Pulmonary Ventilation – Breathing and Lung Mechanics

00:01 Okay, now, let’s go to talk through the different amounts of ventilation that a person has.

00:07 So the classic formula for ventilation, this is minute inhalation or total ventilation is simply taking the breathing frequency times the tidal volume.

00:20 If we want to account for alveolar ventilation, now we need to account for dead space.

00:28 So here, we have the volume of air that reaches out of the mouth.

00:34 It’s the breathing frequency times the tidal volume minus the dead space.

00:39 So alveolar ventilation accounts for dead space while minute ventilation does not.

00:45 Alveolar ventilation is more important in pulmonary medicine than minute ventilation.

00:52 But minute ventilation is easier for us to measure because dead space is a little bit hard to measure.

01:00 So let’s take this example here of a 14-year-old girl with a history of asthma.

01:06 She does a number of pulmonary functions tests after taking a short-acting beta-adrenergic agonist.

01:13 A great example of this is albuterol.

01:16 Her tidal volume is 400 milliliters.

01:19 Her breathing frequency is 10 breaths per minute.

01:23 So if you were calculating per minute ventilation, you simply take 10 times 400 and that would yield 4000 milliliters per minute.

01:35 If you wanted to calculate her alveolar ventilation, we’re going to have to account for dead space.

01:41 So in this case, you’ll take her tidal volume, 400, minus the dead space volume, which is 200, which would yield 200 multiply that by 10, which is her breathing frequency, and you’d get 2000 milliliters per minute.

01:57 So alveolar ventilations will always be less than minute ventilations, but alveolar ventilation tells you the amount of gas that actually gets to the point where gas exchange can occur.

02:11 So there are ways to calculate this in a more complex manner and this is shown with this particular formula.

02:20 But we don’t have to worry about this formula to the same extent because we just want to make sure that the relationship between alveolar ventilation and CO2 is inverse of each other.

02:32 And I’ll show you this from some teeter-totter examples.

02:36 If alveolar ventilation goes up, CO2 has to go down.

02:40 The alternate also occurs as the CO2 goes up, alveolar ventilation rate goes down.

02:48 And where you can use this clinically is all you have to do is either measure the CO2 or measure alveolar ventilation and you’ll be able to predict the other.

02:59 So someone has a high CO2 in their blood, you automatically know that their alveolar ventilation rate is low.

03:07 And the inverse of that is also true.

03:09 If you see that CO2 is low, you automatically know that alveolar ventilation rate is high.

03:18 Ventilation also relies on lung compliance.

03:22 So compliance is the inverse of stiffness or how stiff the lungs are.

03:29 Normally, you have a nice compliant lung.

03:34 However, it can change in certain disease states such as interstitial pulmonary fibrosis.

03:40 In this case, there is scar tissue that forms in the lungs, increasing the amount of collagen fibers and making the tissue less pliable or less stretchable.

03:52 And therefore its compliance has decreased.

03:54 There are other clinical conditions such as emphysema .

03:57 And in emphysema, there’s an increase in compliance.

04:02 And this done because the emphysema process of pathophysiology breaks down elastin fibers.

04:08 And therefore, it’s increased its compliance and it’s less stiff.

04:15 You can think of it as more floppy.

04:19 So if you want to think of these three examples, probably the best way to do that is to think it’s overall compliance.

04:27 Now, with compliance, there are changes that occur even in the lung itself depending upon how much stretch is produced by particular forces.

04:39 So in this case, gravity can cause a decrease in lung compliance in the top portions of the lung.

04:46 There is an increase in compliance at the base of the lung.

04:50 So if you’re a visual person, I think the easiest way to think about compliance is think about two rubber bands.

04:57 If a rubber band is not stretched, it has a higher compliance.

05:04 If you’ve already fully stretched a rubber band, it is hard to stretch it to a greater degree.

05:10 In this case, its compliance has decreased when it is already pretty stretched.

05:16 Now, why is this important for overall ventilation? Because ventilation has a greater propensity to go to areas that have high compliance.

05:27 So an area that is highly compliant, such as the base of the lung, will get more ventilation than areas that are lower compliant such as the apex of the lung.

05:39 If we want to look at this in terms of pathophysiology, if you have fibrotic areas of the lung with low compliance, ventilation or airflow will not want to go to those areas, but will in fact go to others.