From the Guidelines
Distinguishing between types of hypoxemia is crucial for providing appropriate treatment and improving patient outcomes, as it allows clinicians to target the specific physiological problem rather than simply treating symptoms. The main types include hypoxemic hypoxia (low arterial oxygen), anemic hypoxia (reduced oxygen-carrying capacity), circulatory hypoxia (poor blood flow), and histotoxic hypoxia (cells unable to use oxygen) 1. Each type requires different interventions:
- Hypoxemic hypoxia may need supplemental oxygen or mechanical ventilation
- Anemic hypoxia might require blood transfusions or iron supplementation
- Circulatory hypoxia calls for improving cardiac output with fluids or vasopressors
- Histotoxic hypoxia requires addressing the underlying toxin or metabolic issue
According to the BTS guideline for oxygen use in adults in healthcare and emergency settings, hypoxaemia refers to an abnormally low PO2 in the blood, and the target SaO2 (and SpO2) should be ≥94% for most hypoxaemic patients to ensure that the actual oxygen level remains above 90% for most of the time with a 4% margin of safety 1. Recent observational data have shown a step-wise increase in mortality in hypoxaemic acute medical patients breathing air, with the lowest mortality (3.7%) found in patients with saturation ≥96% 1.
Proper identification of the type of hypoxemia allows clinicians to predict complications, determine prognosis, and guide preventive measures, ultimately leading to more efficient use of medical resources. For example, giving oxygen alone won't help a patient with carbon monoxide poisoning (histotoxic hypoxia) or severe anemia 1. Understanding the underlying mechanism also helps minimize unnecessary treatments and reduces potential complications. Mental functioning becomes impaired if the PaO2 falls rapidly to <6 kPa (45 mm Hg, SaO2 <80%) and consciousness is lost at <4 kPa (30 mm Hg, SaO2 <56%) in normal participants 1.
In clinical practice, it is essential to distinguish between types of hypoxemia to provide targeted treatment and improve patient outcomes, as the treatment approach varies significantly depending on the underlying cause of hypoxemia 1.
From the Research
Clinical Significance of Distinguishing Between Types of Hypoxemia
The clinical significance of distinguishing between types of hypoxemia is crucial for guiding management and treatment. According to 2, hypoxemia can be caused by five pathophysiological mechanisms: alveolar hypoventilation, ventilation/perfusion mismatches, diffusion disorders, true shunts, and a decrease in the partial pressure of inspired oxygen.
- Distinguishing between these mechanisms can help clinicians develop a targeted diagnostic approach and treatment plan.
- For example, 3 notes that disturbances of lung function can decrease oxygen partial pressure, oxygen saturation, and oxygen content, resulting in hypoxic hypoxemia.
- In contrast, carbon monoxide poisoning or methemoglobin formation can decrease oxygen saturation and oxygen content, but leave oxygen partial pressure normal, resulting in toxic hypoxemia.
- Anemia, on the other hand, can decrease oxygen content, but leave oxygen partial pressure and oxygen saturation normal, resulting in anemic hypoxemia.
Importance of Accurate Diagnosis and Management
Accurate diagnosis and management of hypoxemia are critical to prevent tissue hypoxia and organ failure. As noted in 4, inadequate effort to correct hypoxia can cause either disturbance of adaptation or undesirable tissue hyperoxia.
- Recent literature supports a rather restrictive strategy of oxygen therapy in critical care, highlighting the importance of precise control of arterial oxygenation.
- 5 proposes two novel strategies for managing hypoxemia: precise control of arterial oxygenation and permissive hypoxemia.
- These strategies aim to avoid the harms associated with inadvertent hyperoxia or hypoxia and minimize the possible harms caused by restoration of normoxemia while avoiding tissue hypoxia.
Regional Differences in Oxygen Saturation
Regional differences in oxygen saturation can also occur, particularly in patients undergoing venoarterial extracorporeal membrane oxygenation.
- As noted in 6, differential hypoxemia can cause clinical problems, particularly in peripheral femoro-femoral venoarterial extracorporeal membrane oxygenation.
- Accurate diagnosis and management of fulminant differential hypoxemia are crucial to prevent deprivation of oxygen to the upper body.
- Possible solutions include implantation of an additional jugular venous return cannula or drainage of venous blood from the superior vena cava.