#kYJ - Valsalva during removal of tubes
I was sent this question:
Rob can you please explain the rationale of holding breath for 10 seconds during removal of CVC!
I understand this action minimizes risk or air embolus .... But how?
...
You may know that this is a common instruction given to patients when we remove CV lines and chest tubes. We do it to minimise the risk of air entering the thoracic cavity when pulling out the tubes.
Ok so here is the physics.
To understand the rationale you must understand 4 concepts.
1 Atmospheric pressure
2 intrathoracic pressure
3 intrapulmonary pressure
4 Pascale's principle
But First, a refresher on the pressures.
All pressure (air pressure, blood pressure, intracranial pressure etc) can be measured using different pressure units.
In medicine we use
Centimetres of water(cmH2O) or millimetres of mercury (MmHg) also called "torr" but only nerds say Torr. Eg blood pressure might be 120/80 MmHg.
Car tyres use pounds per square inch (PSI) or kilo pascals (KPa). Eg a full tyre may be pumped up to 36 psi or 250kpa.
Atmosphere and weather reports use Bars or pascals.
1 bar = 1000 millibars=100000 pascals.
(100 KPa or 1000hectopascals)
Thus a millibar is also a hectopascal.
It's confusing and looks like a horrible highschool maths nightmare but just know that all units of pressure can be converted into each other.
1 Atmospheric pressure is the pressure of air that we breathe. At sea level it is 760mmHg. If you saw it on the evening weather forecast map they would say 1013 hPa.
This atmospheric pressure is all around us and fluctuates slightly. A tropical low might be reported at 1004hPa and a cyclone reported as 890hPa.
1013 hPa converts into 760MmHg
I will use MmHg because we are speaking in the medical context.
Just know that seal level pressure is 760.
2. Intrathoracic pressure is the total pressure inside your thorax. Predominantly influenced by the changing pressure in your lungs as you breathe. This brings us to intrapulmonary pressure.
3. Intrapulmonary pressure is the pressure inside your lungs. As you go to breathe in, your diaphragm contracts, drops, and creates a vacuum or negative pressure change inside you lungs. The pressure in your lungs is less than atmosphere so considered to be a relative negative pressure when compared with the atmosphere. The change is small, only 6-8 MmHg, but enough to allow the atmosphere to rush into your lungs to equalise pressure inside. We call this breathing in, or negative pressure ventilation.
On inspiration
Atmospheric pressure 760
Intrapulmonary pressure 752
A relative vacuum.
Now breathing out is the opposite. I must push air pressure inside my lungs to blow air out to the world. Diaphragm relaxes, rises and puts the lungs under higher pressure than the world.
On expiration
Atmospheric pressure 760
Intrapulmonary pressure 765+
Air rushes out.
Stay with me we have nearly answered the question.
Inside the chest wall, the pressure emulates the fluctuation of the inside of the lungs. Thus intrathoracic pressure rises on inspiration and falls on expiration. This close relationship is described by Pascal's principle which suggests that a pressure increase in one area of a confined space (thorax) is equally transmitted across the whole confined space. I know... Boring!!!
Back to holding your breath.
Take a big breath in now and hold it. Hold it! Hold it!
Feels full hey? Ok breathe normally.
When you held your big breath (valsalva manoeuvre) you held pulmonary (and thoracic pressure) at its highest point relative to the atmosphere.
Your thoracic pressure while breath holding was perhaps 770mmHg, and thus higher than the word. It would be impossible for air to enter your lungs or chest cavity through surgical holes, because the pressure inside you was higher than atmospheric pressure.
Asking the patient to hold their breath during tube removal then makes it impossible for air to invade the hole before you seal it with an opsite or other occlusive dressing.
The pressure inside was too high.
Now the consequence of air getting in through the CVC site is an air embolism. A very small risk, and even if air did get in it enters the venous network and bubbles flow into the right atrium. Bubbly blood is then pumped to the lungs, where most bubbles are filtered out.
In the context of a chest drain tube; well they are sitting in the pleural cavity not a central vein. The tiny bit of air that can get in while pulling out a tube is insignificant and reabsorbs in a hour or so... But safe practice implores you as a nurse to take steps to avoid this introduction of air into a chest when pulling out tubes, drains and CV lines.
Just get them to hold their breath.
Pressure
