Respiratory Anatomy and Physiology: Ventilation made easy

Your Question:  Can you provide some tips on passing "Cardiopulmonary A&P?  The topics this week are:  the process of ventilation: flow, pressure and airway resistance.

My answer:  I will try. This is among the most complicated topics in all of the respiratory therapy program, and perhaps the most important.  I am going to use Egan's Fundamentals of Respiratory Care as my guide.  The chapter I am using is on Ventilation.  You may also reference Egan's cheat sheets here, or DesJardin's Cardiopulmonary Anatomy and Physiology.

Textbook authors do a good job of explaining some things, although they tend to use too many formulas, abbreviations, and information that is simply not necessary and confusing.  For that reason, it's important to have a good teacher to simplify things, and clarify what exactly he or she thinks you need to know.  By my comments here, this is exactly what I will attempt to do.

Below are the elements of the chapter that I think are important. If you understand what I describe here, you should do pretty well on any test.

1.  Ventilation:  This is the process of moving air into and out of your lungs. It's inhaling oxygen (O2) into your lungs, and exhaling carbon dioxide (CO2) out of your lungs. It's determined by a patient's minute ventilation (page 68

2.  Minute Ventilation:  This is determined by the formula: Tidal Volume times Rate.

3.  Rate:  Also called frequency. This is how fast you are breathing.  The faster you breathe, the more O2 is inhaled and CO2 exhaled.  For this reason, patient's who are hypoxemic will tend to breathe faster in search of more oxygen.  Those who have high levels of CO2 will likewise tend to breathe faster in order to blow off CO2.

4.  Tidal Volume (VT):  This is the depth of a single breath.  The deeper the breath the more exchange of gases will occur; i.e. the more O2 inhaled and the more CO2 exhaled.

5.  Residual Volume (RV):  The volume of gas remaining in your lungs after a maximal exhalation. Air is left in your lungs so that the next breath comes in easier.  If all the air was exhaled, it would be very difficult to take in a breath.  For example, it would be like blowing up a balloon for the first time. If you blow up a balloon many times, it becomes easier and easier.  Later we will learn that PEEP, CPAP and EPAP will increase residual volume, thus decreasing a patient's work of breathing.

6.  Inspiratory Reserve Capacity (IRC):  This is the maximal volume of air that can be inhaled after a normal, quiet inspiration.

7.  Expiratory Reserve Capacity (ERV):  This is the volume that can be exhaled from the end-expiratory level.  During normal respiration, ERV is not exhaled.  So, take in a normal breath, and then allow yourself to exhale passively.  Then, at end expiration, force more air out of your lungs.  This additional air you can forcibly exhale is ERV.

Figure 1

8.  Total Lung Capacity (TLC): This includes all the lung volumes combined: VT + RV + IRV  + ERV
(See figure 1)

9.  Vital Capacity:  This is determined by the following formula: IRV + ERV + VT. This is the total amount of air available to inhale and exhale. Generally, this makes up about 80% of total lung capacity.

10.  Surface Tension:  The easiest way to explain this is to say that one surface is attracted to another.  For example, when you first try to blow up a balloon, it's very hard to do. The reason is because the surface tension is high, or the sides of the balloon are attracted to one another. One way to reduce this surface tension is by blowing up the balloon many times.  The more you do it, the less the surface tension is.  Another way to reduce surface tension is by putting soap in the balloon.  This will make it easier to open.  This knowledge will come in handy as you learn that your lungs produce surfactant, a soap-like substance that coats the alveoli (balloon-like structures) in your lungs.  The surfactant helps to reduce surface tension, so that with every breath your lungs easily open.  An example of when surface tension is very high is with premature infants who have yet to develop enough surfactant.  In these instances, they have a hard time inhaling. Artificial surfactant is then dumped into their lungs through an endotracheal tube, and this generally reduces surface tension so they can breathe easier.

11.  Elasticity:  This is the resistance of your lungs to opening. For example, once stretched, a balloon will return to its original shape.  When you inhale, your lungs fill with air.  Exhalation occurs by natural elastic recoil that occurs as your chest returns to its original form.

