Living and Breathing with
Weak Respiratory Muscles
from an article written in 2004 by - Dr (Hal) John Hester Colebatch AM MD FRACP
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Dr John Colebatch was born
in 1928 at Northam, WA. His first year
study of medicine was at UWA. He completed and graduated at |
25 years after I had polio, it became clear to me that muscle
weakness was returning and gradually its severity increased until once
again I was unable to walk and required assistance for showering and
dressing. The relentless progress to
dependence has been a deeply distressing experience to me.
In 1991, after a “Sleep Study” I was advised to use a C-PAP
machine during the night. I soon realized that CPAP was not the appropriate
treatment for anyone with weak inspiratory muscles. In September 1992, after another “Sleep
Study”, I started using a fixed volume ventilator at night. Initially, there was some improvement in
sleep quality, but because it prevented spontaneous breathing, I found this
ventilator intolerably uncomfortable to use.
After three weeks of this nocturnal ventilation, I became troubled
with irregular heart beats. By
decreasing .the amount of ventilation during the night, the irregular heart
beats were abolished and breathlessness decreased.
In November 1992, I started using a BiPAP Ventilator which was
intended to allow me to initiate breathing and be comfortable to use. However,
this early model was unduly sensitive and was triggered into inspiration
because the heart beat caused a small flow of air (about 10 ml) into the
lungs. As a result, my breathing was not
normally regulated, but was abnormally increased because it was initiated by
the heart beat rather than by my own inspiratory efforts. I was soon troubled again with severe
daytime breathlessness and irregular heart beats and, in addition, a
feeling of impending death. These
problems were overcome by the use of a modified BiPAP which was not triggered
by the heart beat.
Recognition of Failing Breathing Muscles
Difficulty with breathing is likely to develop in post-polio subjects
who suffered respiratory muscle paralysis during the acute stage of their
illness. Initially there is increased
breathlessness and fatigue and, later, difficulty sleeping. To assess this problem it is essential to
measure breathing capacity,
and arterial blood gases.
Breathing capacity is usually assessed by measurements of vital capacity
(VC) and the maximum volume expelled in 1 second (FEV1). Before there is a
serious problem with respiratory failure, both VC and FEV1 decrease to less
than 50% of the expected value. But a
more significant measurement is inspiratory capacity (IC) or the volume which
can be inspired from the resting position.
It is this volume which represents how much the subject can breath.
As lung volume increases, inspiratory muscles are shortened and IC
decreases, that is, the ability to breath in decreases. That is why it is for me, more difficult
to breath when sitting or standing compared with lying down. A tight belt around the lower abdomen will
decrease lung volume when standing and increase IC and therefore the ability to
inspire. Arterial blood gases require
collection of arterial blood which is normally done in a hospital.
An increased arterial CO2 level defines the presence
of chronic respiratory failure. By the time this becomes a problem, arterial CO2 will usually have
risen from a normal level of 40-45 mm Hg to around 55 mm Hg. If arterial CO2 remains in the normal range,
assisted ventilation is unlikely to be required. In the presence of normal
lungs, oxygen saturation is well maintained, so that oximetry is unreliable and
of little value in assessing the presence of chronic respiratory failure.
The respiratory muscles and chest wall together make a pump which moves
air into and out of the lungs and is referred to as “ventilation”.
The pump is driven by the nervous system.
When the pump cannot provide adequate ventilation there is a feeling of
breathlessness and the arterial CO2 increases until the quantity produced is
expired in the smaller volume of ventilation. This situation is referred to as
“chronic respiratory failure”.
When weakness of breathing muscles has caused chronic
respiratory failure nocturnal ventilation is required. This ensures adequate ventilation during the
night and allows recovery of fatigued muscles.
The aim of this regimen is to maintain activity and independent
breathing during the day. To minimise
the increase in lung volume with gravity, when sitting or standing, it is
essential to provide abdominal support.
This will help to maintain the ability to inspire.
