#30 Hypoxemia Masterclass
Join Nick and Cyrus as they teach a master class on hypoxemia for everyone and anyone who is ICU adjacent! We discuss why oxygen is so crucial, share some very interesting animal physiology and - perhaps most importantly- discuss the 6 causes of hypoxemia you MUST know… and what you can do about them! Take a listen, let us know what you think, and leave us a review!
ICU OnePager Infographic about Hypoxia & Hypoxemia
Show Notes:
Definitions
Hypoxemia: Low oxygen in the blood, typically measured via ABG (low PaO2.)
Hypoxia: Low oxygen at the tissue level, critical for cell function and survival. Also, hypoxia can occur when there is plenty of oxygen, however, the tissue cannot utilize oxygen properly.
Key Relationships:
Oxygen Content (CaO2):
Formula: CaO2 = 1.34 x Hb x SaO2 + 0.003 x PaO2
CaO2 mostly consists of oxygen bound to hemoglobin and a tiny amount of dissolved oxygen.
DO2 = Cardiac Output (CO = HR x SV) x CaO2
Importance of Oxygen Delivery:
DO2 (Oxygen Delivery): Must be greater than VO2 (Oxygen Consumption) to prevent cellular hypoxia.
Normal Physiology: Saturation of arterial blood (SaO2) is 95-100%, and venous blood (SvO2) is 70-75%, indicating about 25% oxygen extraction by tissues.
Cell respiration is a spectrum: As oxygen utilization increases, anaerobic respiration increases which can be seen as an increase in serum lactate. As more and more oxygen is consumed - faster than it’s being delivered - the rate with which lactate is produced (all else being equal, as a byproduct of anaerobic respiration) will increase.
Physiology Pearls:
Alveolar-arterial (A-a) Gradient / Difference: Helps assess oxygen transfer efficiency from alveoli to blood. Is there (or isn’t there) a difference between oxygen tension within the alveoli and oxygen tension in the blood?
Fick’s Method: Estimates cardiac output by measuring arterio-venous oxygen difference.
CO = HR x SV = VO2/(CaO2-CvO2)
If you know the CO via other means, can use this to calculate VO2
Types of Hypoxia
Hypoxemic Hypoxia:Low oxygen in the tissues due to low oxygen in the blood. Most common type of hypoxia seen in the hospital.
Alternatively stated: a “CaO2 problem”
Ischemic Hypoxia: Inadequate blood flow to the tissues, which can be regional (e.g., stroke, MI) or global (e.g., cardiogenic shock).
Alternatively stated: a “CO problem”
Anemic Hypoxia: Blood carries insufficient oxygen due to low hemoglobin.
Cytopathic Hypoxia: Tissues cannot use the oxygen delivered, often due to toxins like cyanide or severe sepsis.
SIX causes of hypoxemic hypoxa
Low Inspired Oxygen:
Example: High altitude, BVM or ventilator that is not hooked up to oxygen.
Solution: Supplemental oxygen resolves hypoxemia. Return patient from altitude.
Case: Climbing Mount Rainier, leading to hypoxemia due to high altitude.
Hypoventilation:
Example: Sedatives, obesity, sleep apnea (obstruction).
Solution: Increase ventilation, possibly with bi-level / NIPPV. Naloxone when appropriate.
Case: Post-operative patient with history of sleep apnea and recent sedative use.
V/Q Mismatch:
Example: Pulmonary embolism, COPD, pneumonia, starting a heart failure patient on a calcium channel blocker (sometimes).
Can be a “V” problem, a “Q” problem, or both!
Solution: Supplemental oxygen improves hypoxemia. In the case of PE anticoagulation, targeted thrombectomy/thrombolysis or systemic thrombolysis can be helpful in the appropriate patient.
Case: Bed-ridden cancer patient developing PE.
Shunt:
Example: Septal defects, AVMs.
Solution: Address underlying cause, supplemental oxygen less effective.
Note: Shunt & V/Q mismatch can often occur together - for example in the case of pneumonia. Treating the pneumonia and optimizing pulmonary physiology is helpful in these cases.
Case: Patient with PAH and distributive shock revealing a PFO with bubble study.
Diffusion Limitation:
Example: Pulmonary fibrosis, interstitial edema.
Solution: Supplemental oxygen normalizes PaO2. Diuresis in the setting of interstitial edema is helpful.
Case: Retired construction worker with UIP pattern on CT. Minimal-if-any hypoxemia at rest, but much worse when cardiac output increases (i.e. with exertion).
Low SvO2:
Example: Anemia + low cardiac output + increased VO2 due to fever, etc.
Solution: Improve CaO2 with transfusions, increase CO with inotropes, reduce VO2 with antipyretics.
Case: Lupus patient admitted with a flare, presents with anemia, new onset myocarditis, and is septic from a urinary source.
Audio
Video
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Teran F, Prats MI, Nelson BP, et al. Focused transesophageal echocardiography during cardiac arrest resuscitation: jacc review topic of the week. J Am Coll Cardiol. 2020;76(6):745-754.
Arntfield R, Lau V, Landry Y, Priestap F, Ball I. Impact of critical care transesophageal echocardiography in medical-surgical icu patients: characteristics and results from 274 consecutive examinations. J Intensive Care Med. 2020;35(9):896-902.
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Undifferentiated Shock
Cardiac Arrest