Breathe Easy Pediatrics Presents — Tidal Volume Episode 6 – RC Time Constants
Highlights
- Auto-Peep in Mechanical Ventilation
Key takeaways:
- At the beginning of mechanical ventilation, the end expiratory lung volume will increase because the inspired tidal volume is bigger than the expiratory tidal volume.
- Autopeep is not significant if you allow for three time constants when mechanically ventilating a patient.
Transcript:
Speaker 2
So even if we allow for three time constants, where about 95% of the gas will be exhaled, that means that about 5% of the inspired tidal volume will not be exhaled. This means that once you start mechanically ventilating a patient, their end expiratory lung volume will start to increase because their inspired tidal volume is bigger than their expiratory tidal volume.
Speaker 1
Yes, at the beginning of mechanical ventilation, that is true. However, auto-peep is not significant if you allow for three time constants when mechanically ventilating a patient. Even though initially about 5% of the inspired tidal volume will not be exhaled, an equilibrium at a somewhat higher end expiratory lung volume will soon be established. This occurs because as the ex-end expiratory lung volume initially increases, the respiratory system resistance decreases and the respiratory system compliance decreases. (Time 0:11:36)
- The Effects of Expiratory Breaking on End Expiratory Lung Volume
Key takeaways:
- Infants often cry right after birth and during crying, they have a partial adduction of the vocal cords, which increases the expiratory resistance of the respiratory system.
- This increased resistance of the respiratory system is actually useful because it leads to an increase in end expiratory lung volume in these infants, which is needed as they transition from lungs filled with fluid to lungs filled with air.
Transcript:
Speaker 1
Now, there are other scenarios where you actually want to see an increase in the end expiratory lung volumes. Our listeners who have attended births have probably seen a classic scenario many times, but may never have thought to describe it. As we all know, infants often cry right after birth and during crying, they have a partial adduction of the vocal cords. This partial adduction of the vocal cords is called expiratory breaking and increases the expiratory resistance of the respiratory system. The increased resistance of the respiratory system is actually useful because it leads to an increase in end expiratory lung volume in these infants, which is needed as they transition from lungs filled with fluid to lungs filled with air. This breathing pattern of expiratory breaking is even more common in preterm infants compared with healthy infants born at term. (Time 0:14:35)
- The Normal Phenomenon That Increases the End Expiratory Lung Volume in a Spontaneously Breathing Newborn
Key takeaways:
- In a spontaneously breathing newborn, the normal phenomenon of expiratory breaking increases the end expiratory lung volume.
- This is mediated by partial adduction of the vocal cords.
- Expiratory breaking increases the expiratory time constant by increasing the respiratory system resistance.
Transcript:
Speaker 2
It is nice to have a term for this. To help us all remember this term, I'll phrase this as a mini quiz question. The question is, in a spontaneously breathing newborn, what is the normal phenomenon that increases the end expiratory lung volume?
Speaker 1
Expiratory breaking. In a spontaneously breathing infant, the normal phenomenon of expiratory breaking increases the end expiratory lung volume. This is mediated by partial adduction of the vocal cords. Expiratory breaking increases the expiratory time constant by increasing the respiratory system resistance.
Speaker 2
So right after birth, neonates increase the time constant of the respiratory system by expiratory breaking. How do time constants of the respiratory system change with age?
Speaker 1
That's an interesting question. From what we know in infants with healthy lungs, the time constants are shortest at birth and then increase until childhood. After childhood and into adulthood, time constants stay about the same unless the (Time 0:15:36)
- The Effect of Age on Respiratory Time Constants
Key takeaways:
- In spontaneously breathing infants, the normal phenomenon of expiratory breaking increases the end expiratory lung volume.
- This is mediated by partial adduction of the vocal cords.
- Expiratory breaking increases the expiratory time constant by increasing the respiratory system resistance.
- From what we know in infants with healthy lungs, the time constants are shortest at birth and then increase until childhood.
- After childhood and into adulthood, time constants stay about the same unless the child or adult develops a disease that increases the resistance and our compliance of his or her respiratory system.
Transcript:
Speaker 1
Expiratory breaking. In a spontaneously breathing infant, the normal phenomenon of expiratory breaking increases the end expiratory lung volume. This is mediated by partial adduction of the vocal cords. Expiratory breaking increases the expiratory time constant by increasing the respiratory system resistance.
Speaker 2
So right after birth, neonates increase the time constant of the respiratory system by expiratory breaking. How do time constants of the respiratory system change with age?
Speaker 1
That's an interesting question. From what we know in infants with healthy lungs, the time constants are shortest at birth and then increase until childhood. After childhood and into adulthood, time constants stay about the same unless the child or adult develops a disease that increases the resistance and our compliance of his or her respiratory system.
Speaker 2
Interesting. What are the typical time constants across the lifespan?
