Mechanics of Breathing

Question 1

Quiet inspiration is […] using the diaphragm and intercostal muscles; energy is stored in the elastic recoil of the lung and expiration is […].

Answer 1

Quiet inspiration is active using the diaphragm and intercostal muscles; energy is stored in the elastic recoil of the lung and expiration is passiverelaxation.

Question 2

Major Respiratory muscle is …

Answer 2

…the diaphragm.

Question 3

The diaphragm acts to…

Answer 3

1. descend when contracted
2. as it descends it bows outwards due to the abdominal content and thus the lower ribs bow out as well

Question 4

Intercostals

Answer 4

•maintain tone and are very important postural muscles

Question 5

External Intercostals

Answer 5

•oriented so they are inspiratory muscles

Question 6

Internal Intercostals

Answer 6

•oriented so they are expiratory muscles

Question 7

Minute Ventilation

Answer 7

MINUTE VENTILATION = RESPIRATORY RATE (1/min) x Tidal Volume (L)

Question 8

The higher the respiratory rate then the […] the flow and […] work

Answer 8

The higher the respiratory rate then the higher the flow and RESISTIVE work

Question 9

Answer 9

• The above diagram shows that at low respiratory rates and high tidal volumes the elastic work is markedly increased. Conversely, at high respiratory rates and low tidal volumes, the resistive work is markedly increased.
• Total work is the sum of the elastic and the resistive work breathing. It is apparent that at either extreme (high tidal volume or high respiratory rate) the total work of breathing is maximal.
• However, there is an optimal respiratory rate where the work of breathing is at a minimum. This is around 16 to 20 breaths per minute in human adults. This is the normal rate of respiration in adults. It appears that we choose a respiratory rate to minimize work of breathing.

Question 10

Obstructive Disease - how to minimize the work of breathing

Answer 10

• Resistive work is increased because the airways are obstructed.
• To reduce resistive work, the patient should breathe with low flows; i.e. the patient should breathe slowly and deeply.
• asthma and emphysema

Question 11

Restrictive Disease - how to minimize the work of breathing

Answer 11

• Elastic work is increased because the lung is stiff and non-compliant due to scarring (fibrosis) or increased water (edema),
• To reduce elastic work the patient should breathe with small tidal volumes; i.e. rapidly and shallowly
• pulmonary fibrosis, pulmonary edema

Question 12

Spirometry

Answer 12

• maximal forced expiration in the assessment of lung disease
• Forced Vital Capacity:

If a subject takes a full breath in to total lung capacity and then exhales as rapidly and forcefully as possible until residual volume is reached then the volume thus exhaled is called the FORCED VITAL CAPACITY (FVC). If enough force is used during the maneuver then dynamic airway compression is caused and flow limitation is reached. Under these conditions the flows achieved are determined only by lung recoil and lower airway resistance; i.e. the intrathoracic airways. Maximal expiratory flows are independent of effort. Thus, we can use a maximal expiration to test lung function very non-invasively. The FVC gives us information about the lung volume available to be exhaled and is compared to normal values based upon age, gender and height to give a percent of predicted value.

Question 13

Forced Expiratory Volume in 1 Second (FEV1)

Answer 13

• This is a measure of how rapidly the subject can exhale, i.e. the flow that is obtained.
• We also calculate the percent of predicted value for FEV1. In addition, we look at the ratio of FEV1/FVC (normal > 72% in adults). This tells us how rapidly gas was exhaled after correcting for the lung volume. It is a very useful value to calculate.
• For example: A patient with basically healthy lungs has a one lung removed for cancer. After surgery the FVC and FEV1 would both be reduced by 50%; however, their ratio would be normal, thus confirming the basically healthy nature of the remaining lung.

Question 14

Answer 14

• We can also assess flows in a more direct manner by averaging flow over the middle of the FVC to give the FORCED EXPIRATORY FLOW FROM 25 TO 75 PERCENT OF VITAL CAPACITY (FEF25-75%). This value tends to assess lung function determined by the peripheral airways more than does the FEV1.
• It is used to detect the early onset of diseases such as emphysema and cystic fibrosis because they first damage the small, peripheral airways.

Question 15

Obstructive Disease

Answer 15

• diminished forced expiratory flows due to increased airway resistance
• flow relative to lung volume will be decreased
• FVC may be decreased –. RV increased as is the RV/TLC ratio
• FEV1 will be decreased more than FVC —. FEV1/FVC ration will be decreased
• FEF 25-75 will be decreased

Question 16

Restrictive Disease

Answer 16

• loss of lung volume
• flow relative to lung volume remains normal
• FVC will be decreased, but RV will usually be normal as is the RV/TLC ratio
• FEV1 will be decreased proportionallyto FVC —. FEV1/FVC ratio will be normal
• FEF 25-75 will be normal as well

Question 17

Mixed Disease

Answer 17

• combination of obstructive and destructive
• cystic fibrosis
• This is difficult to diagnose with spirometry alone and requires the measurement of lung volume; i.e. one would see an obstructive picture on spirometry, but a reduced total lung capacity as well. This requires the measurement of lung volume with a body plethysmograph or a gas dilution system to measure total lung capacity (TLC) and residual volume (RV).