Physiological Principles

Normal Tidal Breathing
Diaphragm and intercostal muscles contract to increase the size of the thorax. This results in increase in negative pressure in pleura. The gradient between atmosphere and alveoli causes air to enter lung - inspiration. During this process elastic recoil forces increase. Once the inspiration is stopped the elastic recoil forces in the lung causes expiration. Expiration is passive and no muscles contract to produce expiration.
Accessory Inspiratory Muscles
When one takes a deep breath, accessory muscles of respiration are brought into action (Scalene, sternomastoid and trapezium ). Scalene is the first muscle to start contracting and gradually other two muscles are brought into action.

When these muscles contract for tidal breathing - it is abnormal. It is easy to recognize contraction of sternomastoid and trapezium but is a late phenomenon. Routinely one should feel the scalenus muscle during quiet breathing. Stand behind the patient, lay your fingers over the scalenus behind the sternomastoid in the supraclavicular fossa. When the muscle contracts it can be recognized as narrow band of contracting muscle.

If the accessory inspiratory muscles are contracting for quiet breathing most likely FEV volume is close to 30% and is between 1 - 1.5 liters. If a patient is complaining of shortness of breath one should always be able to feel scalene muscle contraction during quiet breathing. In psychogenic or anxiety induced shortness of breath one would not feel the contraction of accessory muscles.

If accessory muscles are hypertrophied, it indicates that the process is long standing.

Accessory Expiratory Muscles
Normal tidal expiration is passive and there is no muscle contraction. Expiratory muscle contraction is always accessory. When you force expiration, expiration muscles come into play. Abdominal muscles and intercostals are expiratory muscles. If a patient is contracting abdominal muscles for quiet respiration it is abnormal and he is attempting to force expiration.
Forced Expiration
Only peak flows can be increased by forced expiration. The flow rates cannot be increased for most of expiratory phase by forcing expiration. The increasing positive pressure in pleura compresses airway and further decreases airway size thus countering the increased force to expire.

In patients with increased airway resistance patients attempt to increase airflow. They have two options either to adopt a rapid shallow breathing or to use pursed lip breathing to counter auto-peep and enhance emptying of lung. When airways are severely narrowed, air trapping occurs and patient may breath with a very high FRC and the only way he can breath is to adopt a rapid shallow breathing.

Negative Pleural Pressure Assessment
To take a deep inspiration one has to increase the negative pressure in pleura. You can clinically detect increased negative pressure in pleura by watching for retraction of supraclavicular fossa, intercostal spaces and downward movement of trachea.

If these features are noted for quiet breathing obviously patient is making extra effort to increase negative pressure in pleura to accomplish tidal breathing, which is abnormal.

Position and Inspiratory Muscles Contribution to Function
In erect position for tidal breathing diaphragm contributes 70% and intercostal 30%. In supine position contribution of diaphragm increases to 90%. Hence patients with diaphragmatic paralysis become severely short of breath in supine position.

Accessory inspiratory muscles work optimally in erect position. They are inefficient in supine position. This is one of the reasons why asthmatics like to stand or sit up with acute exacerbations.

Position and Expiratory Muscles Contribution to Function
The effect of expiratory muscles on diaphragm is optimal in erect position. When abdominal muscles contract, it increases intra-abdominal pressure and passively pushes the diaphragm up to empty air from lungs. In supine position the diaphragm is already high in thorax and there is not much room to push it higher with forced expiration.
Position, Lung, and Airway Size: Effect of Gravity
The size of the lung is large in erect position, decreases in supine position and becomes further smaller in lateral decubitus position (dependent lung).

This phenomenon should be taken advantage of, in patients with partial airway obstruction. If an asthmatic has no rhonchi in standing position, listen to his chest in supine and in the dependent lung in lateral decubitus position. If there is occult airway narrowing this maneuver will bring out the rhonchi. This is also one of the main reasons why asthmatics may not want to lay supine as their airways become narrowed.

In patients with unilateral partial airway obstruction decubitus exam is very useful.

Position and Breath Sounds
In the erect position breath sounds (and thus ventilation) is harsh in bases. In lateral decubitus position the breath sounds are harsher in the dependent lung. This is a gravity induced phenomenon. The resting (FRC) alveolar size is smaller and is in the steep part of compliance curve. As a result larger volume change occurs for a given change in negative pressure.

In unilateral lung disease breath sounds will not increase when the deceased lung is dependent.

