Oxygen saturation - definition, measurement, the norm, and critical value

In medicine, oxygen saturation is a term used to describe the ratio of hemoglobin bound with molecular oxygen in the blood to the total amount of hemoglobin in the body. Every human needs an adequate amount of oxygen in the blood to live. Oxygen saturation is one of the five vital parameters, next to heart rate, body temperature, blood pressure, and respiratory rate.

What is oxygen saturation? 

Oxygen is the most needed element for humans and all living organisms on Earth. All human organs require oxygen to survive and function properly. The brain, heart, and kidneys are most vulnerable to damage caused by insufficient oxygen. Oxygen in the body is transported by hemoglobin, a protein contained in red blood cells that gives blood its red color. Hemoglobin contains four heme groups, in which an iron atom is capable of binding to oxygen. Blood saturation determines the ratio of hemoglobin bound to oxygen at any given moment to deoxygenated hemoglobin. Oxygen enters the body during breathing. When we inhale, we introduce oxygen contained in the air into the respiratory system, which passes it on to the lungs. Then, oxygen is bound to hemoglobin in the alveoli. This combination is called oxyhemoglobin. Oxyhemoglobin along with the blood in the arteries is delivered to tissues throughout the body. A small amount of oxygen is also carried unbound through the blood. Oxygenated blood is delivered to the tissues continuously thanks to the regular functioning of the heart.  

In a healthy patient, one gram of hemoglobin can bind 1.34 ml of oxygen. This means that 100 ml of blood carries about 20 ml of oxygen. The heart pumps 5,000 ml of blood per minute, which means that the tissues and organs receive approximately 1,000 ml of oxygen per minute. Tissues need about 250 ml of oxygen per minute to survive, which means that the amount of oxygen stored in the tissues is very low. Without adequate oxygen supply, tissues can maintain their physiological functions for about 3 minutes. After this time, permanent damage may appear.  

Low oxygen saturation can be caused by various organ defects and respiratory diseases, such as: 

asthma,

chronic obstructive pulmonary disease,

viral or bacterial infections of the respiratory system,

mixing of oxygenated and non-oxygenated blood,

heart defects,

pulmonary leaks

vascular malformations.  

Abnormally low oxygen saturation can also be caused by gas poisoning, for example with nitrogen compounds or carbon monoxide. The structure of blood cells also affects their saturation, as for example, the sickle shape of the blood cells occurring in sickle cell disease does not allow the effective transfer of oxygen to the tissues. Geographic conditions, such as living in places with reduced atmospheric pressure (located above 2,500 meters above sea level), also affect the level of saturation. 

The measurement of oxygen saturation is used in an outpatient setting as one of the vital signs. Hypoxia is often not visible at first glance - the patient shows no symptoms of it even though they are in life-threatening danger.  

The indications for measuring the level of oxygen saturation in a hospital setting are the various situations in which a person may become hypoxic. These include: 

planned or unplanned surgical procedures,

general anesthesia,

surgery performed on the oral or nasal cavity,

endoscopic examination of the digestive system,

cardiac catheterization,

childbirth and antenatal states,

pharmacological coma. 

High mountain climbers also measure their oxygen saturation, as in mountainous areas it is far easier for the body to become hypoxic. Oxygen saturation data is also often used by pilots, parachutists, and athletes.  

Measuring oxygen saturation

Blood saturation is measured using pulse oximetry. The measurement is carried out using a device called a pulse oximeter. It measures the percentage of hemoglobin bound to oxygen in the blood. The principle of operation of the pulse oximeter is based on the absorption of radiation by red cells of red and infrared light. The erythrocyte that carries oxygen is of a different color than an erythrocyte lacking oxygen. The result is calibrated in the device and given as a percentage. Blood saturation is abbreviated as SpO2, and the result is given in writing, for example: "SpO2 98%". Saturation with a pulse oximeter in adults is usually measured on the fingertip or ear lobe. In newborns, oxygen saturation is measured using a pulse oximeter on the wrist or foot.  

Normal blood oxygenation levels in adults 

Life-threatening oxygen saturation (critical value) 

Saturation values lower than 70% are considered life-threatening. Persistently low blood oxygenation values can lead to severe brain damage, impaired perception of stimuli, and then death.  

