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HyPoxia the basics. By deborah dewaay md Acknowledgment: antine stenbit md. Objectives. Knowledge: Understand the difference between hypoxia and hypoxemia Understand physiologic adaptation to hypoxia Understand how hypoxia causes cell death

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Hypoxia the basics

HyPoxiathe basics

By deborahdewaay md

Acknowledgment: antinestenbit md


  • Knowledge:

    • Understand the difference between hypoxia and hypoxemia

    • Understand physiologic adaptation to hypoxia

    • Understand how hypoxia causes cell death

    • Review the different modalities of providing oxygen to a patient

    • Know the differential diagnosis of acute hypoxia in the adult patient

  • Skills:

    • Use the algorithm to determine cause of hypoxia in a particular patient

  • Attitude:

    • Understand the importance of keeping the patient comfortable when they are hypoxic

    • Understand the importance of communicating compassionately with an acutely ill patient

Key messages
Key messages

  • In acute hypoxic respiratory failure, the patient should be put on a non-rebreather.

  • An ABG is crucial to the work up of hypoxia.

  • It is important to continue good communication with a patient during emergent situations like acute respiratory failure.


  • Hypoxia: a reduction of oxygen supply to a tissue below physiological levels despite adequate perfusion of the tissue by blood.

  • Hypoxemia: a decreased partial pressure of oxygen in blood less than 60mmHg on room air or less than 200mmHg on 100% oxygen.

How o 2 gets to tissues
How O2 gets to tissues

  • Inhale oxygen

  • Enters alveoli

  • Crosses alveoli/capillary membranes

  • Diffuses into blood

  • Binds to hemoglobin

  • Is carried to tissues

  • Unbinds from hemoglobin

  • Tissues use oxygen


  • Hypoxic hypoxia: arterial blood Po2is reduced

    • Pneumonia

  • Anemic hypoxia: arterial blood Po2is normal, but the amount of hemoglobin is too low to meet the tissues demands

    • Sickle cell disease

  • Ischemic hypoxia: arterial blood Po2is normal, but the blood flow is too impaired to meet the tissues demands

    • STEMI, compartment syndrome

  • Histotoxic hypoxia: arterial blood Po2is normal but a toxin is preventing the cells to utilize the O2

    • Cyanide poisoning

How hypoxia results in cell death
How hypoxia results in cell death

  • When there is diminished oxygen availability, the tissue changes to inhibit oxidative phosphorylation and increases anaerobic glycolysis

  • As a result less ATP is produced

  • Less ATP leads to in adequate energy to maintain ionic and osmotic equilibrium

  • The cell swells

  • Result: cell death

Adaptations to hypoxia
Adaptations to hypoxia

  • Hypoxia causes:

    • Systemic arteriole dilatation

    • Pulmonary vascular constriction

      • If there is little oxygen in the alveoli  the vascular bed in that area will constrict and send blood to better ventilated areas

      • Results in better ventilation/perfusion matching

      • Results in increased pulmonary vascular resistance

      • Results in increased right ventricular afterload

So they have acute hypoxia now what
So they have acute hypoxia: now what?

  • Assess the patient – ABCs

  • Vital signs

  • Is the oxygen saturation monitor accurate? Wave form?

  • Put on at a facemask (Non-rebreather) on at least 10L

  • Can the patient talk? If so, get a history

  • Physical Exam:

    • Are they wet or dry?

      • Lungs: Crackles? Where is air moving?

      • Extremities: edema (palpate sacrum too)

      • Neck: JVD

    • You will have to make an initial decision without the CXR.

    • Learn to trust your exam.

  • Other signs of chronic hypoxia: clubbing

  • Get an ABG, basic labs, CXR, EKG stat

  • How to give the patient oxygen the nose
    How to give the patient oxygen: The nose

    • Nasal Cannula:

      • Regular: can go up to 6L (39%). After 4L you need add humidity.

      • Oxymizer: gives a more accurate FIO2.Cannot give with humidity. Can give up to 15L (66%)

    How to give the patient oxygen the mouth
    How to give the patient oxygen:the mouth

    • Ventimask: this is a “high flow” mask.

      • Good for “air hunger” and mouth breather

      • Very precise amount of FIO2

      • 24%-50% = 1L – 10L

    • Non-rebreather:

      • Have flow high enough to keep bag open

      • All or nothing: 50 – 66% = 10-15L

    Non invasive ventilation and invasive ventilation
    Non-Invasive Ventilationand Invasive Ventilation

    • Non-invasive Ventilation:

      • CPAP: Continuous Positive Airway Pressure, does not initiate breaths

      • BiPAP: Bilevel Positive Airway Pressure, gives different pressures (high for inhalation, low for exhalation), can time breaths. Chronic use – OSA. Acute use: Acute pulmonary edema (CHF, HTN emergency), COPD exacerbation.

    • Invasive Ventilation: the machine breaths for the patient or supports the patients breath via a tube that is placed through the mouth into the in the trachea.