12.  Compliance: The true definition is the elastic forces and surface tension that try to impede inflation of the lungs.  Or, worded another way, it's a measure of how easy or hard your lungs are to open.  Increased compliance means that your lungs are easily opened and air easily enters your lungs.  Decreased compliance means that your lungs are not easily opened, and air cannot easily enter.  An example of a disease that will increase compliance is emphysema where the lungs become easily opened and too much air gets in (thus becoming trapped).  An example of a disease that decreases compliance is pulmonary fibrosis where the lungs become quite stiff and air simply cannot pass. Also of importance, the lower the compliance of the lungs (the stiffer the lungs are) the greater the peak pressure needed to fill the lungs.  Diseases like like exacerbation of asthma and COPD, along with pulmonary fibrosis, may require high pressures to obtain low volumes.  Yet diseases like emphysema and ARDS may require low pressures to achieve higher volumes. Asthma attacks will decrease compliance making it hard to inhale.  An Albuterol breathing treatment may help relax the muscles of the air passages, thus decreasing compliance and making it easier to take in a breath.  This information will come in handy as you learn about management of BiPAP and ventilator patients.

13.  Spirometer:  The volumes noted above are measured by a spirometer.  The test generally performed is called a pulmonary function test.  Generally, spirometry can help a physician diagnose the various lung diseases.

14.  Work of Breathing:  This is the work needed to get in a breath. This is how difficult it is to breathe, or how hard it is for a person to take in a breath.  If a person is having an asthma attack, his work of breathing is increased (his breathing becomes difficult).  A breathing treatment will decrease work of breathing (make breathing easier). Here is a simple formula:  Change in Pressure X change in flow (WOB = ∆P × ∆V)

Now let us consider pressures.

1.  Atmospheric Pressure:  1 or 760 m Hg.  The is the pressure in the atmosphere (the air you are inhaling)

2.  Respiratory Pressure:  It is expressed as relative to atmospheric pressure.  A respiratory pressure of 0 is the same as 1 atmospheric pressure. Pressures greater than 1 are considered positive pressures. Pressures less than 0 are considered negative pressure. Understanding pressures is essential to understanding how people breathe.  For instance, a person can breathe by creating a negative pressure because this sucks air out of the lungs.  As you inhale, for instance, your lungs create a negative pressure, and this causes you to suck in a breath.  During the polio epidemics of the early 19th century there were breathing machines called iron lungs that were essentially negative pressure ventilators.  They mimicked the natural method of breathing by creating a negative pressure that caused patients to take in a breath.  Modern ventilators are positive ventilators.  They cause a person to breathe by forcing a positive pressure into the lungs.  How much pressure is needed will depend on elasticity, compliance, and the method you use to apply the positive pressure breath (i.e. mask, tracheotomy, intubation, ventilator, BiPAP).

3.  Mouth Pressure:  Well, since your mouth is closest to the atmosphere, respiratory pressure here will be zero unless positive pressure is applied (such as with mouth to mouth breathing, or with a ventilator or BiPAP machine)

4.  Alveolar Pressure:  This is also called intrapulmonary pressure.  It varies through the breathing cycle.

5. Pleural Pressure: It is usually negative during quiet breathing, although the pressure here also varies during the breathing cycle.

6.  Pressure Gradient:  This is the difference between Alveolar Pressure and Pleural Pressure

7. Transrespiratory Pressure Gradient:  The difference between Atmospheric Pressure and Alveolar Pressure or Alveolar Pressure minus Atmospheric Pressure.  This is significant, as the pressure in the lungs is higher than pressure in the atmosphere.  For this reason, air easily flows from the atmosphere to the lungs. Changes in Transrespiratory Pressure, therefore, causes air to go in and out of the lungs.

8.  Breathing cycle:  You may need to read this over more than once, but this is the gist of it.

• Throughout the cycle Atmospheric Pressure (1) and Mouth Pressure (0) stays the same.  
• The only change is inside the lungs.  Pleural pressure is -5 before you take a breath, and alveolar pressure is 0.  Therefore, the Transpulmonary Pressure Gradient is -5 in the resting state.  In this state, no air enters the lungs. 
• As you take in a breath, your thorax is expanded. This causes a decrease in Pleural Pressure and this ultimately increases the Transrespiratory Pressure Gradient. This pressure difference causes the alveoli to open or expand.  As they expand, the pressure falls so there is now a negative pressure in the lungs. Because the pressure in the lungs is now less than atmospheric pressure, a breath is taken in.  
• So when you take in a breath, you are creating a negative pressure in your lungs, causing air to be sucked in.  
• Once a breath is taken in, Intrapleural pressure continues to decrease until it is about -10 at the end of inspiration, and this corresponds with peak inhalation, or tidal volume (VT).  
• Exhalation begins when the thorax naturally recoils, and the Pleural Pressure starts to rise and the Transpulmonary Pressure Gradient decreases.  This causes the Alveolar Pressure to start to drop back to zero.  As this happens, the alveoli become smaller, and air is forced out of the lungs (expiration).  
• In summary, natural inspiration occurs as you expand your chest wall and create a negative pressure inside your lungs.  Natural expiration occurs as your chest naturally recoils, causing the pressure in your lungs to go back to 0.  

