Patients without respiratory disorders who are in the intensive care unit (ICU)—and other patients—may develop hypoxia (oxygen saturation < 90%) during a hospital stay. Hypoxia in patients with known respiratory conditions is discussed under those disorders.
Etiology of Oxygen Desaturation
Numerous disorders cause hypoxia (eg, dyspnea Dyspnea Dyspnea is unpleasant or uncomfortable breathing. It is experienced and described differently by patients depending on the cause. Although dyspnea is a relatively common problem, the pathophysiology... read more , respiratory failure Overview of Respiratory Failure Acute respiratory failure is a life-threatening impairment of oxygenation, carbon dioxide elimination, or both. Respiratory failure may occur because of impaired gas exchange, decreased ventilation... read more —see table Some Causes of Oxygen Desaturation Some Causes of Oxygen Desaturation 
); however, acute hypoxia developing in a patient hospitalized with a nonrespiratory illness usually has a more limited set of causes. These causes can be divided into
Disorders of ventilation
Disorders of oxygenation
Evaluation of Oxygen Desaturation
Total fluid volume given during the hospital stay and, in particular, during the previous 24 hours should be ascertained to identify volume overload Volume Overload Volume overload generally refers to expansion of the extracellular fluid (ECF) volume. ECF volume expansion typically occurs in heart failure, kidney failure, nephrotic syndrome, and cirrhosis... read more . Drugs should be reviewed for sedative administration and dosage. In significant hypoxia (oxygen saturation < 85%), treatment begins simultaneously with evaluation.
History
Very sudden onset dyspnea and hypoxia suggest pulmonary embolus Pulmonary Embolism (PE) Pulmonary embolism (PE) is the occlusion of pulmonary arteries by thrombi that originate elsewhere, typically in the large veins of the legs or pelvis. Risk factors for pulmonary embolism are... read more 
(PE) or pneumothorax Pneumothorax Pneumothorax is air in the pleural space causing partial or complete lung collapse. Pneumothorax can occur spontaneously or result from trauma or medical procedures. Diagnosis is based on clinical... read more 
(mainly in a patient receiving positive pressure ventilation). Fever, chills, and productive cough (or increased secretions) suggest pneumonia Hospital-Acquired Pneumonia Hospital-acquired pneumonia (HAP) develops at least 48 hours after hospital admission. The most common pathogens are gram-negative bacilli and Staphylococcus aureus; antibiotic-resistant... read more . A history of cardiopulmonary disease (eg, asthma Asthma Asthma is a disease of diffuse airway inflammation caused by a variety of triggering stimuli resulting in partially or completely reversible bronchoconstriction. Symptoms and signs include dyspnea... read more , chronic obstructive pulmonary disease Chronic Obstructive Pulmonary Disease (COPD) Chronic obstructive pulmonary disease (COPD) is airflow limitation caused by an inflammatory response to inhaled toxins, often cigarette smoke. Alpha-1 antitrypsin deficiency and various occupational... read more 
, heart failure Heart Failure (HF) Heart failure (HF) is a syndrome of ventricular dysfunction. Left ventricular (LV) failure causes shortness of breath and fatigue, and right ventricular (RV) failure causes peripheral and abdominal... read more 
) may indicate an exacerbation of the disease. Symptoms and signs of myocardial infarction may indicate acute valvular insufficiency, pulmonary edema Pulmonary Edema Pulmonary edema is acute, severe left ventricular failure with pulmonary venous hypertension and alveolar flooding. Findings are severe dyspnea, diaphoresis, wheezing, and sometimes blood-tinged... read more 
, or cardiogenic shock Cardiogenic and obstructive shock Shock is a state of organ hypoperfusion with resultant cellular dysfunction and death. Mechanisms may involve decreased circulating volume, decreased cardiac output, and vasodilation, sometimes... read more . Unilateral extremity pain suggests deep venous thrombosis Deep Venous Thrombosis (DVT) Deep venous thrombosis (DVT) is clotting of blood in a deep vein of an extremity (usually calf or thigh) or the pelvis. DVT is the primary cause of pulmonary embolism. DVT results from conditions... read more 
(DVT) and hence possible PE. Preceding major trauma or sepsis requiring significant resuscitation suggests acute respiratory distress syndrome Acute Hypoxemic Respiratory Failure (AHRF, ARDS) Acute hypoxemic respiratory failure is defined as severe hypoxemia (PaO2 (See also Overview of Mechanical Ventilation.) Airspace filling in acute hypoxemic respiratory failure (AHRF) may result... read more 
. Renal failure indicates higher risk of fluid overload. Preceding chest trauma suggests pulmonary contusion Pulmonary Contusion Pulmonary contusion is trauma-induced lung hemorrhage and edema without laceration. (See also Overview of Thoracic Trauma.) Pulmonary contusion is a common and potentially lethal chest injury... read more .
Physical examination
Patency of the airway and strength and adequacy of respirations should be assessed immediately. For patients on mechanical ventilation, it is important to determine that the endotracheal tube is not obstructed or dislodged. Findings are suggestive as follows:
Unilateral decreased breath sounds with clear lung fields suggest pneumothorax or right mainstem bronchus intubation; with crackles and fever, pneumonia is more likely.
