Human life relies on respiration: the intake of oxygen and the release of carbon dioxide. Respiration provides a constant flow of oxygen molecules to our brain, organs and tissues while providing a way to remove the waste CO2 molecules that are created in the cells.
Without respiration, our bodies will shut down within seconds. For this reason, regardless of the procedure, wound, disease or illness, one of the first priorities of a physician is to insure continuous respiration.
In order to measure respiration, physicians and health care workers use several oxygen and carbon dioxide gas sensors. These gases are measured in 2 areas: the breath and the blood.
Breath Gas Monitoring and Analysis
Measuring the gases in a patient’s breath gives an indication of the effectiveness of the lungs to take in oxygen and expel CO2. Because breath gas analysis is non-evasive, it can be done safely and provides instant feedback about the effectiveness of respiration. For this reason, breath gas monitoring is one of the first procedures performed on a patient in an ambulance or hospital to verify their overall health in an emergency situation.
Fresh air contains approximately 21% oxygen by volume. During respiration, only a portion of the oxygen is used by the lungs. Exhaled breath still contains 13% to 16% oxygen, with the difference made up of an increase in water vapor and carbon dioxide.
To measure the change in oxygen during breathing oxygen sensors capable of taking a reading every second are used. For oxygen control, ventilator manufacturer’s typically use electro galvanic oxygen sensors capable of measuring up to 100% oxygen enriched air from compressed oxygen tanks.
Capnography is the monitoring of CO2 levels expelled from the lungs. Inhaled ambient air has about 400 parts per million (0.04%) by volume of CO2. Exhaled breath contains about 4% to 5% CO2.
A capnograph is a device that displays the CO2 levels in the expelled air in real-time, either as an End Tidal CO2 (ETCO2) graph or as a numerical readout. The advantage of capnography is that it is an indirect monitor of the CO2 levels in blood. Conversely, a difference between the CO2 levels in expired air and the blood is an indicator of lung disease.
To measure CO2 levels in expelled air, a capnograph relies on fast carbon dioxide sensors that can measure minute CO2 changes in real time.
Volatile Organic Compounds
In ancient times, physicians would smell a patient’s breath to diagnose disease. It was believed that breath with a sweet odor indicated diabetes, while a fish-like smell indicated problems with the kidneys.
In the last century, scientist have identified 300 different VOCs or chemicals in an exhaled breath. While still in the research phase, the study of volatile organic compounds through the use of VOC sensors is a promising new field of medicine.
Blood Gas Monitoring
While measuring CO2 and oxygen in the breath is instantaneous, arterial blood gas (ABGs) analysis give a more accurate view of the effectiveness of respiration. ABG analysis uses blood drawn from a patient and inserted into a blood gas analyzer. This device contains sensors that give a readout of the levels of oxygen, carbon dioxide and the pH (alkalinity or acidity) measured in the blood sample.
Oxygen is measured 3 ways, as partial pressure of oxygen (PaO2), as oxygen saturation (O2 Sat) and for oxygen content (O2CT) in the blood. Each of these measurements are derived from the PaO2 measured via a Clark electrode type of oxygen sensor in the blood gas analyzer.
Carbon Dioxide (PCO2)
is measured using a Severinghaus electrode, a modified glass electrode. This type of sensor does not measure the CO2 level directly, instead it measured a change in pH from a reaction between CO2 and sodium bicarbonate which is proportional to the CO2 level.
A deviation from normal pH (7.35 – 7.45) is an important measurement physicians use to determine the health of a patient. pH is measured in a blood gas analyzer via a glass electrode suspended in the blood sample. The blood sample acts as a conducting electrolyte, where the electrical change is proportional to the pH difference.
Gas Sensors in Hospitals
Oxygen and CO2 sensors are used throughout hospitals and medical facilities in many applications beyond patient care:
CO2 Sensors in HVAC systems. Clean air is critical in a hospital environment. Because CO2 is a good indicator of overall ventilation, wall-mounted CO2 sensors can be used to determine indoor air quality.
Oxygen Sensors in Oxygen Generators. Most large hospitals do not purchase oxygen in cylinders, but instead have an oxygen generator in-house to provide their own oxygen. 100% oxygen sensors like this SST 100% Oxygen Sensor are used to control industrial oxygen generators.
Oxygen Sensors in Safety Alarms. Because pure oxygen is piped throughout a hospital, Oxygen Enrichment Safety Alarms use an oxygen sensor to monitor room air. An oxygen level above 21% indicates a leak in the oxygen gas lines which is a potential fire hazard.
While not technically gas sensors, the next generation of non-invasive medical sensor technology features wearable bio-sensors. Technologies like Clark electrode oxygen sensors for portable glucose monitoring, fingertip infrared pulse oximeters for measuring blood oxygen levels and photoelectric pulse rate sensors built into watches are commonplace.
However, next generation wearable sensors for health monitoring can sample fluids such as sweat, tears and saliva for the electrochemical detection of biomarkers. They measure these fluids using potentiometric ion selective electrodes and amperometric enzymatic sensors. The goal is to noninvasively and continuously screen body fluids for the diagnosis and management of diseases as well as monitoring fitness.
The overall trend in sensors over the last several years is a reduction in size. The next generation of gas sensors will be even smaller.
For example, micro oxygen sensors incorporating an LED light source, a custom integrated circuit with a light detector are being developed that can be inserted into the body. These sensors detect how much oxygen an organ is getting from inside the body and can give doctors an early warning of danger for transplanted organs.
Another example is temperature sensors that are small enough to be injected into the body via a hypodermic needle.
As new sensors like these are combined with AI and wireless technology, in the future our health could be remotely and continuously monitored, with our physician being notified should a breathing problem occur.