Speaker 1
In healthy infants, tau increases from about 3-10ths of a second to about 6-10ths of a second during the first year of life. Thereafter, tau remains about the same with normal values for tau in adults found by other investigators to be in the range of 5-10ths to 8-10ths seconds. (Time 0:15:50)
- The Effects of Extrinsic Peep on Respiratory System Function
Key takeaways:
- Adding extrinsic peep (water added to the ventilator circuit) will increase the endexpertory lung volume, which will decrease the respiratory system's resistance, compliance, and time constant.
Transcript:
Speaker 1
Exactly. If the increase in tau is large enough that the patient cannot fully exhale after each inspiration, then the patient's lung volume at end expiration will increase. This will result in auto-peep. If the expiratory time is more than 3 tau, there will be no significant increase in auto-peep.
Speaker 2
Okay, so now that we've discussed auto-peep, let's move on to talking about how extrinsic peep and how adding extrinsic peep can affect this patient's end-expertory lung volume and how adding extrinsic peep can affect this respiratory system's resistance, compliance, and time constant. Say we added an extrinsic peep of 6 centimeters of water to the ventilator circuit. What effects would this additional extrinsic peep have on this patient's end-expertory lung volume?
Speaker 1
Well, this one is pretty intuitive and something that we all have experience with. Adding extra peep will increase the end-expertory lung volume.
Speaker 2
And what effect will that increase in end-expertory lung volume have on this respiratory system's resistance, compliance, and time constant?
Speaker 1
As we discussed earlier, when the end-expertory lung volume increases, the resistance and compliance of the system usually both decrease. Since the time constant is the product of the resistance times the compliance, the time constant will also decrease. Adding extrinsic peep will ultimately decrease the respiratory system's resistance, compliance, and time constant. (Time 0:26:27)
- The Effect of Extrinsic Peep on the Respiratory System
Key takeaways:
- Adding extrinsic peep to a circuit of a patient with low lung volumes due to adlectasis will decrease the respiratory system's resistance, compliance, and time constant.
- In some cases, the product of the two may remain constant or might increase or decrease.
Transcript:
Speaker 1
Well, this one is pretty intuitive and something that we all have experience with. Adding extra peep will increase the end-expertory lung volume.
Speaker 2
And what effect will that increase in end-expertory lung volume have on this respiratory system's resistance, compliance, and time constant?
Speaker 1
As we discussed earlier, when the end-expertory lung volume increases, the resistance and compliance of the system usually both decrease. Since the time constant is the product of the resistance times the compliance, the time constant will also decrease. Adding extrinsic peep will ultimately decrease the respiratory system's resistance, compliance, and time constant. On the other hand, our cases in which peep is added to the circuit of a patient with low lung volumes due to adlectasis. The adlectasis may resolve due to the addition of peep and due to positive pressure mechanical ventilation. In this case, the respiratory system compliance may increase due to the loss of anelectasis while the respiratory system's resistance decreases. Therefore, the product of the two may remain constant or might increase or decrease.
Speaker 2
That's a good reminder that adlectasis decreases pulmonary compliance, so by resolving it, you can increase the compliance of the lungs. Yes. (Time 0:27:16)
- The Risks of Scuba Diving for People with Lung Disease
Key takeaways:
- Pediatric pulmonologists are sometimes consulted on scuba diving because people with lung diseases with long time constants are at significantly increased risk of experiencing a pneumothorax, pneumomediastinum, pneumoparacardium, endorph, and intravascular air embolism as they ascend from the depths of the water to the surface.
- The time constant of the lung sis or bola may be very long, which means that those areas of the lung do not reach pressure equilibrium with each breathing cycle.
Transcript:
Speaker 1
Now let's talk about an activity outside of the hospital that can be really risky for people who have lung disease with long time constants. What were you thinking of? One activity that pediatric pulmonologists are sometimes consulted on is scuba diving. This is because people who have lung diseases where there are areas of their lungs with long time constants are at significantly increased risk of experiencing a pneumothorax, pneumomediastinum, pneumo-paracardium, endorph, and intravascular air embolism as they ascend from the depths of the water to the surface. Yes, none of those things are good.
Speaker 2
Can you talk about how that relates to long time constants in scuba diving?
Speaker 1
This has to do with the dramatic changes in ambient pressure involved with scuba diving. As you can imagine, people with lung sis, apical boli, or diseases where there is heterogeneity in how well different parts of the lungs are ventilated, such as asthymocystic fibrosis or COPD, will have areas of their lungs which have prolonged time constants. This means that certain areas of their lung will have longer time constants compared with other areas. In fact, for people with lung sis or boli, the time constant of the lung sis or bola may be very long. This means that those areas of the lung do not reach pressure equilibrium with each breathing cycle. (Time 0:28:35)