Inpatient Evaluation of Hypoxia


  • Understand causes of hypoxia and hypoxemia (~10-15 min)
  • Basic evaluation of acute onset dyspnea or hypoxia in a patient in the hospital (~10 min)

Hypoxia and Hypoxemia
Causes of Hypoxemia

There are five causes of hypoxemia (decreased PaO2 or SaO2). The A-a gradient and response to 100% FiO2 can be used to differentiate between these causes. The A-a gradient is the difference between the PaO2 and PAO2 (alveolar oxygenation). PaO2 is directly measured on an ABG, but PAO2 is calculated:

PAO2 = 0.21 (Patm– PH2O) – [PaCO2/RQ]

Patm – partial pressure of the atmosphere, at sea level is 760 mmHg
PH2O – partial pressure of the water, 47 mmHg
RQ – respiratory quotient, for most patients on a regular diet is 0.8

Normal A-a gradient is determined by age. An easy estimate is ~ 15, but is formally calculated as:

Expected A-a gradient = (age/4) + 4

V/Q mismatch, diffusion limitation and shunt are associated with an elevated A-a gradient; where as, hypoventilation and decreased FiO2 are associated with a normal A-a gradient.

Hypoventilation, V/Q mismatch and shunt are the main causes of hypoxemia that occur acutely in the hospital setting. Click to understand the relationship between shunt fraction and response to oxygen therapy (~ 1 min).

Basic Differential and Evaluation of Inpatient Dyspnea and Hypoxia

Dyspnea and hypoxia occur frequently in hospitalized patients. Dyspnea and tachypnea can occur without associated hypoxia such as patients with a metabolic acidosis or those with psychogenic causes of dyspnea (e.g. anxiety). The below diagram is a an anatomic way to organize different inpatient causes of dyspnea and hypoxia in the hospital (though should not be taken as a comprehensive list).


Now you’re ready apply what you’ve learned with some practice cases!

Learn more about oxygen delivery systems here.


Click on each diagram for the teaching script. 

Board set up

Part 1:

Part 2

EXAMPLE CASES (~ 5 min each)


A 45 year old man admitted for AKI and management of back pain after recently diagnosed renal cell carcinoma, develops increased shortness of breath on POD2. He reports pleuritic bilateral chest pain and denies fevers, chills or cough. Vitals are notable for T 36.7, HR 125, BP 125/75, RR 24, 96% on 3L NC. On exam, he is tachypneic and uncomfortable but able to speak in full sentences. Lungs are clear to auscultation and JVD is 9 cm H2O.

What additional work-up do you want right now?

How would you interpret his ABG? 

What is your differential for his hypoxemic respiratory failure? How would you further evaluate your differential? 

Final Diagnosis and Teaching Points 


A 64 year old man with HFrEF of 40% and severe COPD (FEV1 0.9L) presents to the ED with increased cough and SOB over the past three days. He denies fever or chest pain but reports sore throat, increased sputum production and “chest congestion”. He has been using his inhalers around the clock and he doubled his dose of diuretics for the past three days without improvement in his symptoms. On exam, he is afebrile, RR 32, has poor air movement on auscultation of his lungs but no appreciable wheezing.

What additional work-up would you want?

How would you interpret this ABG? 

What is the most likely diagnosis? 

How would you manage this patient?


A 20 yo female with a history of developmental delay was admitted for acute hypoxemic respiratory failure requiring intubation for parainfluenza infection. She was extubated 3 days ago and has been stably on 3-4L NC. She has had thick secretions and weak cough but has not been aggressively suctioned because of agitation. She was found to be acutely hypoxic to 80% FiO2 with moderately increased work of breathing. On exam, her RR is ~ 25-30 with no accessory muscle use. She was placed on 100% NRB with improvement in her saturations to 90%. A CXR showed:

What is the differential for this finding? 

Final Diagnosis and Take Home Points: 


A 75 year old woman with a history of mitral stenosis and atrial fibrillation is POD1 from cystoscopic resection of a bladder mass. Intraoperatively, she recieved 1U pRBCs and 3L of IVF for atrial fibrillation with rapid ventricular response (RVR) with heart rates in 150s. Immediately post-operatively, she continued to have heart rates in the 120s and was started on metoprolol. Over the past few hours, she has reported increasing shortness of breath and has had an increase in oxygen requirement from 1L NC to 60% high flow nasal cannula.  She reports some mild chest tightness but no fevers, cough, or pleurisy.

What additional evaluation and work-up do you want? 

What is the most likely diagnosis? 

How do you want to manage the patient? 


A 68 yo woman with metastatic breast cancer admitted for cancer-related back pain was found to be acutely hypoxemic to 70% FiO2 and RR in the 30s. She was placed on 100% NRB with improvement in her saturations to 85%. On exam, she has wet upper airway sounds that are transmitted throughout all lung fields. There is little airway movement at the L base. She is unable to respond to any questions because of respiratory distress. She is DNR/DNI.

What is your differential diagnosis?

What additional evaluation would you want?

Final Diagnosis and Take Home Points:


  1. Pittman RN. Regulation of Tissue Oxygenation. San Rafael (CA): Morgan & Claypool Life Sciences; 2011. Chapter 7, Oxygen Transport in Normal and Pathological Situations: Defects and Compensations. Available from:
  2. Chapter 8. Acid-Base Balance. In: Levitzky MG. eds. Pulmonary Physiology, 8e New York, NY: McGraw-Hill; 2013. Accessed July 10, 2017.
  3. Sarkar M, Niranjan N, Banyal P. Mechanisms of hypoxemia. Lung India : Official Organ of Indian Chest Society. 2017;34(1):47-60. doi:10.4103/0970-2113.197116.
  4. Johannes, J & Saggar R. Arterial Hypoxemia. In: Vincent, JL et al, eds. Textbook of Critical Care. 7th ed. Philadelphia, PA. Elsevier Inc. 2017.