1	Hypoxia Evaluation and Management
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3	"A 78 year old male..." A 78 year old male is seen in the pre-op clinic with a six month history of shortness of breath, ankle edema, orthopnea, productive cough, hoarseness and 30-40lb weight loss. He has a palpable mass in his abdomen, suspicious for colonic cancer and is scheduled for resection. As part of your preop work up, a blood gas is performed: pH 7.42 PaCO2 36 PaO2 41 SaO2 78%. He is put on 100% oxygen, has a chest x-ray performed, which is normal, and a spiral CT of his thorax, which also appears normal. His blood gas on 100% oxygen is: pH 7.46, PaCO2 36, PaO2 42, SaO2 78%. What is going on?
4	Definition A relative deficiency of oxygen in arterial blood. Classical: PaO2<80mmHg or SpO2 < 95% on RA Clinical: PaO2 < 60mmHg or SpO2 < 90% Because the content of O2 is minimally changed by increases in oxy-hemoglobin (HbO2) above 90%
5	Factors controlling the Affinity of Oxygen for Hemoglobin Increased Affinity (Leftward Shift): Reduction in temperature Alkalosis Increased 2,3-DPG Decreased Affinity ( Rightward Shift): Acidosis Increased Temperature Decreased 2,3 - DPG
6	Oxygen-Hgb Dissociation Curve
7	"I’m my own grandpa" I’m my own grandpa.
8	"The leftward shift of the..." The leftward shift of the oxyhemoglobin dissociation curve caused by hypocarbia is known as the A). Fick Principle B). Bohr Effect C). Haldane Effect D). Law of Laplace E). None of the above
9	PaO2 and SpO2 Correlation The magnitude of change in SpO2 is greater at PaO2 levels of < 60mHg (SpO2 < 90%) and decreases with increasing PaO2 levels > 60mmHg Spo2 is a sensitive monitor of arterial oxygenation in critical levels of PaO2 < 60mmHg but relatively insensitive at higher levels
10	"Each of the following will..." Each of the following will cause erroneous readings by dual-wavelength pulse oximeters EXCEPT: A). Carboxyhemoglobin B). Methelyne Blue C). Fetal Hemoglobin D). Methemoglobin E). Nail polish
11	PaO2 and Arterial Blood Oxygen Correlation CaO2 = Hgb x 1.34 x SaO2 + PaO2 x 0.003 Assuming a PaO2 of 100mmHg and a hemoglobin concentration of 15 g/100ml, the CaO2 is about 20.3 ml/dl….20 ml of which is in combination with hemoglobin and 0.3ml is in solution with plasma Normal PaO2 decreases with age: PaO2 = 102 – age (yr) / 3
12	Causes of Hypoxemia 1). Low fraction of inspired oxygen (FiO2) 2). Hypoventilation 3). Decreased barometric pressure 4). Ventilation- Perfusion (V/Q) mismatch 5). Left to Right shunt 6). Diffusion Defect (controversial)
13	"A 62 yo male is..." A 62 yo male is brought to the ICU after elective repair of a AAA. His vital signs are stable, but he requires a sodium nitroprusside infusion at a rate of 10 mcg/kg/min to keep the SBP below 110mmHg. The SaO2 is 98% with controlled ventilation at 12 breaths/min and an FIO2 of 0.60. After 3 days his SaO2 decreases to 85% on the pulse oximeter. CXR and physical exam are unchanged. Which most likely accounts for the desaturation? A). Cyanide toxicity B). Thiocyanate toxicity C). O2 toxicity D). Thiosulfate toxicity E). Methemolobinemia
14	Evaluation of Hypoxia Clinically assess the pt to determine need for emergent intervention: Vital signs, breath sounds ( bronchospasm), mental status, PMH, PSH, etiology of admission, labs, If intubated: determine mechanical integrity of ventilator, BBS, ETT location, secretions, current vent settings
15	Evaluation of Hypoxia…CXR Determine ETT location R/O pneumothorax Atelectasis Pleural Effusions DHT/NGT/CVL locations
