Introduction

Oxygen is crucial for our survival, and our bodies need a balance between the oxygen we consume and the oxygen we receive. Two main systems, the lungs and the heart, help deliver oxygen and maintain a balance. If these systems do not work properly, it can lead to a lack of oxygen in the blood, called hypoxemia.

The most common reason for hypoxemia is when air doesn't flow well to parts of the lungs or when blood does not pick up enough oxygen.¹ This common clinical occurrence can cause tissue oxygen deficiency and needs quick diagnosis to guide treatment. Hypoxemia means the level of oxygen in the blood is lower than it should be (normal is 80–100mmHg).²

Hypoxemia, defined as low arterial oxygen levels, can result from a variety of medical conditions affecting the respiratory, cardiovascular, neurological, haematological, metabolic, and environmental systems. Respiratory disorders such as pneumonia, chronic obstructive pulmonary disease (COPD), and pulmonary embolism can reduce the oxygen intake or its exchange in the lungs. Cardiovascular illnesses such as congestive heart failure (CHF) and cardiogenic shock can reduce the oxygen flow to tissues.

Neurological problems, such as stroke or traumatic brain injury, can impair the respiratory drive or its control. Haematological disorders, such as anaemia or carbon monoxide poisoning, can reduce the oxygen-carrying capacity of blood. Metabolic abnormalities such as diabetic ketoacidosis (DKA) and sepsis can cause tissue hypoxia.

Environmental factors including high altitude exposure and smoke inhalation can also cause hypoxemia. When someone experiences acute hypoxemia, their heart tends to beat faster, which raises their cardiac output. This happens because the heart rate increases while the amount of blood pumped with each beat (stroke volume) stays about the same. With the heart pumping faster, more blood returns to the heart from the veins (venous return).³

A comprehensive diagnostic approach is essential to identify the underlying cause of hypoxemia and guide appropriate management strategies tailored to the specific condition.⁴

Diagnosis

• Arterial blood gas (ABG) measurement: This test directly measures the oxygen levels in the blood, helping confirm hypoxemia
• Assessment of tissue oxygenation: Signs of tissue hypoxia, like cyanosis (blue skin), confusion, or rapid heart rate, can indicate low oxygen levels
• Identifying underlying causes: Understanding why hypoxemia occurs is crucial. It could result from conditions like pneumonia, lung diseases, heart problems, or altitude sickness

In a study, we found that among patients experiencing low oxygen levels, 51% had mild hypoxemia, 40% had moderate hypoxemia, and 9% had severe hypoxemia. The primary cause identified was pneumonia, accounting for 53% of cases. Interestingly, most patients had multiple factors contributing to their low oxygen levels.

Additionally, about 21% of patients met the criteria for acute respiratory distress syndrome (ARDS), with the prevalence increasing as the severity of hypoxemia increased. However, it's worth noting that less than half of severely hypoxemic patients actually had ARDS.⁵

The diagnosis for hypoxia in patients can include conditions like pneumothorax, pleural effusion, and pneumonia. Pneumothorax is a condition where air collects between the lung and chest wall. It can be detected when lung sliding is absent on ultrasound.

Pleural effusion, where fluid accumulates between the lung and chest wall, can also be identified with ultrasound by visualising hypoechoic spaces. Additionally, pneumonia, which causes inflammation in the lungs, can be indicated by attenuated lung sliding and other abnormalities in lung imaging. These conditions affect the ability of the lungs to exchange oxygen properly leading to hypoxia in patients.⁴

Lung sliding

Ultrasound imaging can reveal the movement of the pleural layers during normal breathing, known as lung sliding. Normally, these layers appear as a bright horizontal line on the ultrasound video, moving rhythmically with each breath. Lung sliding is absent in conditions like pneumothorax or pleural effusion, where the layers are separated.

In cases of inflammation like pneumonia or acute respiratory distress syndrome (ARDS), lung sliding may be reduced. M-mode imaging can further clarify lung sliding, showing a grainy pattern called the seashore sign, indicating normal pleural movement.⁴

A-line

A-lines appear on ultrasound when there's a lot of gas in the respiratory system below the pleural layer. Since gas doesn't let ultrasound pass through, most of the signals bounce back at the pleural line and create a repeating pattern. These patterns look like horizontal lines on the ultrasound image. A-lines typically show up when there's air in the lungs or if there's a pneumothorax (collapsed lung).

When A-lines are seen along with lung sliding, it indicates normal air-filled lungs. But if A-lines are present along with a specific pattern called a lung point, it suggests a pneumothorax. If lung sliding disappears after a procedure, it can also indicate a pneumothorax.⁴

Treatment

The treatment used for hypoxemia includes various modalities depending on the severity of the condition. For patients with mild-to-moderate hypoxemia, non-invasive methods like nasal cannula or face mask oxygen therapy are commonly used. In more severe cases, high-flow oxygen therapy or non-invasive ventilation (NIV) are used.

