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Respiratory Measures

Many respiratory measures can be assessed in individuals with SCI (see table 2).

Table 2. List of repiratory measures aften assessed in individuals with SCI.

Measure

Acronym

Definition

Lung Volume Measures

Tidal Volume

TV

Volume of air inhaled or exhaled during breathing – at rest or during exercise

Inspiratory Reserve Volume

IRV

Maximum volume of air that can be inhaled to total lung capacity over and above the tidal volume

Expiratory Reserve Volume

ERV

Maximum volume of air that can be exhaled from the functional residual capacity to residual volume

Residual Volume

RV

Volume of air remaining in the lungs after a maximal expiration

 

Lung Capacity Measures

Inspiratory Capacity

IC

Maximal volume of air that can be inhaled – sum of tidal volume and inspiratory reserve volume

Functional Residual Capacity

FRC

Volume of air remaining in the lungs at the end of an ordinary expiration

Vital Capacity

VC

Maximum volume of air that can be expelled after a maximum inspiration (i.e., from total lung capacity to residual volume)

Total Lung Capacity

TLC

Total amount of air in the lungs after a maximal inspiration TLC= RV + ERV + TV + IRV

Respiratory Muscle Strength Measures

Maximum Expiratory

Pressure

MEP

Pressure exerted by the respiratory muscles during a forceful exhalation

Maximum Inspiratory Pressure

MIP

Pressure exerted by the respiratory muscles during a forceful inhalation

The Forced Expiratory Volume in one second (volume of air that can be exhaled in the first second = FEV1) and Forced Vital Capacity (maximum volume of air that can be exhaled = FVC) are two measures taken in healthy people to detect airways obstruction. Due to reduced inspiratory muscle force, these measures are diminished in people after SCI with higher lesions and especially in people with tetraplegia and demonstrate moderate correlation with injury level. Longer duration of injury and smoking are two factors associated with greater loss while incomplete lesions (compared to complete lesions) have greater loss of respiratory functional measures of FEV1 and FVC.

People with SCI at most levels affect innervation of the abdominal muscles which may severely compromise the ability to generate cough and clear respiratory secretions. The respiratory system has other important roles such as speaking and posture-related activities which can also be negatively impacted by the SCI, especially with higher lesions. See figure 6 and 7 for a prototypical output of a spirometer and flow-volume loop, respectively.

Figure 6.This is a prototypical output of a spirometer. The vertical axis signifies the volume and the horizontal axis signifies time. The bottom left corner equals zero lung volume at the start of the spirometer recording session. The first small amplitude part of the sinusoid depicts repeated resting state involuntary breathing. The amplitude of this small sinusoid corresponds to the 'Tidal Volume'. The large positive amplitude spike represents voluntary inspiration to maximal volume or 'Total Lung Volume'. The large negative amplitude spike represents forced expiration to the lowest possible physiological lung volume called 'Residual Volume'.

Figure 7.This is a flow-volume loop of a normal Forced Vital Capacity, one of the most common functional tests of the lungs. The patient sits or stands and inspires fully then expires fully as fast as possible. Along the x-axis of a flow-volume loop is the rate of volume expired or inspired (L/s). Along the y-axis is the volume inspired and expired.

For more information please see: Respiratory Management.