9.  Peak Pressure, or Peak Airway Pressure (PIP):  This is the maximum pressure and flow required to fill the lungs with air.  Take in as deep of a breath as you can and hold it tight.  This is your peak pressure.  

10.  Static or Plateau Pressure.  This is the maximum pressure minus the flow needed to fill your lungs with air.  Take in as deep of a breath as you can and hold it (peak pressure). Then, while still holding your breath, relax your chest and shoulders.  This is static pressure.  Static pressure is used in a variety of formulas because it takes out flow but still gives you a measure of inspired pressure.  So you will definitely need to know what static pressure is.  You will see it used when you learn about ventilators.

11.  Positive Pressure:  A breath can be given to a patient by artificial means by providing positive pressure breaths.  This is when you artificially increase the Alveolar Pressure so that it is greater than atmospheric pressure. This can be done with a mouth to mouth breathing, an AMBU bag, BiPAP, or a ventilator.

As long as we've gone this far, I'd like to slip in a few additional definitions:

1.  Airway Resistance (Raw):  This represents the forces that impede the flow of air into your lungs, or anything that impedes the movement of air through the airways.  For instance, as you inhale, air rubs against the sides of your mouth, larynx, pharynx, corina, bronchi, and bronchioles.  A better term for this rubbing is friction.  The greater the friction the greater the resistance.  I will give some examples.  1) If you have a patient intubated, or if you try to breathe through a straw, this narrow airway causes a lot of resistance to breathing.  To decrease this resistance you will apply a positive pressure.  This positive pressure is said to decrease the work of breathing due to resistance.  Removing the endotracheal tube or straw from the airway is another way of decreasing resistance.  CPAP, BiPAP, and mechanical ventilation are all methods of increasing positive pressure to reduce Raw.  2)  A patient has an asthma attack.  His air passages become narrow.  This increase airway resistance.  Albuterol will help relax the air passages, thus reducing resistance and decreasing work of breathing. Congested Heart Failure results in increased fluid in the lungs, and this acts to increase airway resistance. One way to reduce this resistance is with CPAP or BiPAP. Increased secretions in airway may also increase resistance, and one way to reduce it is to cough and spit, or also to suction the airway. There are many formulas to determine airway resistance, but the simplest is this: Peak Pressure - Static Pressure/ Flow; or, simply Peak Pressure - Plateau. I usually prefer the later because it's the simplest to do (keep it simple).

Note:  Volume also affects airway resistance. Higher volumes = increased airway diameter = decreased airway resistance.  Lower volumes = decreased airway diameter = increased resistance. This goes along the same principle that there is increased resistance when you are breathing through a narrow airway like a straw, as compared to a larger airway like your mouth.  Worded a simpler way, resistance is highest at end expiration, and lowest at peak inspiration.

2.  Flow:  It's flow.  It can be fast or slow, laminar or turbulent. Airway resistance is affected by the three types of flow: laminar, turbulent, tracheobronchiolar.

Figure 2

3.  Laminar Flow:  The official definition is "gas moving in discrete layers or streamlines."  A simpler way of saying this is smooth, easy flowing flow that produces little friction.  The flow of air is so smooth that it does not rub onto the walls of the air passages; it produces very little friction, or very little resistance.  Ideally, in order for air to get into the lungs, you want a smooth, laminar flow.  For example, in order for albuterol to get deep into the lungs where you want it, you want the patient to take normal, easy inhalations in order to create a laminar flow. You do not, in other words, a child to be crying, which creates fast, choppy breathing that increases resistance and results in a poor treatment. (see figure 2)

4.  Turbulent flow:  Flow is not streamlined.  It is fast, and disorderly, and therefore it bangs into the air passages. There is a high degree of rubbing or friction, and there is a high degree of airway resistance. For example, if a nurse tells a patient to take a deep breath in order to get in more of the medicine, what she is ultimately doing is telling the patient to create a turbulent flow.  This will cause more of the medicine to impact in the upper airway, and less of it will get to the lungs.  During asthma attacks, occluded air passages increase turbulent flow through these air passages, and this therefore increases airway resistance.  Turbulent airflow, therefore, increases work of breathing.  Turbulent flow is greatest in the larger airways, such that 90% of resistance occurs in the nose.  Air naturally slows down as it gets deeper into the lungs, and thus becomes more laminar.