    If wet:

    Stop the fluids!

    Give nitroglycerin 1st to venodilate (SL or paste).

    Lasix: Dilate now, pee later. ESRD: give it anyway (dilate now, dialyze later)


    If dry: stabilize with oxygen

    When in doubt get a spiral CT (once stable enough).

    Throughout all of this mess, don’t forget your ABC’s, ask RTs help with CPAP/BiPAP…

    The art of multitasking
    The Art of Multitasking

    So you know they are hypoxic/hypoxemic: but you need to know


    V q normal
    V/Q - normal

    • Normal physiology:

      • V = ventilation (How well O2 gets into alveoli)

      • Q = perfusion (How well Blood gets to capillaries)

      • Blood vessels and alveoli are preferential to the bases, BUT the blood vessels > alveoli

      • V/Q is highest in the apices

      • V/Q is lowest in the bases

    5 causes of hypoxia
    5 Causes of Hypoxia

    • Reduced inspired oxygen tension

      • not enough O2 is in the air the patient is breathing, for example: high altitude

    • Hypoventilation

      • broken pump

    • Ventilation-Perfusion Mismatch

      • V/Q mismatch

    • Shunt

      • Really bad V/Q mismatch

    • Diffusion impairment

    On the abg
    On the ABG

    • “A-a O2 Gradient = [ (FiO2) * (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] - PaO2 from ABG]


        • Use the online calculators

        • Can also estimate.

          • If you put a “normal” person on 10L their PaO2 should be around 300.

    • A normal A-a gradient = 4 +age/4. 

    Reduced inspired oxygen tension
    Reduced inspired oxygen tension

    • Normal A-a gradient

    • Altitude:

      • Bad air - breathing air that has a low FIO2

    Hypercarbic respiratory failure broken pump
    hypercarbic Respiratory Failure~ Broken Pump ~

    • There is NO difference between the Alveolar O2 and the arterial O2 [No A-a gradient] & increase PCO2 then the problem is a matter of HYPOVENTILATION. The air isn’t moving = Pump failure.

    • CNS depression: drugs, CNS infection, metabolic alkalosis, stroke, hypothyroidism.

    • Myopathies: diaphragm, myositis, dystrophies, electrolytes (phosphorus).

    • Neuropathies: cervical spine, phrenic nerve, GBS, ALS, polio

    • Neuro-muscular junction: Myasthenia, botulism

    If there is an a a gradient
    If there is an A-a Gradient

    • If there is an A-a gradient  Hypoxic respiratory failure  V/Q problem.

    • What is a V/Q mismatch? All it means is the blood and the oxygen are not going to the same places.

    • If you put the patient on oxygen and they get better…

    • “V/Q mismatch”

    • DDx: airway problem (asthma, COPD), alveolar problem (PNA, CHF), Vascular problem (PE).

    If there is an a a gradient1
    If there is an A-a Gradient

    • If there is an A-a gradient and you give the patient O2 and the hypoxemia doesn’t improve = “Shunt”.

    • This is confusing – just remember 

      Shunt = Really Bad V/Q mismatch

    • DDx Shunt: Alveolar collapes (atalectasis), Alveolar filling (CHF, PNA), RL intracardiac shunt (VSD), intrapulmonary shunt (AVM).

    Diffusion limitation
    Diffusion limitation

    • Usually characterized by exercised-induced or exacerbated hypoxemia

      • During exercise  less time for diffusion. Healthy lungs will have capillary dilation to increase the surface area available so oxygenation is not affected

      • Lungs with alveolar or interstitial inflammation/fibrosis (ILD) can’t recruit additional surface area so hypoxia occurs

    • If causing acute hypoxia it is usually occurring concurrently with V/Q mismatch

    • Need PFTs to diagnose

    Hypoxemia recap
    Hypoxemia - recap

    Low FIO2 (altitude) No A-a gradient

    Hypoventilation Yes

    (CNS↓, MM, Nerve, NMJ) Corrects w/ O2?

    No Yes

    Shunt V/Q mismatch

    (atalectasis, CHF, PNA (Asthma, COPD, PNA, CHF,PE)

    PE, intracard or intrapul shunt)

    ***if diffusion limitation is suspected in addition to the above, get PFTs after patient is stable

    Don t forget the patient
    Don’t forget the patient

    • Patients feel like they are drowning

    • Give reassurance

    • Don’t forget to talk to them about what is happening and what you are going to do for them as you try to stabilize them

    • Assume they can hear you

    • Give low dose IV morphine (1-2mg) if the patient is awake and suffering to help with the “drowning” feeling


    • Lung anatomy picture: http://www.nhlbi.nih.gov/health/dci/images/lung_anatomy.jpg

    • Pocket Medicine, Sabatine, Lippincott 2008

    • Uptodate.com: oxygenation and mechanisms of hypoxemia

    • Harrison’s Online: chap 35

    • Ganong’s Review of Medical Physiology: chap 36