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HyPoxia the basics. By deborah dewaay md Medical University of South Carolina May 29, 2012 Acknowledgment: antine stenbit md. Objectives. Knowledge: Understand the difference between hypoxia and hypoxemia Understand physiologic adaptation to hypoxia

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hypoxia the basics

HyPoxiathe basics

By deborahdewaaymd

Medical University of South Carolina

May 29, 2012

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