Oxygen & Hypoxia: Physiological Tidbits · 2019-09-27 · Case Report 24 year old women (with...
Transcript of Oxygen & Hypoxia: Physiological Tidbits · 2019-09-27 · Case Report 24 year old women (with...
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Oxygen & Hypoxia: Physiological Tidbits
Arthur S. Slutsky, MD
Keenan Research Center, Li Ka Shing Knowledge Institute, St. Michael’s Hospital;
Interdepartmental Division of Critical Care Medicine,
University of Toronto
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Overview
Unusual cause of increased (A-a)O2
difference
Apneic oxygenation
Mechanisms of cardiovascular effects of
hypoxia
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Case Report
24 year old women (with perfectly normal lungs) to ED with hyperventilation syndrome. As she walks in her PaCO2 is 24 mmHg, and PaO2 118 mmHg
– (A-a)O2=(150-24/0.8)-118=2
MD comes to see her; less anxious; markedly decreased ventilation; apneic for 1.5 minutes
– PaCO2 increases to 32 mmHg;
– PaO2 decreases from 118 to 60 mmHg
Calculation of (A-a)O2: PAO2=PIO2-PCO2/R =150-32/0.8=110:
– Therefore (A-a)O2=110-60=50 mmHg
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What is the reason for the increased (A-a)O2
PAO2=PIO2-PaCO2/R
R=Respiratory quotient=VCO2/VO2
– VCO2 = CO2 elimination ~200 ml/min
– VO2 = O2 uptake ~250 ml/min
R=200/250 = 0.8
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(~20 ml/min)
(~250 ml/min)
CO2
O2
What is the Respiratory Quotient (R)? There are 2 R’s!
Rtiss=VtCO2/VtO2
= 200/250=0.8
R=VCO2/VO2
= 0.8
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(~20 ml/min)
(~180 ml/min)
R=VCO2/VO2
The 2 R’s are NOT necessarily equal in non-steady state conditions
Apnea
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(~20 ml/min)
(~180 ml/min)
R=VCO2/VO2
The 2 R’s are NOT necessarily equal in non-steady state conditions
In this situation, the alveolar air equation is not
valid
Apnea
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When are changes in R relevant?
Changes in diet
– Protein vs fat vs sugar
During periods of non-steady state
– Acute hypoventilation
– Acute hyperventilation
Dialysis with a non-bicarbonate buffer
– Dialysis-associated hypoxemia
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Apneic Oxygenation
Originally called diffusion respiration
(Draper and Whitehead, 1940’s)
Technique
– Washout of N2 by ventilation with 100% O2
– 100% O2 @ airway opening
– adequate oxygenation for >1 hr
– CO2 accumulation
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6.6
6.7
6.8
6.9
7
pH
1 2 3 4 5
Patient Number
Apneic Oxygenation in Man Frumin et al Anesthesiology 1961
0
50
100
150
200
250
PaCO
2 (
mm
Hg)
1 2 3 4 5
Patient Number
•Patients in OR: AO for 45-60 minutes •SaO2 > 95% in all patients
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How is it possible to obtain adequate oxygenation without ventilation?
(~20 ml/min)
(~180 ml/min)
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(~180 ml/min)
(~20 ml/min)
(~250 ml/min)
How is it possible to obtain adequate oxygenation without ventilation?
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(~180 ml/min)
(~20 ml/min)
(~250 ml/min)
Negative
Pressure
How is it possible to obtain adequate oxygenation without ventilation?
Animals can survive ~90 minutes during Apneic Oxygenation With PO2~80 mmHg
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Why do animals die as apneic oxygenation continues despite adequate PaO2s?
7.5 7.1
6.7
6.3
6.5
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Physiological Responses to Hypoxemia
Cardiovascular responses to hypoxemia
– Heart rate response
– Respiratory sinus arrhythmia
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Constant Flow Ventilation
Lehnert, Oberdoster, Slutsky Journal of Applied Physiology 1982:53: pp 483-489
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Impact of Constant Flow Ventilation on Heart Rate Response to Hypoxia
Rationale:
– Remove effect of cyclic changes in lung volume on hemodynamics
Methods:
– Studies in dogs made hypoxic
Spontaneous breathing
Constant Flow
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Kato, Menon, Slutsky Circulation 1988;77:407-14
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Kato, Menon, Slutsky Circulation 1988;77:407-14
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Kato, Menon, Slutsky Circulation 1988;77:407-14
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Kato, Menon, Slutsky Circulation 1988;77:407-14
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Heart
Rate
b/m
in
Oxygen Saturation (SaO2)
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Heart
Rate
b/m
in
Oxygen Saturation (SaO2)
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Respiratory Sinus Arrhythmia
Shykoff, Naqvi, Menon, Slutsky J Clin Invest 87:1621-27,1991
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Respiratory Sinus Arrhythmia
Shykoff, Naqvi, Menon, Slutsky J Clin Invest 87:1621-27,1991
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Shykoff, Naqvi, Menon, Slutsky J Clin Invest 87:1621-27,1991
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Conclusions
Interpretation of alveolar-air equation requires a consideration of respiratory quotient
Apneic oxygenation – Interesting technique with potential
implications for assessing brain death and tolerance to hypercapnia
Heart-lung interactions not simply related to impact of pressures