Control of Ventilation in Health & Disease

CONTROL OF VENTILATION IN
HEALTH & DISEASE - PART 2
Dr M A Nazrul
1
Control of ventilation: Health & Disease- Dr M A Nazrul

CONTROLLER
EFFECTOR
LUNG VOLUME
SENSOR
rCPG
Inspiration/Expiration
Muscles of
respiration
Chemoreceptors
Mechanoreceptors
Reference- Fishman’s Pulmonary Diseases and Disorders Fifth Edition –Chapter 11 Control of
Ventilation
2

CONCEPT OF LOOP GAIN
• Loop gain is an engineering term that is used to define the stability or instability of a negative feedback
control system.
• It represents the overall response of the plant (representing the lung and respiratory muscles), the controller
(representing the ventilatory control centers and the chemoreceptors), and the delay inherent in transferring
the signal between the plant and the controller.
• Controller gain, or chemoresponsiveness, is the change in ventilation, to hypercapnia and hypocapnia, that is
ΔV₁/ΔPaCO₂ where ΔV₁ is minute ventilation.
• Plant gain is determined by the magnitude of the reduction in Paco2 resulting from a given change in ventilation
, that is ΔPaCO₂/ ΔV₁, the efficiency with which carbon dioxide is eliminated.
3

4

5
• In the case of ventilation, the propensity for CO2 fluctuations is a function of an individual’s loop gain.
That is, an individual with a high loop gain is prone to developing periodic breathing or Cheyne–Stokes
breathing, even with minimal perturbation. On the other hand, an individual with low loop gain will
maintain relatively stable breathing patterns even with major perturbations.
• Increased controller gain (eg, due to sustained hypoxia, acute intermittent hypoxia, or heart failure) or
increased plant gain due to metabolic alkalosis or disorders of hypoventilation, or both, in an individual
narrows the carbon dioxide reserve to increase breathing instability.
• If loop gain is less than 1, then the system corrects itself (i.e., breathing normalizes), but if loop gain is 1
or greater, then the system remains unstable.
• The importance of loop gain is receiving increasing attention given the recognition of its importance in
obstructive sleep apnea, central sleep apnea, periodic breathing at high altitude and other conditions.

Control of ventilation: Health & Disease- Dr M A Nazrul 6

Cheyne–Stokes Breathing was first observed in patients with cardiac or CNS disease, but it has since been reported in
seemingly normal humans. The appearance of Cheyne–Stokes breathing can occur during wakefulness although often masked
by behavioral influences, but is more common during nonrapid eye movement (NREM) sleep. Arousal tends to occur during the
hyperpneic phase of the respiratory pattern, a finding which is often associated with paroxysmal nocturnal dyspnea in patients
with heart failure.

9
SLEEP
• Sleep is classified on the basis of the electroencephalogram(EEG) and electro-oculogram (EOG) into
rapid eye movement(REM) and non-REM (stages N1–N4) sleep
• Non-REM sleep is often thought of as the “restorative”nondreaming phase of sleep. It is promoted and
sustained by a system of neurons that inhibit the brain-arousal systems of wakefulness.
1. Stage N1 is dozing, from which arousal easily takes place. The EEG is low voltage, and the frequency is
mixed but predominantly fast.
2. In Stage N2,the background EEG is similar to stage N1 but with episodic sleep spindles (frequency 12–
14 Hz) and K complexes(large biphasic waves of characteristic appearance). Slow, large-amplitude
(delta) waves start to appear in stage N2
3. In Stage N3 Delta waves become more dominant, in which spindles are less conspicuous and K
complexes become difficult to distinguish.
4. In Stage N4, which is often referred to as deep sleep, the EEG is mainly high voltage (more than 75 mV)
and more than 50% slow (delta) frequency
• REM sleep- The EEG pattern is the same as in stage N1, but the EOG shows frequent rapid eye
movements that are easily distinguished from the rolling eye movements of non-REM sleep. Skeletal
muscle tone generally decreases, and dreaming occurs during REM sleep.

11

12
BREATHING IS DEPENDENT ON FEEDBACK REGULATION
IN SLEEP
• There are mutually opposing interactions between the wakefulness-promoting neuronal systems and the
non-REM sleep-promoting neuronal systems
• This organization leads to wakefulness
being associated with both a relatively
high level of activity in the wake-
promoting neuronal arousal systems
combined with a relatively low level of
activity in the opposing sleep promoting
GABA system and vice versa.

