Key Considerations in Respiratory Measurements & Instrumentation Involved

Airflow

The airflow in the respiratory system is normally laminar. However, during heavy breathing or when there is an obstruction, the airflow may become turbulent. The airflow is laminar unless the Reynolds number of the flow becomes 50,000 or more. When the Reynolds number is small, the viscous force in the flow dominates over the inertial force, resulting in the flow being laminar. When the Reynolds number is less than one, then the inertial force can be neglected and viscous force totally controls the airflow. Low Reynolds number is the characteristics of airflow in alveolar passages of diameter less than a few hundred microns.

The Lung Volume and Capacities

The lung volume and capacities are determined to assess the state or conditions of the breathing mechanism. There are certain parameters related to breathing mechanism which are to be defined and understood as they used in determining the lung volume and capacities.

These important lung parameters or capacities are:

Tidal Volume (TV)

It is the volume of gas inspired or expired during each respiration cycle. It is the resting tidal volume between the inspiratory and expiratory level. The tidal volume is about 500 ml for a normal adult.

Inspiratory Reserve Volume (IRV)

It is the extra volume that can be inspired above the tidal volume. The normal value of IRV is about 3000 ml.

Expiratory Reserve Volume (ERV)

It is the extra volume that can be expired by a forceful expiration at the end of the tidal volume. The normal value is about 1100 ml.

Residual Volume (RV)

It is the volume of air, remaining in the lungs despite forceful expiration. The normal value is about 1200 ml.

Inspiratory Capacity (IC)

It is the maximum volume of air that can be inspired after reaching the end of the expiratory level. The normal value is about 3500 ml.

IC = TV + IRV

Functional Residual Capacity (FRC)

It is the volume of air remaining in the lungs at the end of expiratory level.

FRC = RV + ERV

OR

FRC = TLC – IC

FRC can be regarded as the baseline from which other volumes and capacities are specified. The normal value of FRC is about 2300 ml.

Vital Capacity (VC)

It is the maximum volume of gas that can be expelled from the lungs by forceful expiration after maximum inspiration. It is actually the difference in volume of the maximum inspiration and the residual volume.

VC = IRV + TV + ERV

The normal value is about 4600 ml

Total Lung Capacity (TLC)

It is the amount of gas contained in the lungs at the end of maximum inspiration.

TLC = TV + IRV + ERV + RV

The normal value is about 5800 ml

Respiratory minute volume

It is the amount of air inspired during one minute while resting. It can be obtained by multiplying the tidal volume (TV) by the number of respiratory cycles per minute.

Terminology for Respiratory Measurements

Compliance

The volume increase of lung per unit increase in lung pressure is called compliance. It is expressed as litres per cm H₂O. The compliance of the normal lungs is 0.22 litres per 1 cm of H₂O .

Compliance depends upon the size of the lungs hence a child has a smaller compliance than an adult.

Compliance Work

The work required to expand the lungs against elastic forces.

Airway Resistance

It is the resistance of the air passage and expressed as the ratio of pressure to flow i.e. cm of H₂O per litres/sec.

Hypoventilation

It is insufficient ventilation to maintain normal partial pressure of carbon dioxide.

Hyperventilation

It is an abnormally prolonged, rapid or deep breathing which produces the condition of over breathing.

Dyspnoea

It is the shortness of breath or distress in breathing usually associated with serious diseases of lungs.

Related: How a Ventilator is used to deliver oxygen to a patient with a breathing difficulty

Hypercapnia

It is the excess amount of carbon dioxide in the lung system which results from inadequate ventilation.

Hypoxia

It is the shortage of oxygen due to inadequate ventilation.

How a Spirometer is used in the Respiration Measurement

Spirometer is the commonly used instrument for respiratory volume measurements. It is possible to determine all lung volumes and capacities by measuring the amount of gas inspired or expired under a set of conditions or during a specified time interval using a spirometer.

