Airway Resistance II Notes

Airway Resistance II

Instructor: Peter P. Sayeski, PhD

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Objectives

  • Understand how and why airway resistance changes with asthma. Airway resistance is how hard it is for air to move in and out of your lungs. We'll learn why this resistance gets much higher for people with asthma.

  • Learn about the various types of asthma. There isn't just one kind of asthma; we'll look at the differences between them.

  • Comprehend the appropriate treatments for asthma and the rationale behind these treatments. We'll cover the best ways to treat each type of asthma and understand why those treatments work to help people breathe better.

  • Understand why airway resistance changes with lung volume. We'll explore how the amount of air in your lungs (lung volume) affects how easy or hard it is to breathe.

  • Define the equal pressure point. We'll explain a specific point in your airways that's important for understanding how your lungs work during breathing.

Asthma Overview

  • Definition of Asthma: Asthma is a long-lasting lung disease where breathing becomes significantly harder because the airways (the tubes that carry air to and from your lungs) become narrow and have much more "resistance" to airflow. Think of it like trying to suck a thick milkshake through a very narrow straw.

    • Normal Bronchiole vs. Asthmatic Bronchiole: Imagine your normal airways as wide, open pipes that air can easily flow through. In someone with asthma, these pipes become inflamed (swollen and irritated), produce extra sticky mucus, and the muscles around them tighten up (called bronchoconstriction). All of this makes the pipes much narrower, like a squeezed straw, making it very difficult for air to pass through.

Increased Airway Resistance: Asthma

  • Mechanisms Influencing Airway Resistance:

    • Your body has an automatic control system, called the Autonomic Nervous System, which has two main parts: the Sympathetic and Parasympathetic nervous systems. They work like a seesaw, balancing each other to control your airways.

      • The Sympathetic system is like your body's "fight or flight" response; it helps to open up your airways, making it easier to breathe in stressful situations or during exercise. It causes the muscles around the airways to relax.

      • The Parasympathetic system is for "rest and digest"; it tends to constrict (narrow) your airways. In asthma, this system can sometimes act too strongly.
        The balance between these two systems affects how relaxed or constricted your airways are, which directly changes how much airway resistance you experience.

  • Types of Asthma:

    • Mild Asthma: This type of asthma is often triggered by specific factors like intense exercise or breathing in very cold, dry air.

      • Symptoms: When cold/dry air irritates the upper airways, it can cause them to narrow slightly, leading to coughing, wheezing, or shortness of breath.

      • Treatment: The common treatment is a sympathetic agonist, like Albuterol. An agonist is something that mimics the action of another substance. So, a sympathetic agonist acts like the sympathetic nervous system, causing your airway muscles to relax and open up your airways quickly. It's often called a "rescue inhaler."

    • Moderate Asthma: This type is usually triggered by common allergens, which are substances that cause an allergic reaction, such as animal dander (tiny flakes of skin from pets), pollen, or dust mites.

      • Treatment: This is often treated with Albuterol (to open airways) and/or anti-histamines (like Benadryl). Anti-histamines work by blocking the effects of histamines, which are chemicals your body releases during an allergic reaction that can cause inflammation and airway narrowing.

    • Severe Asthma: This is the most serious type and involves extensive inflammation (swelling and irritation) throughout the upper airways. This inflammation is caused by a strong release of powerful inflammatory chemicals in the body, such as histamines, prostaglandins, and leukotrienes.

      • Treatment: Simply opening the airways (bronchodilation) with Albuterol isn't enough here because the main problem is the deep inflammation. Therefore, the treatment must treat the underlying inflammation. This usually requires powerful anti-inflammatory medications called corticosteroids (like prednisone). Corticosteroids reduce the swelling and irritation in the airways, allowing them to open up properly again.

Physiological Considerations
When Airway Resistance is Acceptable

  • Our bodies are smart! At very low lung volumes, especially when you breathe out as much air as you possibly can (maximal expiration), your body actually increases airway resistance on purpose. This prevents your lungs from completely emptying of air, which would cause tiny air sacs (alveoli) to collapse and make it harder to breathe efficiently. This limited deflation is actually beneficial.

    • Mechanism:

      • During a forced exhale, the space around your lungs (called the pleural space) decreases in volume. This causes the pressure in this space, called pleural pressure, to significantly increase (it can be estimated to be around +30 ext{ cm H}_2 ext{O} – a pretty high positive pressure).

      • This increased pleural pressure acts from outside your airway tubes, squeezing and compressing them. This regulated compression of the airways helps to control and limit how fast air can leave your lungs, preventing them from emptying too much.

    • Conclusion: This is a natural protective mechanism. As the amount of air in your lungs (lung volume) gets lower during a forced exhale, the resistance to airflow actually increases to keep some air in your lungs.

Dynamic Airway Compression

  • Dynamic Airway Compression happens specifically during a forced expiration (when you push air out forcefully, like blowing out candles), not during normal, quiet breathing (tidal volume breathing). During forced expiration, there's a strong pressure acting to collapse your airways.

    • Equal Pressure Point (EPP): This is a specific point within your airways where the pressure inside the airway (the air pressure) becomes exactly equal to the pressure outside the airway (the pleural pressure).

    • Implications of EPP:

      • In the parts of the airway that are "downstream" from the EPP (meaning closer to your mouth, past the EPP), the pressure outside the airway becomes greater than the pressure inside. This difference in pressure causes the airway walls to tend to collapse.

      • This partial collapse significantly increases the airway resistance in those areas. This increased resistance traps some air in your lungs, which contributes to what's called residual volume (the air that always remains in your lungs even after you exhale as much as you can).

  • In a typical, healthy person, the EPP is usually found in the larger breathing tubes of your lungs, specifically within the segmental bronchi.

Summary of Key Points

  • Asthma is a breathing problem where your airways become much narrower, leading to increased airway resistance and making it hard to breathe.

  • There are different types of asthma (mild, moderate, severe), and they are identified by what triggers them and how serious the symptoms are.

  • The resistance in your airways increases when you have less air in your lungs, especially during a forceful exhale. This is a built-in protective mechanism.

  • Dynamic airway compression and the equal pressure point (EPP) are important during forced exhalation. Past the EPP, your airways can partially collapse, trapping some air and contributing to the residual volume (air always left in your lungs).