5.  Tracheobronchiolar flow:  This a combination of laminar and turbulent flow that occurs in real airways. Around turns, such as the chorina, you may have more turbulent flow.  When the airway is straight, you will have more laminar flow.  Around obstructions in the airway you will have turbulent flow.

6.  Anatomic Deadspace:  This is the amount of air that is inhaled that does not participate in gas exchange. This happens because the air does is not exhaled, instead it is rebreathed.  In other words, this is air that is wasted.  There are certain ways that you can increase deadspace.  One is to place extra long tubing between the endotracheal tube and the Y on a patient who is on a ventilator.  The greater the deadspace, the higher will be the CO2.  Therefore, if you have an intubated patient with a low CO2, creating more deadspace will get it back to normal.  Deadspace averages about 1 ml per pound of body weight. So if you weight 200 pounds, you have 200 ml of deadspace.

7.  Alveolar Ventilation:  This is the volume of air that reaches the alveoli per breath.  It is determined by the following formula:  Tidal Volume - Deadspace x rate.  Example: Vital Volume 500.  The person weights 200 pounds.  His respiratory rate is 10.  500-200 = 300 X 10 = 3000 Alveolar Ventilation.  Increased airway disease will decrease alveolar ventilation. When a patient requires a high rate, and can only draw in a low tidal volume, Alveolar Ventilation will decrease.  For example, say the same patient above can only draw in 400 tidal volume due to airway disease and his CO2 is 65 (normal is 35-45).  400-200= 200 X 10 = 2000 Alveolar Ventilation.

• Patient's normal Alveolar Ventilation = 3,000
• Patient's current Alveolar Ventilation = 2,000

This decrease in Alveolar Ventilation should explain the increase in CO2. You can also use this equation to see how a patient on a ventilator improves over time.  As when you put him on a ventilator his aleolar ventilation is 2,000.  You should track this every day and watch the trends.  As the Alveolar Ventilation improves, this can show the patient is improving.  If the Alveolar Ventilation gets worse or stays the same, this indicates the patient is not getting better.   

The way to improve Alveolar Ventilation is to increase tidal volume.  Methods to do this are BiPAP and ventilator. By increasing tidal volume to 500, the patient will blow of more CO2 and increase PO2.  It likewise decreases work of breathing, and should also result in a decrease in rate.  As the disease condition is resolved, the Alveolar Ventilation should improve, and the patient may be taken off BiPAP or ventilator.

8.  Alveolar Deadspace:  There is very little in a normal, healthy person.  However, some disease states increase deadspace in the alveoli. These alveoli are ventilated (air gets in and out) but they are not perfused (no contact with capillary blood).  Pulmonary embolism is a disease condition that causes a significant amount of deadspace.

Figure 3
Distribuion of gases throughout
the lungfields is similar to when
you hold up a slinky.  The apices
are more distended, and this results
in less resistance in the apices, and
more space for gas exchange.. The
 bases are less distended, and this
results in more resistance in the
 bases, resulting in less space for
gas exchange to occur.  In other
words, air is better distributed in
the apices as compared to the bases.

9.  Physiologic Deadspace:  Anatomic Deadspace plus Alveolar Deadspace = Physiologic Deadspace. This is the total amount of wasted air, or deadspace. This is the total amount of deadspace of the air passages and the alveoli.

10.  Deadspace/ Tidal Volume Ratio:  This tells you how much deadspace there is per breath. It tells you how efficient the patients' ventilation is.  So, if you take the example from above, you have an anatomic deadspace of 200.  You have a tidal volume of 500.  The Deadspace/ Tidal Volume Ration = 0.4.  The normal range is anywhere between 0.2 and 0.4.  So our patient would be in the normal range.

10.  Distribution of Ventilation:  Think of the lungs as a slinky (see figure 3).  The lungs are expanded more at the bases than at the top.  This is due to gravity. According to Egan, "expansion of the lower chest is about 50% greater than that of the upper chest.  Distribution is affected by compliance, resistance, and respiratory muscles

• Resistance: Lungs more distended in apices, resulting in greater surface area for alveolar contact with capillaries, and less resistance to air flow
• Compliance:  There is a higher compliance in the bases because they are expanded more. 
• Respiratory Muscles:  Distribution of air through the lungs can be altered depending on what muscles of respiration are used.  

Conclusion:  Now it's time to study. Once you understand what is listed in this post, you should do pretty well on any test.

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