Measurements like Vital capacities and Forced expiratory volumes can be performed on a spirometer. However a spirometer (on its own) cannot measure volumes and capacities which require the measurements of the gas that cannot be expelled from the lungs under any condition. Such measurements include the measuring of:

• Residual volume
• Functional residual capacity
• Total lung capacity

A Spirometer consists of a movable bell inverted over a chamber of water as shown in the diagram below:

The bell contains air above the waterline which is used for breathing. The inverted bell is counter-balanced by a weight to ensure that the air inside the bell remains at atmospheric pressure. The amount of air inside the inverted bell above the waterline is proportional to the height of the bell above the waterline. As the patient breathes into the tube connected to air under the inverted bell through soda ash canister, the bell moves down with inspiration and it moves up with expiration. The height of the bell above the waterline can be directly calibrated to the volume of the air breathed in and out. A pen that can write on a drum (called kymograph) can be attached to the counter balancing mechanism to trace the breathing pattern of the patient.

How a Gas Analyzer is used with a Spirometer to measure: Residual Volume (RV), Functional Residual Capacity (FRC) and Total Lung Capacity (TLC)

We have 2 techniques that are used to measure RV, FRC and TLC namely:

• Closed circuit
• Nitrogen washed out technique

The Closed Circuit Technique

This technique involves breathing from a Spirometer which has a known volume of air with a fixed concentration of helium gas (Known as marker gas) under the inverted bell. The patient is made to breathe this air for a considerable period of time so that a complete mixing of gas in the spirometer and the lungs can be assured. Thereafter, the concentration of helium gas in the spirometer is ascertained with the help of a gas analyzer. The concentration of helium gas in the spirometer is a measure of the residual volume (RV).

The Nitrogen Washed Out Technique

This method involves the inspiration of pure oxygen and expiration into a spirometer which has oxygen in the inverted bell. During breathing, we have almost 78 % Nitrogen remaining in the lungs. Hence on expiration, a certain amount of Nitrogen will wash out from the lungs which can be measured with expired breath.

A Nitrogen washout curve can be obtained with each breath. The residual volume (RV) can be found from this curve.

How Plethysmograph is used to measure Intra-thoracic Pressure

Plethysmograph is an airtight box in which the patient is made to sit. The patient breathes air from within the airtight box through a tube provided inside the box.

The tube has a shutter to close the tube and a pressure transducer to measure the air pressure in the breathing tube. Another pressure transducer is provided in the airtight box. When we consider the patient and the box, then the total air in them is the sum of the air inside the box (V_b) and the air in the lungs of the patient (V_P) i.e. the intra-thoracic volume:

V_total = V_b + V_P

On small change, we get:

Δ(V_total) = Δ(V_b) + Δ(V_p)

But Δ(V_total) = 0 as the air box is airtight

Δ(V_b) = – Δ(V_p) — equation (1)

As per the Boyle’s law, we have

P_pV_P = Constant, here Pp = Intra-thoracic pressure

Δ(P_p) = – Δ(V_p) — equation (2)

From equation (1) and (2), we have:

Δ(P_p) = Δ(V_b)

The above relation shows that during expiration, when the pressure in the lungs is high, the air volume of the box increases while it decreases when the patient inspires. Boyle’s law is also applicable to air in the box. Hence, pressure inside the box increases during inspiration and decreases during expiration. These changes in the intra-thoracic pressure (lung’s pressure), corresponds to changes of the box’s volumes and pressures. If the test performed at the end of the expiratory level, the intra-thoracic volume is equal to Functional residual capacity (FRC). If the shutter in the breathing tube is closed so that the airflow to the mouth is stopped, then the pressure transducer in the tube measures the mouth pressure which is almost equal to alveolar pressure.

Airway Resistance Measurements

The airway resistance is determined by simultaneously measuring:

• Intra-alveolar pressure
• Airflow in the body plethysmograph

The airflow resistance is given by the following equation:

Where, R = airway resistance

P_ia = Intra-alveolar pressure

P_atm = Atmospheric pressure

f = airflow