Thoraco-Abdominal Partition
Normal males have dominant abdominal component, while women have dominant thoracic component. If there is acute abdomen, breathing will become primarily thoracic.
Function of Lung
Is to provide oxygenation, get rid of CO2 the metabolic end product and maintain pH. When the lungs fails to perform adequately patients can become hypoxic, retain CO2 and develop respiratory acidosis. Hypoxia leads to central cyanosis. Central cyanosis can be detected if conjunctive, mucous membrane of tongue, lips, nose, and fingers are blue. The hands will be warm, fingers may be clubbed. Erythrocytosis could be detected in conjunctive in chronic hypoxic states.

CO2 retention in extreme state can be recognized by papilledema

pH changes are not easily detectable. Our ability to clinically detect hypoxia, CO2 retention and pH changes is very poor and should not be relied on. Blood gases is the best way to assess pulmonary function.

Respiratory Rate
Normal baseline respiratory rate is between 10-14 per minute. Rate should be counted after patient is comfortable and without his knowledge. Decreased rate would suggest suppression of respiratory center. (sedatives, raised intracranial tension) Tachypnea is also abnormal and can be seen in a variety of disorders.
Pattern of Breathing
Normal respiration is regular with occasional sighing. Abnormal patterns are Kussmaul's, biot's, Cheyne-Stokes etc. Multiple sighs would suggest anxiety state.
Pattern of Breathing in Sleep
In normal, the rate slows but is regular. In patients with sleep apnea syndrome, one gets cyclical respiration. Pattern varies depending whether it is obstructive or central type.
Size of Thorax
The size of thorax is determined by the balance between elastic recoil of lungs and chest wall compliance. In normal, the FRC position is usually at 60% of the TLC. At this position muscle length tension curve is optimal for muscle contraction. If the elastic recoil of lung decreases the resting position of thorax will be larger, it maybe 80% of the TLC position. This position is very inefficient to generate force by muscles and leads to shortness of breath.
Symmetry of Hemithorax
Both sides are equal in size and asymmetry is abnormal. Unilateral lung or pleural disease alters negative pressure in pleura, affecting the resting size of hemithorax. e.g. In pneumothorax the negative pressure in pleura is lost and there is nothing to hold chest wall down. Hemithorax on that side will assume TLC position. In patients with atelectasis the negative pressure in pleura increases and the size of hemithorax will become smaller.

It is best to assess symmetry of hemithorax with patient laying flat in bed without pillows. Stand either at head or foot end and look tangentially at the thorax level to assess asymmetry.

Has intra and extra thoracic components. The extra thoracic component narrows during inspiration and widens during expiration. The intrathoracic component narrows during expiration and widens during inspiration. If there is obstruction it gets worse during the phase of inspiration, when the airway size is smaller.
AP Diameter of Thorax
The AP diameter of the thorax is usually less than transverse diameter in resting state i.e.; in FRC position. If one takes a deep breath and attains the TLC position, AP diameter is equal to transverse diameter giving the barrel shape. In patients with COPD where the FRC is elevated they end up having barrel shape to thorax in resting position. As we age and loose some elastic recoil to lungs similar situation arises.
Chest Expansion
Chest expansion can be measured with a tape encircling around nipple in males and under breast in women. Normal chest expansion from complete expiration to inspiration varies between 3 to 5 cms. In patients where the FRC is high and in disease states of lung, pleural or chest wall the chest expansion will be decreased.

Chest expansion is symmetrical. Asymmetrical chest expansion is abnormal. The hemithorax with decreased expansion is the abnormal side.

Expiration Time
Expiration time is measured by listening with stethoscope over Trachea. Expiration even though is physiologically longer than inspiration, on auscultation over lung fields it will be shorter. The air moves away from alveoli towards central airway during expiration, hence you can hear only early third of expiration. However over Trachea the entire duration of expiration can be heard. The normal forced expiration time is less than 5 seconds. In patients with obstructive lung disease forced expiration time is prolonged and is longer than 5 seconds.
Expiratory Force
Force of expiration is crudely measured by the ability to blow a lit matches at a length of 12" from mouth. This corresponds to peak expiratory flow. Inability to blow the matches at this length would imply decreased peak flows.
Shortness of Breath
Exercise, FRC... effort, position of comfort, lack of ... effort and CO2 retention.
Changes with Aging
As we age, the thorax tends to assume a barrel shape. The lung gradually looses elastic recoil resulting in higher FRC. The cartilages calcify and the compliance of the chest wall decreases. Osteoporosis results in kyphosis. All of these factors contribute to barrel shaped chest and decreased chest expansion. Expiratory force decreases as result of decreased elastic recoil of lungs. Arteriosclerotic changes occur to cerebrovascular system. This results in alteration of the respiratory center and can lead to Cheyne stokes respiration.