13
• A prevailing level of tonic excitation into the respiratory network is essential to drive respiratory rhythm
and muscle activation
• The brain arousal systems of wakefulness provide a major source of such excitation to modulate
breathing volitionally and/or non-volitionally termed behavioral influences or the wakefulness
stimulus
• Such behavioral influences on respiratory network activity are reduced or withdrawn as one moves
from wakefulness to non-REM sleep.
• As a result, the respiratory system becomes
dependent upon feedback regulation in non-
REM sleep to sustain sufficient activity.
• Tonic activity of the peripheral and central
chemoreceptors is normally sufficient to sustain
effective breathing in non-REM sleep
• However, any reduction or defect in feedback
chemoreceptor control, for any reason, causes
severe respiratory disturbance in non-REM
sleep

Importance of feedback regulation: Example 1
Hypocapnia is a potential cause of central apnea. Interestingly, hypocapnia, by itself, is not sufficient to
elicit central apneas in wakefulness or in REM sleep because of the concomitant presence of behavioral
influences on breathing.
However, in non-REM sleep, hypocapnia can elicit central apneas because the stimulatory effects of brain
arousal
(i.e., behavioral influences) to the respiratory network are absent.
Hypocapnia may be present at the onset of sleep as a result of chronic hyperventilation in wakefulness
caused, for example, by heightened chemoreceptor drive (e.g., resulting from congestive heart failure) or
by exaggerated behavioral influences on breathing (e.g., caused by anxiety).
Hypocapnia at sleep onset can also result from the transient hyperventilation caused by sleep disturbance
and brief arousal from sleep: the hyperventilation predisposing to unstable breathing by depleting the CO2
reserve .

ONDINE’S CURSE
In Greek mythology, the nymph Ondine was an immortal water
spirit who became human after falling in love for a man, marrying
him, and having a baby. In one of the versions of the tale, when
she caught her husband sleeping with another woman, she
cursed him to remain awake in order to control his own
breathing.
During the 19th century, the rare syndrome characterized by loss
of autonomic breath control, while voluntary respiration remains
intact, was cleverly named “Ondine's curse”. Nowadays, the term
Ondine's curse is usually associated with Congenital central
hypoventilation syndrome

CONGENITAL CENTRAL HYPOVENTILATION SYNDROME
 a rare neurological disorder
 characterized by inadequate breathing during NREM sleep
 ventilate normally during wakefulness and during REM sleep
 Patients congenitally lack or have poor chemosensitivity to both hypercapnia and hypoxia, and suffer from
inadequate ventilation
 In more than 99% of CCHS patients, mutations have been found in the paired-like homeobox 2b (PHOX2B)
gene, a gene expressed by neurons involved in peripheral and central chemoreception
 PHOX2B is not expressed by respiratory neurons can perhaps explain the ability of these patients to
breathe in wakefulness, because the mutation does not affect respiratory neurons. the state of wakefulness
itself provides sufficient excitatory drive to the respiratory system to mask the major defect in
chemoreceptor activity caused by the PHOX2B mutation.
 The ability of the CCHS patients to breathe normally in REM sleep further reinforces the principle that the
relatively high levels of brain activation inherent to REM sleep can provide sufficient restoration of
behavioral drives to the respiratory network to reinstitute breathing.

Wakefulness as an independent driver of respiratory activity
• Deep non-REM sleep and anesthesia are the most vulnerable states for respiratory rate depression by opioids at the pre-Bötzinger
complex.
1. Opioids depress the rate and depth of respiration at the pre-Böt C to induce ventilatory depression; this was reversed by injection of
naloxone into the pre-Böt C.
2. Opioids also hyperpolarize the KF neurons, contributing to opioid-induced loss of the post inspiration phase of the respiratory cycle and
induction of apneusis.
3. Opioids alter the discharge properties of cranial motoneurons of the larynx and pharynx and the bulbospinal neurons controlling the
diaphragm, chest wall, and expiratory abdominal muscles
-to induce chest and abdominal wall rigidity,
-reduce genioglossus muscle activity and upper airway patency,
-acutely blunt hypoxic and hypercapnic responsiveness
• Breathing can be sustained by wakefulness even when opioids are present at the pre-Bötzinger complex.
• However, loss of this important wakefulness stimulus to breathing can lead to hazardous respiratory depression in non-REM sleep
and anesthesia.
• The clinical relevance of this principle is that sedating agents can be deemed well tolerated in the initially alert patient but, when the
stimulating effects of wakefulness are withdrawn during sleep, the patient may suffer significant respiratory depression
• Particularly dangerous when patients with sleep-related breathing problems use opioids for pain management