Distended neck veins with bilateral lung crackles suggest volume overload with pulmonary edema, cardiogenic shock, pericardial tamponade Cardiac Tamponade Cardiac tamponade is accumulation of blood in the pericardial sac of sufficient volume and pressure to impair cardiac filling. Patients typically have hypotension, muffled heart tones, and distended... read more (often without crackles), or acute valvular insufficiency.
Distended neck veins with clear lungs or unilateral decrease in breath sounds and tracheal deviation suggest tension pneumothorax.
Bilateral lower-extremity edema suggests heart failure, but unilateral edema suggests DVT and hence possible PE.
Wheezing represents bronchospasm (typically asthma or allergic reaction, but it occurs rarely with PE or heart failure).
Decreased mental status suggests hypoventilation.
Testing
Hypoxia is generally recognized initially by pulse oximetry. Patients should have the following:
A chest x-ray (eg, to assess for pneumonia, pleural effusion, pneumothorax, heart failure, or atelectasis)
ECG (to assess for arrhythmia or ischemia)
Arterial blood gases (ABGs), to confirm hypoxia and evaluate adequacy of ventilation
Bedside echocardiography done by an intensivist may be used to assess for hemodynamically significant pericardial effusion or reduced global left ventricular or right ventricular function until formal echocardiography can be done. Bedside ultrasonography can also be done to identify pneumothorax. Elevated serum levels of brain (B-type) natriuretic peptide (BNP) may help differentiate heart failure from other causes of hypoxia. If diagnosis remains unclear after these tests, testing for PE Diagnosis Pulmonary embolism (PE) is the occlusion of pulmonary arteries by thrombi that originate elsewhere, typically in the large veins of the legs or pelvis. Risk factors for pulmonary embolism are... read more should be considered. Bronchoscopy may be done in intubated patients or in patients with a tracheostomy to rule out (and remove) a tracheobronchial mucous plug.
Treatment of Oxygen Desaturation
Identified causes are treated as discussed elsewhere in THE MANUAL. If hypoventilation persists, mechanical ventilation Overview of Mechanical Ventilation Mechanical ventilation can be Noninvasive, involving various types of face masks Invasive, involving endotracheal intubation Selection and use of appropriate techniques require an understanding... read more via noninvasive positive pressure ventilation or endotracheal intubation is necessary. Persistent hypoxia requires supplemental oxygen.
Oxygen therapy
The amount of oxygen given is guided by arterial blood gases (ABG) or pulse oximetry to maintain PaO2 between 60 and 80 mm Hg (ie, 92 to 100% saturation) without causing oxygen toxicity. This level provides satisfactory tissue oxygen delivery; because the oxyhemoglobin dissociation curve is sigmoidal, increasing PaO2 to > 80 mm Hg increases oxygen delivery very little and is not necessary. The lowest fractional inspired oxygen (FiO2) that provides an acceptable PaO2 should be provided. Oxygen toxicity is
Concentration-dependent
Time-dependent
Sustained elevations in FiO2 > 60% result in inflammatory changes, alveolar infiltration, and, eventually, pulmonary fibrosis. An FiO2 > 60% should be avoided unless necessary for survival. An FiO2 60% is well tolerated for long periods.
An FiO2 < 40% can be given via nasal cannula or simple face mask. A nasal cannula uses an oxygen flow of 1 to 6 L/minute. Because 6 L/minute is sufficient to fill the nasopharynx, higher flow rates are of no benefit. Simple face masks and nasal cannulas do not deliver a precise FiO2 because of inconsistent admixture of oxygen with room air from leakage and mouth breathing. However, Venturi-type masks can deliver very accurate oxygen concentrations.
An FiO2 > 40% requires use of an oxygen mask with a reservoir that is inflated by oxygen from the supply. In the typical nonrebreather mask, the patient inhales 100% oxygen from the reservoir, but during exhalation, a rubber flap valve diverts exhaled breath to the environment, preventing admixture of carbon dioxide and water vapor with the inspired oxygen. Nonetheless, because of leakage, such masks deliver an FiO2 of at most 80 to 90%.
High-flow nasal cannula (HFNC) oxygen therapy, in contrast to traditional nasal oxygen, delivers oxygen at rates from 20 to 60 L/minute; the oxygen is humidified. Humidification helps prevent airway desiccation and inflammation, maintain mucociliary function, and improve mucus clearance. HFNC therapy tends to wash out upper airway dead space and decrease the work of breathing more than non-rebreather masks. This therapy may help patients with hypoxemic respiratory failure not due to heart failure and who are not hypercapneic.
Refractory hypoxia may require neuromuscular blockade, recruitment maneuvers, prone ventilation, or venovenous extracorporeal membrane oxygenation (ECMO).
Key Points
Hypoxia can be caused by disorders of ventilation and/or oxygenation and is usually first recognized by pulse oximetry.
Patients should have a chest x-ray, ECG, and arterial blood gas measurements (to confirm hypoxia and evaluate adequacy of ventilation); if diagnosis remains unclear, consider testing for pulmonary embolus.
Give oxygen as needed to maintain PaO2 between 60 and 80 mm Hg (ie, 92 to 100% saturation) and treat the cause.
