Blood Oxygen Saturation

Rescue worker measures oxygen saturation of patient

Oxygen saturation is a key parameter for EMS providers to correctly assess the patient’s condition. Inhaled oxygen is transported to the body’s cells via the blood. Too low an oxygen saturation in the blood can have life-threatening consequences in extreme cases. Measuring oxygen saturation provides an indication of how well the blood is being supplied with vital oxygen.

Oxygen saturation in the blood is not only measured in emergencies; it is also indicated, among other scenarios, for monitoring respiration, for the diagnosis of various symptoms, and for monitoring the progression of certain diseases.

MEDUCORE Standard² from WEINMANN enables quick, user-friendly measurement of oxygen saturation, anytime, anywhere – whether in hospital, by EMS field providers, or in military medical corps services.

What does blood oxygen saturation (SaO2) mean?

Oxygen saturation describes the oxygen content in the blood. It indicates what percentage of the hemoglobin in the blood is loaded with oxygen molecules. Normally, this is between 95% and 99% in arterial blood.

Hemoglobin is a protein found in red blood cells (erythrocytes). During gas exchange in the alveoli, the hemoglobin in the erythrocytes absorbs the oxygen molecules from the air we breathe. This oxyhemoglobin transports oxygen into the tissues via the pulmonary vessels and bloodstream, and releases it into the cells. The discharged deoxyhemoglobin can then take up carbon dioxide molecules, and transport them via the bloodstream back to the alveoli, where the CO2 is added to the exhaled air.

Based on the measured oxygen saturation in the blood, the efficacy of oxygen transport and the functionality of the lungs can be determined.

Types of blood oxygen saturation 

A distinction is made between the following types of oxygen saturation:

  • SO2 - Oxygen saturation in general
  • SpO2 - Oxygen saturation measured by pulse oximetry
  • SaO2 - Arterial oxygen saturation
  • SvO2 - Venous oxygen saturation
  • SzvO2 - Central venous oxygen saturation
  • SO2 - Mixed venous oxygen saturation

This distinction is important, because oxygen saturation varies depending on the section of the cardiovascular system and the method used to measure it.

For example, arterial blood is responsible for transporting oxygen from the lungs to the tissues and organs, while venous blood transports carbon dioxide from the tissues and organs back to the lungs. Consequently, arterial oxygen saturation is normally significantly higher than venous oxygen saturation.

What is the partial pressure of oxygen (paO2)?

The oxygen partial pressure is determined by blood gas analysis. It indicates the amount of oxygen dissolved in the arterial blood. In the normal range, it is between 65 mmHg and 100 mmHg, depending on age and gender. The following rule of thumb applies:

♂ 100–0.33 × age ± 10 in mmHg

♀ 98–0.32 × age ± 10 in mmHg

How does oxygen saturation correlate with partial pressure of oxygen? 

The correlation between the partial pressure of oxygen and oxygen saturation is described by the oxygen binding curve. The higher the partial pressure of oxygen in the blood, the higher the blood oxygen saturation. This correlation is non-linear, because the oxygen affinity of hemoglobin depends on the number of oxygen molecules already bound. The oxygen binding curve shows an oblique s-shaped progression.

How is blood oxygen saturation measured?

Blood oxygen saturation can be determined invasively by blood gas analysis, or non-invasively by pulse oximetry.

1. Blood gas analysis

Blood gas analysis (BGA) is an invasive method for assessing the oxygenation capacity of the lungs or the circulatory status and acid-base balance in the blood. BGA measures the paO2, pCO2 and sO2 values of an arterial or venous blood sample.

2. Pulse oximetry

Pulse oximetry is a non-invasive procedure for determining SpO2 levels and pulse rate. It is performed using a special kind of spectrophotometer known as a pulse oximeter.

In adults, the skin on a finger or earlobe – in newborns mostly on a heel – is transilluminated with a specific wavelength of light. Since hemoglobin absorbs more or less light depending on how heavily it is loaded with O2 molecules, the proportion of oxyhemoglobin in the total hemoglobin of the blood can be determined on the basis of the absorbed light.


In emergency medicine, measurement of blood oxygen saturation is a basic parameter for quickly checking the oxygen supply and assessing the patient’s circulatory status. It provides indications as to metabolic activity, oxygen uptake and blood flow to the tissue, and allows conclusions to be drawn about cardiac output.

SpO2 measurement by pulse oximetry is also used to monitor ventilation or oxygen therapy, and for monitoring respiration during anesthesia or in the intensive care unit.