Additionally, adjunctive therapies aimed at improving oxygenation are administered, such as neuromuscular blockers, extracorporeal membrane oxygenation (ECMO), inhaled nitric oxide, and prone positioning. Patients on invasive mechanical ventilation receive lung-protective ventilation strategies, including low tidal volume ventilation and positive end-expiratory pressure (PEEP) titration. However, despite these interventions, some patients require invasive ventilation during their ICU stay. Overall, the choice of treatment depends on the severity of hypoxemia and the patient's response to different modalities.⁵

Oxygen therapy: This is a fundamental treatment for hypoxemia, involving the administration of supplemental oxygen to increase oxygen levels in the blood.⁷

Treatment of hypoxemia in obstructive sleep apnea

Many people with obstructive sleep apnea (OSA) experience drops in their oxygen levels during sleep, which can be harmful. Treatment usually focuses on fixing the apnea to prevent low oxygen levels. However, some patients can't use standard treatments like CPAP machines or surgery. In a study, 43 such patients were given oxygen at night instead.

Their symptoms improved, and they felt less sleepy during the day. Tests showed their oxygen levels improved while using oxygen. So, oxygen therapy could be a helpful option for OSA patients who cannot use CPAP or have surgery.⁶

Treatment strategies for hypoxia during one-lung ventilation (OLV)

The treatments for hypoxia during one-lung ventilation (OLV) include immediate and simultaneous actions to relieve hypoxemia and identify and address its underlying cause.

1. Increasing fio2 (Fraction of Inspired Oxygen): This is an effective immediate treatment for hypoxemia during OLV. Elevating the Fio2 helps increase oxygenation levels, particularly when the shunt fraction is below 40%. It's essential to monitor oxygen saturation closely and adjust the Fio2 accordingly. However, maintaining Fio2 at 1.0 throughout OLV may lead to collapse of the lung and prevent the use of nitrous oxide
2. Reexpansion of the nonventilated lung: If increasing Fio2 does not improve oxygenation, informing the surgeon and expanding the nonventilated lung becomes necessary. This can be achieved through methods like applying Continuous Positive Airway Pressure (CPAP). CPAP helps expand the nonventilated lung and maintain its expansion, thereby improving oxygenation. Repeated hand ventilation or high-frequency jet ventilation of the non-dependent lung are other techniques used to reexpand the nonventilated lung
3. Identifying and addressing the underlying cause: Alongside immediate treatment, it's crucial to identify and address the root cause of hypoxemia. A fiberoptic bronchoscopy is a valuable tool for correcting tube position and clearing secretions or blood in the ventilated lung. This helps correct the cause of hypoxemia during OLV and ensures effective management⁷

These strategies aim to promptly alleviate hypoxemia during OLV while also addressing its underlying cause to ensure optimal patient care and safety.

Treatment of refractory hypoxemia in adult with acute respiratory distress syndrome

1. Lung-protective ventilation strategies: This may involve the use of low tidal volumes, higher positive end-expiratory pressure (PEEP), and prone positioning to optimise lung recruitment and improve oxygenation
2. Recruitment manoeuvres: Techniques such as sustained inflation or intermittent increases in airway pressure may be employed to re-expand collapsed alveoli and improve ventilation-perfusion matching
3. Neuromuscular blockade: Temporary paralysis with neuromuscular blocking agents can facilitate lung-protective ventilation and improve oxygenation by reducing patient-ventilator asynchrony and preventing ventilator-induced lung injury
4. Inhaled pulmonary vasodilators: Agents such as inhaled nitric oxide (iNO) or prostacyclin may be used to selectively vasodilate the pulmonary vasculature, reducing pulmonary vascular resistance and improving oxygenation by optimising ventilation-perfusion matching
5. Extracorporeal membrane oxygenation (ECMO): In severe cases of refractory hypoxemia, ECMO can provide temporary support by oxygenating and removing carbon dioxide from the blood outside the body, allowing time for the lungs to heal
6. Prone positioning: Placing patients in the prone position can improve oxygenation by redistributing lung perfusion, promoting better ventilation of dependent lung regions, and reducing the ventilation-perfusion mismatch
7. High-frequency oscillatory ventilation (HFOV): HFOV delivers very rapid, small tidal volumes at high frequencies, potentially reducing ventilator-induced lung injury and improving oxygenation in select patients with refractory hypoxemia⁹

These treatments aim to address the underlying pathophysiology of refractory hypoxemia in ARDS and may be used alone or in combination depending on the individual patient's condition and response to therapy.

Management of hypoxemia

• Check blood gases to confirm low oxygen levels
• Look for signs of oxygen deprivation in tissues
• Identify why oxygen levels are low
• Start oxygen therapy based on the cause
• Check blood gases again to ensure oxygen levels improve
• Treat any underlying health issues¹⁰

Summary

Hypoxemia, or low blood oxygen levels, can result from various medical conditions affecting the lungs, heart, or other body systems. Diagnosis involves tests like arterial blood gas measurement and assessment of tissue oxygenation. Treatment options include oxygen therapy, non-invasive methods like nasal cannula, and more advanced interventions like lung-protective ventilation and extracorporeal membrane oxygenation (ECMO).

In patients with acute respiratory distress syndrome (ARDS), strategies such as neuromuscular blockade and prone positioning are used to improve oxygenation. During one-lung ventilation (OLV), increasing the fraction of inspired oxygen and re-expanding the nonventilated lung are key approaches. Overall, prompt diagnosis and appropriate management tailored to the underlying cause are essential for effectively treating hypoxemia and ensuring patient safety.