing extraction when delivery drops
Conditions that may affect tissue extraction
Excessive tissue requirements
Demand > Supply
Inability of body to use oxygen
Impaired oxygen unloading at the capillary level
Demand > Supply
oxygen demand higher than system can provide
hypermetabolism/systemic inflammatory response syndrome (post CPB)
sepsis/ malignant hyperthermia
Why would the body not be able to use oxygen?
Toxic hypoxia
sepsis/cyanide or ethanol toxicity
Why would the body have impaired oxygen unloading at the capillary level?
Alkalemia/Hypocarbia
Administration of large amounts of banked blood
Conditions that may affect tissue oxygenation
Inadquate blood flow
decresaed oxygen saturation of arterial hemoglobin
severe anemia
Inadequate blood flow
ischemic hypoxia
-obstruction lesions of blood vessels/peripheral vascular disease
Decreased oxygen saturation of arterial hemoglobin
hypoxemic hypoxia
inadequate oxygen transfer from lungs to blood
-hypoxemia most common cause (PaO2 less than 60 mmHg) / carbon monoxide poisoning / methemoglobinemia
Severe anemia
anemic hypoxia
deificiency ofh emoglobin molecules
hemorrhage / nutritional deficiencies/dilution
Oxygen Reserve (RO2)
oxygen reserve can be described as what’s left after consumption has taken place
venous gases returning ot the heart
it is essentially a build in phsyiological buffer
Normal Oxygen Reserve
450 cc/min/m2
700-800 cc/min
Carbon Dioxide
produced as part of the metabolic process and has an emission rate that is dependent on the level of activity
while co2 is not normally harmful, the concentration of CO2 can act as a marker for the adequacy of ventilation
CO2 is produced where
at the tissue
CO2 is picked up by what
the capillary
ECMO will provide
oxygenation
carbon dioxide removal
maintenace of PH
ECMO will have the ability to _______ and ______ both oxygenation and carbon dioxide removal.
Assist and control
V-V ECMO
Oxygenated blood returned to the right side of hte heart (preoxygenated)
V-A ECMO
oxygenated blood returned to the left side of the heart (as normal)
Oxygenation is controlled on ECMO via ….
an O2 blender
Control of FIO2
Monitor PaO2
If you increase FiO2 % = ______ PaO2 (mmHg)
increase
Carbon Dioxide Removal
controlld by gas flowing across a membrane (LPM)
Blows off CO2 from the membrane
Monitor pCO2 (mmHg)
Increase gas flow = ____ pCO2 (mmHg)
decrease
Gas to BF ratio of ____ when intiating ECMO
1
An increase in GBFR (more sweep) results in _____ pCO2
lower
A decrease in GBFR (less sweep) results in _____ pco2
higher
Cool stuff to know about gases
The body attempts to control PH
Respiratory centers control pCO2
metabolic processes control carbonic acid cycle
Hence ABG problems are referred to as metabolic or respitaory
Modified Henderson Hasselbach Equation
pH = 6.1 + log ([HCO3-]/0.03 x pCO2)
pH is proportional to …..
HCO3-
pH inversely proportion to….
CO2
Carbonic Acid Cycle
Carbonic acid is an intermediate step in the transport of CO2 out of the body via respiratory gas exchange
H20 + Co2(g) H2CO3 (carbonic acid) H+ + HCO3-
The goal of ECMO is to….
rest the body
