872: Pulmonary Physiology and Pharmacology

For Air to enter the lungs during inhalation the pressure must drop below 760 mmHg

The pressure inside the lungs must drop below this level

Inspiration at rest

Begins when the diaphragm contacts and flattens and the external intercostal muscles contract, lifting the rib cage

As the volume of the lungs increases

The intrapulmonary pressure drops below atmospheric pressure and air flows into the lungs

Expiration at rest is a passive process

It relies on the elastic recoil of the lungs

Once the diaphragm and external intercostals relax, the thoracic cavity returns to its resting size

As lung volume decreases

Intrapulmonary pressure rises above atmospheric pressure and air flows out of the lungs

Forced inspiration

Involves a greater contraction of the diaphragm and external intercostals and may recruit accessory muscles such as the SCM and scalenes

These efforts further expand the throacic cavity and create a larger drop in intrapulmonary pressure

Forced expiration

Becomes an active process where the internal intercostals and abdominal muscles contract to forcefully decrease thoracic volume and expel more air from the lungs than elastic recoil alone can achieve

Emphysema

In pts with this the elastic fibers of the lungs are damaged, reducing the lungs ability to recoil during expiration

This makes passive expiration less efficient, leading to air trapping, increased residual volume, and the characteristic barrel chest appearence

Restrictive lung disease

With this lung tissue becomes stiff or scarred, reducing lung compliance

As a result inspiration becomes more difficult even though expiration may remain relatively normal

These pts often take rapid shallow breaths to reduce the work of breathing

Neuromuscular disorders

Diseases that affect the diaphragm or intercostal muscles can impair both inspiration and expiration

In ALS for example the loss of motor neurons weakens the diaphragm leading to hypoventilation, reduced lung volumes and eventually respiratory failure

Ventilation-perfusion, V/Q

Refers to the relationship between the air that reaches the alveoli via pulmonary capillaries

Average V/Q ratio in the lungs at rest

0.8

Low V/Q

Indicates poor ventilation relative to perfusion

High V/Q

Suggests adequate ventilation but poor perfusion

Hypoxic pulmonary vasoconstriction

Areas of the lung that are poorly ventilated will experience vasoconstriction of the pulmonary arterioles to divert blood to better ventilated regions

Pulmonary embolism

Blood clot blocks perfusion to parts of the lung

Ventilation may be normal but perfusion is absent

This causes a high V/Q mismatch aka dead space ventilation

COPD

Mucus and inflammation obstruct airflow to the alveoli, creating areas where perfusion is intact but ventilation is impaired

A low V/Q mismatch, aka shunt like state

Pneumonia

Fluid or exudate in the alveoli prevents normal ventilation

Even though blood continues to flow through the capillaries, gas exchange is impaired, another low V/Q situation

Oxygenated is transported in the blood in two forms

1. Bound to hemoglobin

2. Dissolved in plasma

Carbon dioxide is transported in the blood in three forms

1. As bicarb

2. Bound to hemoglobin

3. Dissolved in plasma

Anemia

There is reduced hemoglobin available to bind to oxygen even if PaO2 is normal

Oxygen hemoglobin dissociation curve

Illustrates the relationship between the partial pressure of oxygen and the percentage of hemoglobin saturated with oxygen

How the oxygen hemoglobin saturation curve works

In the tissues where PaO2 is lower, hemoglobin releases oxygen to meet metabolic demands (so that oxygen can be used by the tissues that need it like skeletal muscles)

A rightward shift (reduced affinity for oxygen, more unloading tissues)

Is caused by increased temperature, increased CO2, decreased pH, and increased 2,3 BPG

Benefit of a rightward shift

Helpful during exercise or in metabolically active tissues because it allows more oxygen to be released from hemoglobin and used by the tissues for metabolism

A leftward shift

Occurs with decreased CO2, increased pH, and decreased temperature

Exercise and fever (rightward shift)

The curve shifts to the right

This promotes oxygen unloading at tissues, helping meet metabolic demand

Hypothermia or alkalosis (leftward shift)

Curve shifts to the left, meaning hemoglobin holds onto oxygen more tightly

Supplemental O2 indication

Hypoxemia

Short AND long acting beta 2 agonist inhaler indication

Airway obstruction

Inhaled corticosteroids indication

Airway inflammation and obstruction

Muscarinic antagonists

Airway obstruction

PT implications/possible adverse side effects of supplemental O2

Long term use may cause cytotoxic damage and depression of ventilation

Short acting beta 2 agonist inhaler mechanism of action

• stimulate the B-2 receptors in smooth muscle cells that line the bronchi, thus causing relaxation and dilation of the airways

• These medications act quickly and are used to relief from sudden attacks/symptoms

Long acting beta 2 agonist inhaler mechanism of action

• stimulate the B-2 receptors in smooth muscle cells that line the bronchi, thus causing relaxation and dilation of the airways

• Are longer acting and are used regularly in chronic asthma and COPD

Inhaled corticosteroids mechanism of action

• suppress airway inflammation

• They also enhance B-2 adrenergic receptor expression and function and are often used with LABAs in the treatment of airway obstruction

Muscarinic antagonists mechanism of action

Activate bronchoconstriction therefore inhibiting this mechanism causes bronchodilation

PT implications/possible adverse side effects of short and long acting beta 2 agonist inhaler

Increased sympathetic activation and HR

PT implications/possible adverse side effects Of inhaled corticosteroids

At high doses they may increase the risk of HTN, hyperlipidemia, and glucose intolerance

PT implications/possible adverse side effects Of muscarinic agents

• dry mouth/skin

• Headache

• Confusion

• Dizziness

• Tachycardia

• Blurred vision

• Delirium

• Decreased GI activity