SLEEP DISORDERED
BREATHING
• This term is used to describe a continuum of respiratory abnormalities seen during sleep, ranging from
simple snoring to life-threatening obstructive sleep apnea.
• All are characterized by periods of apnea, with or without episodes of airway narrowing or obstruction,
that lead to repeated episodes of subcortical arousal from sleep and arterial hypoxia.
• Four syndromes are described,but there is considerable overlap between them-
1. Upper airway resistance syndrome in which tidal volume and arterial oxygen saturation (SaO2)
remain normal, but at the expense of extensive respiratory effort, which causes over 15 arousals per
hour.
2. Obstructive sleep hypopnea involves frequent (.15 per hour) episodes of airway obstruction of
sufficient severity to reduce tidal volume to less than 50% of normal for over 10 seconds. There may be
small decreases in SaO2.
3. Obstructive sleep apnea is characterized by more than five episodes per hour of obstructive apneas
lasting over10 seconds and associated with severe decreases in SaO2. In fact, durations of apnea may
be as long as 90 seconds, and the frequency of the episodes as high as 160 per hour. In severe cases,
50% of sleep time may be spent without tidal exchange.
4. The last two syndromes are commonly grouped togetheras Sleep apnea/hypopnea syndrome
(SAHS).

The Mechanism of Airway Obstruction
There are four components contributing to airway obstruction during sleep-disordered breathing:
1. An anatomically narrow airway,
2. Inadequate control of airway muscles,
3. The ease of arousal during apnea (the arousal threshold)
4. Instability of the respiratory control system

CENTRAL APNEA
• Central apneas arise from complete or partial reductions in central neural outflow to the respiratory
muscles during sleep
• Central apneas are distinguished from obstructive apneas by the absence of respiratory effort, which
can be detected by routine monitoring techniques such as respiratory inductive plethysmography
combined with assessment of nasal pressure
• Mechanisms which can produce central apneas include
(1) reduced excitation of chemoreceptors as a consequence of hypocapnia and hyperoxia
(2) functional or actual structural medullary damage which may result in a pattern of grossly irregular
ataxic breathing
(3) loss of nonspecific respiratory excitatory stimulation (noise, light, tactile stimuli) in the absence of
adequate
chemical drive
(4) active suppression of breathing by respiratory inhibitory reflexes, which is observed in a number of
clinical
situations including pharmacological therapy with methadone and other opiate medications. Reflex
inhibition may arise from the cardiovascular system, from the lung and chest wall, or from somatic and
visceral afferents.

AGE
CSA is more prevalent in older individuals than in middle-aged adults. Sleep state oscillations may precipitate
central apnea in older adults
Children have risk of obstructive sleep apnea if they have adenotonsillar hypertrophy
SEX
Women are less susceptible to the development of hypocapnic central apnea during NREM sleep compared with
men.
Administration of testosterone to healthy premenopausal women for 12 days resulted in an elevation of the
Apnea Threshold(AT) and a diminution in the magnitude of hypocapnia required for induction of central apnea
during NREM sleep.
Conversely, suppression of testosterone with leuprolide acetate in healthy men decreased the AT.
GENETIC FACTORS
Several transcription factors are responsible for control of breathing.
The transcription factor Dbx1 is essential for pre-Böt C development; deletion of Dbx1 eliminated all pre-Böt C
glutamatergic respiratory neurons, with complete elimination of inspiratory activity
RTN development is particularly vulnerable to a PHOX2B mutation that causes congenital central hypoventilation
syndrome (CCHS) in humans

OBESITY
The obesity-related disorders of
obstructive sleep apnea and
hypoventilation are fairly well defined,
obesity also impacts other conditions
such as asthma and chronic
obstructive pulmonary disease.

Summary
1. History
2. Controller
3. Effector
4. Sensor
5. Loop gain
6. Sleep state and disorders
7. Several factors

THANK YOU

Control of Ventilation in Health & Disease

Recommended

Recommended

More Related Content

Similar to Control of Ventilation in Health & Disease

Similar to Control of Ventilation in Health & Disease (20)

Recently uploaded

Recently uploaded (20)

Control of Ventilation in Health & Disease