Managing Nocturnal Ventilation
When breathing is failing, one of the symptoms is
difficulty sleeping. Sleep studies may be undertaken to help define
the breathing problem. However, these studies are complex and in my view may
lead to errors in management. Notwithstanding evidence of obstructive sleep
apnoea, which is commonly observed, treatment with CPAP is not appropriate
for anyone with weak respiratory muscles.
This is because an increase in airway pressure increases lung volume and
shortens respiratory muscles making it more difficult to further increase lung
volume and maintain breathing. It is as if the subject were sitting up all
night instead of lying down and instead of resting the inspiratory muscles it
makes their work more difficult. Breathing is driven by the acidity of
fluids around the brain which in turn reflect metabolic activity.
The best type of breathing assistance is one which allows this
regulation to continue in its natural state. That is, the subject’s brain
should drive the ventilator as part of the chest pump, the ventilator should
not determine the amount of ventilation independently of the subject’s
metabolism.
It follows that the subject initiates inspiration to which the machine
responds and supports – a demand-driven machine. A pressure support
ventilator fits this criterion and such machines are in common use.
My preference is for the BiPAP Ventilator operated in
demand mode. I have used this ventilator every night for
more than eleven years. Fixed-volume
ventilators are uncomfortable to use at night for anyone who retains some
ability to breathe independently. They
also do not permit the subject to regulate his breathing. A similar disadvantage applies to a
pressure-assist ventilator set to a fixed time, which excludes the subject from
initiating inspiration. If there is
concern about the occurrence of apnoea during sleep, this possibility can be
overcome by setting the ventilator to a slow back-up rate, say 8 per minute,
which will maintain sufficient ventilation if needed, but not interfere with
the subject’s own breathing.
Improving Daytime Activity
One of the objects of nocturnal ventilation is to improve daytime
activity. For this to be achieved the level of ventilation during the night
must not exceed what the subject can maintain when breathing independently
during the day. This is best achieved by using a ventilator in demand mode with
the subject retaining regulation of breathing.
There should be no attempt to decrease to “normal” an
increased arterial CO2. The level of
CO2 is set by the ability of the chest pump to respond to the demands for
breathing during daytime activity. To
lower CO2 during the night, in these circumstances by excessive ventilation,
will increase daytime breathlessness.
It may also be dangerous by causing irregularity of the heart beat. Normally CO2 is a little higher during the
night than during the day and this situation should be allowed to continue.
If excessive nocturnal ventilation is suspected it may be worthwhile to
measure the electrolytes. The risk is an abnormal loss of potassium
resulting in irregularity of the heart beat. This disorder may be fatal. Potassium
supplements will correct this problem. Weakness of post-polio muscles is an
ongoing problem which cannot be overcome by exercise. It is sufficient to try to maintain the
activities of daily living. To do this
it is essential to have adequate periods set aside for rest during the day. (Exercise programs are for an earlier stage of
life.) Post-polio paralysis is not a
disease of muscle, rather it reflects a loss of those motor nerve cells and
fibres on which initial recovery depended.
The surviving muscles with their remaining nerve supply are normal. It follows that nutritional supplements or
anabolic steroids aimed at improving muscle function are unlikely to have any
lasting effect. On the other hand an
adequate intake of the vitamin B group may be helpful for their essential role
in the nervous system.
Besides exercise, metabolism and with it the demand for breathing is
increased by intake of food. Breathlessness
during the day, while at rest, is distracting and can be decreased by limiting
intake of food to the smallest amount that can be tolerated. This regimen has the additional benefit of
discouraging an increase in body weight when any increase is a serious
disadvantage for continuing activity. In
situations where exertion cannot be avoided such as toileting, it is an
advantage to have the use of a ventilator.
This can make the difference between dependence and independence.
The slowly progressive nature of the post polio syndrome means that new
challenges to activity and independence arise year by year and even each few
months. To meet the particularly
difficult challenge of respiratory failure and yet maintain some independent
activity, it is essential to consider carefully all possible adjustments. Simple things such as limiting daytime intake
of food and ensuring rest periods can help to reduce discomfort and make the
days a little more enjoyable.