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20	Evaluation of Hypoxia…ABG’s Assess oxygenation, ventilation, saturation, and acid base status If PaO2 is below 60 torr on RA calculate the alveolar to arterial difference for O2… PAO2 = FIO2 (PBP – PH2O) - PaCO2 / R A-aDO2 = A-a gradient = PAO2 – PaO2
21	A-a Gradient Clinical usefulness limited because values are dependent on FIO2 A-a gradient increases with increasing FIO2. Ex. The A-a gradient may increase to 70mmHg in normal individuals with FIO2 of 100%; approx 5-7 mmHg for every 10% increase in FIO2
22	A-a Gradient Norm A-a gradient < 10mmHg at FIO2 .21 Normal A-a gradient: Hypoventilation Decreased FIO2 Increased gradient: V/Q mismatch Shunt Diffusion Barrier
23	a-A PO2 Ratio Minimally effected by FIO2 because the alveolar PO2 is both numerator and denominator: a/A PO2 = 1 – (A-a PO2)/PAO2 FIO2 Norm a/A PO2 0.21 0.74 – 0.77 1.0 0.80 – 0.82
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25	Evaluation of Hypoxia ….Mixed Venous PO2 Identifies a systemic delivery/ consumption (DO2/VO2) imbalance Norm approx 40 mmHg Normal Venous PO2: Indicates solely V/Q abnormality. If >40mmHg then lungs may be the source of the hypoxia. If CXR unrevealing consider acute PE
26	Evaluation of Hypoxia…Mixed Venous PO2 cont. Low Venous PO2: Indicates a systemic DO2/VO2 imbalance If below 40mmHg indicates a low rate of O2 delivery (anemia, low CO) or increased O2 consumption.
27	Low FIO2/Hypoventilation FIO2: initial 100% with rapid wean to FIO2 that maintains PaO2 60-80mmHg or SpO2 of 90% – 94% FIO2 of 50% or less to avoid: Denitrogenation Atelectasis Oxygen Toxicity ( O2 radicals) Retinopathy of Prematurity
28	Low FIO2/Hypoventilation cont. Titrate respiratory rate to adequate MV; target pH not PaCO2 If the patient is on room air (.21), determine the A-a gradient
29	Decreased Barometric Pressure Altitude Associated Hypoxemia At the top of Pikes Peak at an altitude of approximately 14000 feet, PB = 450 PAO2 = .21 (450-47) – (40/0.8) = 37mmHg Typical response is hyperventilation that decreases PaCO2 and PACO2, which increases the FIO2 within the alveolus
30	Right to Left Intra-cardiac shunt True shunt VA/Q = 0 Result in mixed venous blood entering the L heart with a resulting decreased CaO2 The magnitude of hypoxemia depends on the size of the shunt and the CVO2. Examples: Tetralogy of Fallot Tricuspid Atresia Transposition of the Great Arteries Total Anomalous Venous Return Truncus Arteriosus Hypoplastic Left Heart Syndrome
31	Right to Left Intra-cardiac Shunts cont. Eisenmengers Syndrome: a reversal of an L to R intra-cardiac shunt to a R to L secondary to elevated R heart pressures Blood passing through shunt is not exposed to alveolar gas = true shunt or VA/Q = 0 Hypoxemia is refractory to oxygen therapy Consider refractory hypoxemia when PaO2 is < 55mmHg and the FIO2 is > 0.35
32	"A 78 year old male..." A 78 year old male is seen in the pre-op clinic with a six month history of shortness of breath, ankle edema, orthopnea, productive cough, hoarseness and 30-40lb weight loss. He has a palpable mass in his abdomen, suspicious for colonic cancer and is scheduled for resection. As part of your preop work up, a blood gas is performed: pH 7.42 PaCO2 36 PaO2 41 SaO2 78%. He is put on 100% oxygen, has a chest x-ray performed, which is normal, and a spiral CT of his thorax, which also appears normal. His blood gas on 100% oxygen is: pH 7.46, PaCO2 36, PaO2 42, SaO2 78%. What is going on?
