May 22, 2024

Have you ever wondered how those little waves on a capnography reading represent the different phases of respiration? Well, today we’re going to talk about one particular phase – dead space.

What is dead space, you ask? Simply put, it’s the volume of air in the respiratory system that doesn’t participate in gas exchange. So why is it important to know about dead space?

Well, because it can affect your capnography readings! 

What is capnography?

Capnography is the waveform produced by the monitoring of CO2 concentrations in the respiratory gases. The shapes of the waveforms differ depending on whether it is a ‘normal’ or an ‘abnormal’ capnography reading.

Normal capnography readings have three distinct phases. The first phase (phase I) represents the CO2 that is in the alveolar air (that is, air that has been recently exhaled from the lungs). The second phase (phase II) represents the CO2 that is in the airway dead space (that is, air that has not been in contact with respiratory tissue). The third phase (phase III) represents the CO2 that is in the inspired air (that is, air that has not been exhaled from the lungs).

Abnormal capnography readings may only have two phases, or may have only one phase. If there are two phases, it indicates that there is some obstruction to airflow, such as from kinking of the endotracheal tube or from secretions in the trachea. If there is only one phase, it means that there is complete obstruction to airflow and no CO2 is being exhaled.

How does capnography work?

Capnography is a means of measuring the concentration of carbon dioxide in respiratory gases. It is generally used as a measure of ventilation, both during anaesthesia and in intensive care.

In order to understand how capnography works, it is first necessary to understand a little about how respiration works. When we inhale, air flows into our lungs and down into tiny air sacs called alveoli. The alveoli are where gas exchange takes place – that is, where oxygen from the inhaled air diffuses into the blood, and carbon dioxide diffuses out of the blood into the alveoli so that it can be exhaled.

The process of gas exchange is not 100% efficient, however, and there is always some residual carbon dioxide present in the exhaled air. This is known as ‘end-tidal carbon dioxide’ or ‘etCO2’. Capnography measures the concentration of etCO2 in exhaled respiratory gases.

Which phase of the capnography waveform represents dead space

There are three phases to the capnography waveform: inspiratory baseline, expiratory plateau andInspiratory upstroke. The inspiratory baseline represents dead space – that is, the volume of gas in the respiratory system which does not take part in gas exchange (i.e. it does not come into contact with the alveoli). The expiratory plateau represents alveolar ventilation – that is, the volume of gas which does take part in gas exchange (i.e. comes into contact with the alveoli). Finally, the inspiratory upstroke represents uptake of oxygen by the lungs (this can be seen as a positive deflection on the waveform).

What is dead space?

Dead space is the area of the lungs that is not involved in gas exchange. This can be due to anatomical factors, such as the size of the airways, or physiological factors, such as airway obstruction.

The implications of dead space on capnography readings are two-fold. First, dead space can affect the shape of the waveform. This is because the CO2 in inhaled air mixes with the CO2 in dead space, resulting in a lower concentration of CO2 in alveolar gas. This can cause the waveform to appear flattened or even reversed.

Second, dead space can affect the reading itself. This is because the CO2 sensor only measures CO2 concentration in exhaled gas, not in inhaled gas. Thus, a higher percentage of dead space will result in a lower reading.

How does dead space affect capnography readings?

Dead space is the volume of air in the respiratory system that is not involved in gas exchange. When you breathe in, some of the air goes into the lungs and some of it goes into the dead space. When you breathe out, the air in the dead space is exhaled without exchanging any gas.

The amount of dead space in your respiratory system can affect your capnography readings. If you have a lot of dead space, it will take longer for carbon dioxide to reach your lungs and be exhaled. This will delay the peak of the waveform and make it smaller. If you have a small amount of dead space, carbon dioxide will reach your lungs more quickly and the waveform will peak sooner and be taller.

Dead space can also affect the shape of the waveform. If you have a lot of dead space, it will take longer for carbon dioxide to reach your lungs and be exhaled. This will delay the peak of the waveform and make it smaller. If you have a small amount of deadspace, carbon dioxide will reach your lungs more quickly and the waveform will peak sooner and be taller.

What are the implications of dead space on capnography readings?

Dead space is the part of the respiratory system where gas exchange does not take place. This can result in inaccurate readings on a capnography waveform.

The implications of dead space on capnography readings depend on which phase of the waveform is affected. If dead space is present during the inspiratory phase, it will cause a decrease in the peak amplitude of the waveform. If dead space is present during the expiratory phase, it will cause an increase in the plateau duration.

How can we account for dead space when interpreting capnography readings?

While capnography is a useful tool for monitoring patients during anesthesia, it is important to understand the limitations of the technique. One of the main limitations is that it cannot account for dead space.

Dead space is the portion of the respiratory system that is not perfused with fresh blood. This can be due to anatomical factors (e.g., respiratory bronchioles) or functional factors (e.g., alveolar hypoventilation). Because capnography only measures gas exhaled from the lungs, it cannot account for gas in the dead space.

This can lead to misinterpretation of capnography readings, as the readings will reflect only partial ventilation of the lungs. For example, if a patient has a high degree of dead space, their capnography reading will be artificially low (reflecting only exhaled gas from the well-ventilated portions of the lungs). Conversely, if a patient has a low degree of dead space, their capnography reading will be artificially high (reflecting all exhaled gas, including gas from the dead space).

It is important to keep this limitation in mind when interpreting capnography readings. If possible, clinicians should try to estimate the degree of dead space in a patient before relying on capnography readings alone.

What are the clinical implications of dead space on capnography readings?

Dead space is the portion of the respiratory system that is not involved in gas exchange (i.e. the alveoli). When a patient is ventilated, some of the air that is delivered to the lungs does not reach the alveoli and therefore does not participate in gas exchange. This ‘dead space air’ mixes with exhaled air in the conducting airways and is exhaled.

The implications of dead space on capnography readings are two-fold. Firstly, an increase in dead space results in a decrease in the amount of oxygen that is available for gas exchange. This can lead to hypoxia (insufficient oxygenation of blood).Secondly, an increase in dead space also results in an increase in carbon dioxide levels in the exhaled air. This can lead to hypercapnia (excessive carbon dioxide levels in blood).

What are the potential implications of dead space on capnography readings in the future?

The potential implications of dead space on capnography readings are two-fold. First, if dead space is not taken into account, it could lead to inaccurate readings. Second, if dead space is not taken into account, it could lead to under-ventilation and/or hypoventilation.