|
Advances in Critical Care 'Mechanical ventilation'
Years of research in ventilator designs and close co-operation
with leading clinical researchers have shown us the importance of improving
the flow delivery and regulation to suit the unique needs of individual patients
Mr.Jan Sandberg
Maquet Critical Care AB
Solna,Sweden |
Critical care ventilation has evolved extensively over the
past several decades. What started as an air-pumping device following a big
polio epidemic is today a highly developed clinical tool for respiratory support
at the critical care bedside. For the most effective usage of the modern day
ventilators, besides the irreplaceable sound clinical foundation - an in-depth
understanding of the machine's technological capability has become a necessity.
The strength of a 'mechanical' ventilation platform is not determined by a mere
sum total of its various technical specifications, but by the 'harmonised' packaging
of its various component sub-systems aimed at a desired clinical outcome. Like
an orchestra is not judged by the number of musical instruments in it but by
the melody produced, a ventilator is to be judged by its impact on possible
favourable patient outcomes, and not just by the number of microprocessors,
displays or modes.
Today's high-speed signal processing and actuation technology together with
an intelligent 'on-board' firmware allows us several clinical benefits like:
- Quicker weaning
- Minimal lung damage
- Effective oxygenation and gas exchange
- Minimal influence on pulmonary and systemic circulation
- Lesser side-effects
Years of research in ventilator designs and close co-operation with leading
clinical researchers have shown us the importance of improving the flow delivery
and regulation to suit the unique needs of individual patients. The newer ventilation
platforms can enhance interaction between the patient and the ventilator by
offering unprecedented levels of speed in sensing and control as well as a range
of ventilation modes and treatment features, which will help clinicians address
the specific needs for a wide array of patient characteristics. Forward-looking
ventilation platforms such as the Servo provide the clinicians with newer possibilities
in critical care ventilation therapy, such as:
- NAVA - Neurally Adjusted Ventilatory Assist for improved
patient-ventilator interaction
- Heliox option: for reducing work of breathing
NAVA
NAVA senses activity in the diaphragm and responds by providing the requested
level of ventilatory assist. The Edi signal is obtained by an electrode
array mounted close to the distal tip of the Edi catheter |
The NAVA breakthrough clinical application allows a ventilator
to respond to the body's own respiratory demands leading to improved synchrony
between the patient and the ventilator.
The act of breathing depends on rhythmic discharge from the respiratory centre
of the brain. This discharge travels along the phrenic nerve, excites the diaphragm
muscle cells, leading to muscle contraction and descent of the diaphragm dome.
As a result, the pressure in the airway drops, causing an inflow of air into
the lungs.
Neuro-Ventilatory Coupling: NAVA senses the electrical activity of the diaphragm
(Edi), the earliest respiratory signal that can be detected. Conventional technology
is limited to sensing patient effort at the final stage of the respiratory process.
NAVA senses activity in the diaphragm and responds by providing the requested
level of ventilatory assist. The Edi signal is obtained by an electrode array
mounted close to the distal tip of the Edi catheter. This catheter can also
serve as a conventional nasogastric feeding tube. Conventional mechanical ventilators
sense a patient effort by either a drop in airway pressure or a reversal in
flow. The last and most slow reacting step in the chain of respiratory events
is used to sense the patient effort. Hence, creating a system that is sensitive
to hyperinflation, intrinsic PEEP and secondary triggering problems.
With NAVA, the electrical activity of the diaphragm (Edi)
is captured, fed to the ventilator and used to assist the patient's breathing.
As the ventilator and the diaphragm work with the same signal, mechanical coupling
between the diaphragm and the ventilator is practically instantaneous.
Properly positioning of the catherter is confirmed by a prominent P-wave
in the uppermost channel with a continued decline of P-wave amplitude
in the lower leads. |
|
|
|
|
Servo-i with Heliox option |
NAVA senses activity in the diaphragm and responds by providing
the requested level of ventilatory assist. The Edi signal is obtained by an
electrode array mounted close to the distal tip of the Edi catheter.
NAVA at bedside
Maquet currently offers full NAVA capability as an optional
upgrade on new as well as existing Servo-i ventilators. The only equipment required
in addition to Servo-i ventilator is NAVA software, and Edi module with cable
and Edi catheter. The same module can be used interchangeably with different
Servo-i ventilators. The Edi catheter also functions as naso-gastric feeding
tube, and comes in different dimensions to cover all patient categories from
neonatal to adult.
Easy Application and Connectivity
The NAVA Edi catheter is as simple to apply as any standard
nasogastric tube. However, positioning of the Edi catheter takes on added importance
to ensure a strong Edi signal and accurate readings.