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34	Ventilation-Perfusion Mismatch The normal pulmonary ventilation to perfusion (V/Q) is 0.8 Normal lung segment V/Q = 1 Absolute Shunt V/Q = 0 ( perfused but not ventilated) Deadspace V/Q = infinity ( ventilated but not perfused)
35	Ventilation-Perfusion Mismatch Dead space
36	Deadspace V/Q = infinity; ventilated but not perfused Examples: Pulmonary Embolus Low Cardiac Output Hypovolemia Lung Hyperventilation Excessive PEEP Breathing Circuits Extended neck/Protruded jaw Erect Posture General Anesthesia Rapid Short Inspirations
37	VD/VT The fraction of the tidal volume composed of dead space volume VD/VT = alveolar PCO2 – expired PCO2 ________________________________________________________________________________________________ alveolar PCO2
38	"Which of the following statements..." Which of the following statements correctly defines the relationship between minute ventilation (VE), dead space ventilation (VD), and PaCO2? A). If VE is constant and VD increases, then PaCO2 will increase. B). If VE is constant and VD increases, then PaCO2 will decrease C). If VD is constant and VE increases, then PaCO2 will increase D). If VD is constant and VE decreases, then PaCO2 will decrease
39	Shunt Effect V/Q < 1 but > 0 Diminished V relative to Q results in reduced PAO2 and reduced PO2 in pulmonary capillaries. Decreased pulmonary capillary blood O2 creates decreased CcO2 which creates decreased CaO2 in the L heart = hypoxemia
40	True Shunt V/Q = 0; perfused but not ventilated Approx. 2-5% of CO is normally shunted through pulmonary shunts; this accounts for the normal A-a gradient Significant shunts 20-30% Fatal Shunts are those > 30%
41	Types of Shunt Physiologic shunt: most common, include atelectasis or consolidation of alveoli. Postpulmonary Shunt: secondary to bronchial, mediastinal, pleural, and thesbian veins. Pathoanatomic Shunt: congenital or traumatic anomalies and intrapulmonary tumors.
42	Calculation of Shunt Fraction Fraction of the CO that passes through a shunt Qs/Qt = (Cc – Ca) (Cc - Cv) Cc = O2 content of end pulm. capillary blood Ca = O2 content of arterial blood Cv = O2 content of mixed venous blood Calculated assuming an FIO2 of 1.0 in order to separate true shunt from shunt effect; FIO2 > 0.5 can increase shunt from loss of HPV
43	Hypoxic Pulmonary Vasoconstriction Local response of PA smooth muscle to a decreased regional alveolar PO2. It acts to decrease Q to underventilated regions of the lung and maintain a normal V/Q. Inhibited by volatile anesthetics and potent vasodilators.
44	Increased FIO2 on shunt effect Denitrogenation associated with FIO2 of 1.0 increases PAO2 in the underventilated alveolus to ensure saturated Hgb thus correcting hypoxemia The lower the V/Q the higher the required FIO2. Hypoxemia from shunt effect responds to O2 therapy
45	Increased FIO2 on true shunt V/Q = 0 Blood passing through the shunt is not exposed to alveolar gas…..increased FIO2 has minimal effect Consider shunt refractory when the PaO2 is < 55mmHg and FIO2 is > 0.35
46	Oxygen Challenge Used to differentiate refractory versus non-refractory hypoxemia to O2 while avoiding FIO2 > 0.5 Obtain baseline ABG Increase FIO2 by 0.2; repeat ABG in 30 min If the PaO2 has increased by > 10mmHg from baseline the hypoxemia is responsive to O2
47	The End….. Fool!