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Oxygen Desaturation

(Hypoxia)

by Soumitra R. Eachempati, MD

ICU (and other) patients without respiratory disorders may develop hypoxia (O 2 saturation < 90%) during a hospital stay. Hypoxia in patients with known respiratory conditions is discussed under those disorders.

Etiology

Numerous disorders cause hypoxia (eg, dyspnea, respiratory failure—see Table: 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

Some Causes of Oxygen Desaturation

Mechanism

Examples

Disorders of ventilation

Decreased ventilatory drive

Decreased mental status (eg, caused by head injury, oversedation, sepsis, shock, or stroke)

Obstructed ventilation

Bronchospasm

Dislodgement of endotracheal tube

Mucus plugging of the airways or endotracheal tube

Severe pain in the chest, abdomen, or both

Rib fractures

Thoracic or abdominal surgery

Disorders of oxygenation

Pulmonary causes

Acute respiratory distress syndrome

Atelectasis, pneumonia, pneumothorax, pulmonary embolus, pulmonary contusion, aspiration pneumonitis

Nonpulmonary causes

Iatrogenic fluid overload

Heart failure (eg, due to exacerbation of underlying disease or to acute MI)

Evaluation

Total fluid volume given during the hospital stay and, in particular, the previous 24 h should be ascertained to identify volume overload. Drugs should be reviewed for sedative administration and dosage. In significant hypoxia (O 2 saturation < 85%), treatment begins simultaneously with evaluation.

History

Very sudden onset dyspnea and hypoxia suggest pulmonary embolus (PE) or pneumothorax (mainly in a patient on positive pressure ventilation). Fever, chills, and productive cough (or increased secretions) suggest pneumonia. A history of cardiopulmonary disease (eg, asthma, COPD, heart failure) may indicate an exacerbation of the disease. Symptoms and signs of MI may indicate acute valvular insufficiency, pulmonary edema, or cardiogenic shock. Unilateral extremity pain suggests deep venous thrombosis (DVT) and hence possible PE. Preceding major trauma or sepsis requiring significant resuscitation suggests acute respiratory distress syndrome. Preceding chest trauma suggests pulmonary contusion.


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 (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, or atelectasis)

  • ECG (to assess for arrhythmia or ischemia)

  • ABGs (to confirm hypoxia and evaluate adequacy of ventilation)

Bedside intensivist-performed echocardiography (see Hand-held echocardiography) may be used to assess for hemodynamically significant pericardial effusion or reduced global left ventricular function until formal echocardiography can be done. 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 (see Pulmonary Embolism (PE) : Diagnosis of Pulmonary Embolism) should be considered. Bronchoscopy may be done in intubated patients to rule out (and remove) a tracheobronchial plug.


Treatment

Identified causes are treated as discussed elsewhere in T he M anual . If hypoventilation persists, mechanical ventilation via noninvasive positive pressure ventilation or endotracheal intubation is necessary (see Respiratory Failure and Mechanical Ventilation). Persistent hypoxia requires supplemental O 2 .

O 2 therapy

The amount of O 2 given is guided by ABG or pulse oximetry to maintain Pa o 2 between 60 and 80 mm Hg (ie, 92 to 100% saturation) without causing O 2 toxicity. This level provides satisfactory tissue O 2 delivery; because the oxyhemoglobin dissociation curve is sigmoidal, increasing Pa o 2 to >80 mm Hg increases O 2 delivery very little and is not necessary. The lowest fractional inspired O 2 (F io 2 ) that provides an acceptable Pa o 2 should be provided. O 2 toxicity is both concentration- and time-dependent. Sustained elevations in F io 2 > 60% result in inflammatory changes, alveolar infiltration, and, eventually, pulmonary fibrosis. An F io 2 > 60% should be avoided unless necessary for survival. An F io 2 < 60% is well tolerated for long periods.

An F io 2 < 40% can be given via nasal cannula or simple face mask. A nasal cannula uses an O 2 flow of 1 to 6 L/min. Because 6 L/min is sufficient to fill the nasopharynx, higher flow rates are of no benefit. Simple face masks and nasal cannulas do not deliver a precise F io 2 because of inconsistent admixture of O 2 with room air from leakage and mouth breathing. However, Venturi-type masks can deliver very accurate O 2 concentrations.

An F io 2 > 40% requires use of an O 2 mask with a reservoir that is inflated by O 2 from the supply. In the typical nonrebreather mask, the patient inhales 100% O 2 from the reservoir, but during exhalation, a rubber flap valve diverts exhaled breath to the environment, preventing admixture of CO 2 and water vapor with the inspired O 2 . Nonetheless, because of leakage, such masks deliver an F io 2 of at most 80 to 90%.


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 ABGs (to confirm hypoxia and evaluate adequacy of ventilation); if diagnosis remains unclear, consider testing for pulmonary embolus.

  • Give O 2 as needed to maintain Pa o 2 between 60 and 80 mm Hg (ie, 92 to 100% saturation) and treat the cause.

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