Low oxygen saturation value 

Any saturation value below 95% indicates a risk of hypoxia. Hypoxia is a condition in which the blood does not deliver enough oxygen to the tissues. This condition may apply to the entire body or to a single organ. The symptoms of hypoxia vary depending on the rate of its development, the degree of oxygen resilience, and the body's ability to compensate for oxygen deficiency. The main symptoms of hypoxia are: 

dyspnea,

breathing difficulties,

Cheyne-Stokes respiration, i.e. increasing and then decreasing breath frequency,

motor coordination disorders,

slurred speech,

blurred vision

tachycardia,

increase in the frequency and depth of breathing,

loss of consciousness.  

One of the symptoms is also cyanosis - a disease that occurs in states of insufficient oxygen supply. Blood that is low in oxygen is darker in color than oxygenated blood. Cyanosis takes its name from the characteristic cyan color of tissues, skin, and mucous membranes. The discoloration appears mainly on the lips, tongue, and in the mouth when oxygen saturation drops below 85%.  

Correct hemoglobin oxygen saturation 

When hemoglobin is saturated with oxygen at every possible binding site, the saturation equals 100%. In the human body, only a small amount of the hemoglobin from the blood does not pass through the lungs to be oxygenated.  

Oxygen saturation – normal levels 

The blood saturation in the arteries of a healthy adult who breathes air at sea level should equal 95-99%. The main factor influencing this value is the height above sea level a person finds themselves on. The oxygen contained in the air has a lower pressure at altitudes high above sea level, which makes it more difficult for the human body to obtain oxygen. Blood saturation in the veins usually equals about 75%.  

Oxygen saturation in a smoker 

Cigarettes are nowadays one of the most widely used stimulants. Cigarette smoke affects both the smoker and those in their surroundings, i.e. passive smokers. Cigarette smoke contains substances that negatively affect the human body: nicotine, carbon monoxide, and hydrogen cyanide. After reaching the lower respiratory tract, nicotine is absorbed by the alveoli into the bloodstream. Depending on the time of inhalation of the smoke, the depth of breathing, and the pH of the smoke, nicotine concentration in the blood ranges from 31 ng/ml to 41 ng/ml.  

Smoking can cause harm to many organs and tissues in the human body. It can also lead to a number of diseases, such as: 

atherosclerosis,

increased risk of heart attack,

stroke,

emphysema,

increased risk of respiratory infections,

cancers of the lung, throat, esophagus, mouth, pancreas, stomach, and others.  

Cigarette smoking is the main cause of death of 20-40% of people in developed countries and 40-60% in developing countries.  

A study was conducted which analyzed the effect of smoking on the level of oxygen saturation. 406 people took part in the study - both smokers who had not been diagnosed with any diseases (189 people) and non-smokers (217). People participating in the study measured the level of saturation with a pulse oximeter between 4:00 PM and 6:00 PM. Saturation was measured on the finger, and the study's participants were not allowed to have fake fingernails or nails painted with polish or henna. 

The study showed that the oxygen saturation among smokers was 97.28% ± 1.19, and that of non-smokers it was 97.92% ± 1.09. According to the results, the level of saturation in smokers is not much lower than in non-smokers. Smoking cigarettes has little effect on the oxygen saturation of the hemoglobin. Nevertheless, smoking leads to serious diseases that significantly reduce the quality of life of smokers and may cause death.  

The oxygen saturation value in newborns and children 

Saturation is one of the parameters that should be tested in the first 24 hours of a child's life. Saturation is measured again between the 2nd and 24th day after birth. The measurement is made with a pulse oximeter on the child's right lower limb. The measurement takes about 2-3 minutes and is completely painless. Oxygen saturation in a healthy child should exceed 95%. The oxygen saturation result allows you to determine whether the newborn child suffers from any serious concealed heart defects, such as having a single-chamber heart or a common atrioventricular canal. Any oxygen saturation level lower than 95% means that the newborn is hypoxic. It is a very serious condition that can lead to permanent damage to the central nervous system or the development of cerebral palsy. Neonatal saturation value below 95% is an indication for further tests, for example, echocardiography. Usually, when low saturation is found on the baby's leg, a re-measurement is made on the right wrist. 

Higher saturation in the upper limb may indicate such diseases as: 

left heart hypoplasia,

critical aortic stenosis,

mitral valve atresia,

disruption of the aortic arch,

critical aortic coarctation.  

Low saturation in both the upper and lower limbs may indicate such conditions as: 

arthritis of the tricuspid valve or pulmonary artery,

extreme tetralogy of Fallot,

Ebstein’s anomaly.  

To prevent serious and irreversible damage, the baby is given additional oxygen and connected to a breathing apparatus.