Blood gas analysis is also performed in case of the following indications:

  • Early detection of cardiovascular or pulmonary diseases
  • Gastrointestinal diseases
  • Hemoglobin determination
  • Hyper-/hypokalemia
  • Hyperventilation
  • Monitoring pH value
  • Kidney failure
  • Pre-operative diagnostics
  • Sepsis, shock, circulatory insufficiency
  • Monitoring the child at birth
  • Suspected hypercapnia
  • Monitoring the progression of metabolic disorders
  • Monitoring the progression of chronic lung diseases

Hypoxemia/hypoxia: low oxygen saturation

If the oxygen saturation in the blood is too low, hypoxemia or hypoxia is present. Hypoxemia refers to a reduced oxygen content in the arterial blood; hypoxia to a lack of oxygen in a tissue or in the entire organism. 

Mild hypoxemia


approx. 80 mmHg

Moderate hypoxemia


approx. 60 mmHg

Severe hypoxemia

< 85%

< 50 mmHg

Symptoms of hypoxemia

Too low an oxygen saturation in the blood is usually manifested by the following symptoms:

  • Shortness of breath
  • Dizziness
  • Chest pain
  • High pulse, tachycardia, increased blood pressure
  • Anxiety, restlessness

A special form of hypoxemia – so-called silent hypoxemia or happy hypoxemia – has recently occurred more frequently with COVID-19 infections. In this, patients exhibit neither shortness of breath nor accelerated breathing, despite severe hypoxemia with oxygen saturation values down to below 70%. Although the typical symptoms of such low oxygen saturation levels are absent, decompensation of the lungs progresses rapidly with such severe hypoxemia, and is life-threatening.

Causes of hypoxemia

Causes of low blood oxygen saturation include:

  • Impaired breathing, such as due to head injuries, sleep apnea/snoring, or stroke
  • Pulmonary diseases such as pulmonary embolism, pneumonia, asthma, or chronic obstructive pulmonary disease (COPD)
  • Blood disorders such as anemia or blood formation disorders
  • Cardiovascular diseases such as heart failure or myocardial infarction
  • Certain environmental factors such as extreme altitude
  • Poisoning, such as from medications, drugs, or carbon monoxide

What to do when blood oxygen saturation is low 

If the oxygen saturation in the blood is too low, additional oxygen can be supplied to the lungs by oxygen therapy.

Pulse oximetry or blood gas analysis is used to determine oxygen saturation, and the amount of supplemental oxygen required. The required oxygen is then supplied to the lungs, usually via a nasal cannula.

Oxygen therapy can be used in acute therapy as well as for chronic care.

As part of chronic care therapy, 3 types of system are usually used: Oxygen concentrators, compressed gas systems, and liquid oxygen systems. Compressed gas and liquid oxygen systems require a tank of oxygen in a liquid or gaseous compressed state, respectively. Oxygen concentrators do not require an oxygen tank. They merely separate oxygen from nitrogen, thereby increasing the oxygen content in the inhaled air.

Increased oxygen saturation and its effect on the body

Increased oxygen saturation can occur, for example, during hyperventilation, or when oxygen therapy is administered even though the blood is already sufficiently oxygenated.

Excessive oxygen saturation in the blood can cause damage to the lungs as well as the cardiovascular and nervous systems. It also increases the risk of lung failure, heart attacks, cardiac arrhythmias, and organ failure. Even at an initial SpO2 value of 94-96%, additional oxygen in the blood can have harmful consequences.1

That is why the guidelines recommend reducing the oxygen concentration after ROSC when performing resuscitation.

It is therefore strongly advisable to check the actual oxygen saturation in the patient’s blood before administering oxygen.

Oxygen saturation with WEINMANN MEDUCORE Standard² at a glance

In an emergency, rapid and reliable measurement of blood oxygen saturation is essential for assessing the patient’s condition. MEDUCORE Standard² from WEINMANN enables user-friendly monitoring of oxygen saturation by means of SpO2 measurement.

MEDUCORE Standard² is robust yet lightweight and handy, which allows it to be used in hospitals as well as by EMS providers, in air rescue services, and in military medical corps services. The compact monitor/defibrillator has all the necessary functions for pre-hospital and in-hospital patient monitoring and advanced diagnostics.

WEINMANN has been developing reliable, intuitive emergency medical equipment for more than 45 years. All WEINMANN devices are perfectly mutually compatible, and can be combined on specific portable units.

MEDUCORE Standard² can be installed as a stand-alone device on the LIFE-BASE portable unit, and can be used in combination with an EMS and transport ventilator. Installing both devices on a LIFE-BASE perfectly combines monitoring, defibrillation, and ventilation.

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1Derek K Chu, MD et al.: Mortality and morbidity in acutely ill adults treated with liberal versus conservative oxygen therapy (IOTA): a systematic review and meta-analysis. The Lancet (April 2018) Vol. 391, Issue 10131: 1693-1705.