The Edi catheters come in different sizes ranging from 6 to 16 Fr. and are chosen
according to the patient's height. A Nose-Ear-Xiphoid (NEX) measurement protocol
distance is used for each patient to determine insertion length for both oral
as well as nasal insertion. The Edi catheter is dipped in water to activate
its lubricant prior to insertion while taking due precautions to avoid wetting
the connectors and then inserted like any other feed tube.
With the Edi catheter inserted and positioned, all that remains
is to plug the Edi module into the Servo-i and connect the Edi catheter to its
outlet. The esophageal ECG now showing on the Servo-i screen can help confirm
proper Edi catheter positioning.
Setting up NAVA
NAVA ventilation mode setting is quite simple and is done
as follows:
- Basic settings: NAVA level is set in cm H2O/µV
and the default setting is 1.0 cmH2O/µV.
PEEP and O2 Concentration - as usual.
Edi trigger level is set in µV ( in a range of 0.1 and 2.0 µV).
70 per cent of the peak Edi signal for normal and high Edi signals (40 per cent
for low Edi signals).
Values for pneumatic trigger level, inspiratory cycle off
and pressure support level are selected.
During NAVA, the amount of pressure delivered (in cm H2O)
is adjusted by multiplying the Edi (which is expressed in µV) by a proportionality
factor, called the 'NAVA level' (expressed as cm H2O/µV). The NAVA level
expresses a type of exchange rate, that is, how many cm H2O the patient will
receive per µV Edi. For example a NAVA level of 1 cmH2O/µV will
give 5 cm H2O when the Edi signal is 5 µV. Following the same example,
for the same diaphragm activity of 5 µV, increasing the NAVA level to
2 cm H2O / µV doubles the pressure delivered and provide 10 cmH2O.
Benefits with NAVA
- Improve patient ventilator interaction:
Synchrony by proportionally assisting each breath with minimum
time delay.
- Enhance respiratory monitoring:
Monitoring the electrical activity of the diaphragm (Edi).
Information about the inspiratory drive and the brain signaling
Heliox option:
Helium is an inert gas present in the atmosphere (0.00052 per cent). It was
discovered as early as 1868 by a French astronomer named Jansen. As a noble
gas, helium is an unreactive, colorless, odorless, tasteless, non-toxic, monoatomic
gas. Although it is the second most abundant element in the universe (after
hydrogen), it is relatively rare on earth, where it is extracted from natural
gas.
Because helium is less dense than both oxygen and nitrogen,
it has advantages in terms of maintaining laminar gas flow. For a given pressure
difference, laminar gas flow provides a greater flow rate compared to turbulent
gas flow, making it more efficient. The lower density of the helium in Heliox
leads to a lower pressure drop in obstructive airways than would be the case
with air/ oxygen. This improves gas distribution and facilitates expiration.
The lower density and better flow conditions should also mean that patients
who are experiencing difficulty breathing need to make less effort to overcome
airway resistance and ventilate themselves adequately. It should be noted, however,
that if one uses the same pressure when ventilating with Heliox as when administering
oxygen/ air, the pressure in the alveoli will be higher with Heliox.
The applications for helium within respiratory care are thus
related to its physical rather than its chemical properties. Heliox mixtures
of helium and oxygen have been used in a medical context since the 1930s. They
are typically three times less dense than air, helping the patient to breathe
more freely. There are no known contraindications for the use of Heliox. At
a given flow, transairway pressure will be lower with Heliox than with air or
air-oxygen mixtures. The higher the helium concentration, the lower the pressure.
When transairway pressure is constant, the flow through the airways will be
higher with Heliox, and the higher the helium concentration, the higher the
flow compared to air or air-oxygen.
This means that Heliox decreases the effort involved in breathing
and potentially relieves dyspnea. In addition, CO2 diffuses four to five times
better through Heliox, which should increase its elimination and improve gas
exchange. It also potentially decreases air trapping (auto PEEP) and hyperinflation
of the lungs, as well as transairway pressure, and improves distal airway deposition
of aerosol particles.
Heliox has thus been shown to reduce work of breathing and
lead to a better clearance of carbon dioxide. In addition there have been a
number of claims regarding its respiratory benefits, and there is considerable
interest in the scientific community in further studies of both benefits and
clinical applications.
Heliox is used on both pediatric and adult patients, and even in neonates. Its
use was initially adopted by the medical community to alleviate symptoms of
upper airway obstruction, such as croup (respiratory problem, mainly pediatric,
often due to a virus, that produces a harsh crowing sound during inhalation),
epiglottitis (inflammation/infection of the epiglottis), laryngitis (inflammation/infection
of the larynx), tracheitis (inflammation/infection of the trachea) and tumors.
Since then, however, its range of medical uses has expanded to include lower
airway obstruction, such as asthma (episodic reversible attacks of airway obstruction),
Chronic Obstructive Pulmonary Disease (COPD, chronic obstruction of the airflow
in the airways), bronchiolitis (inflammation/infection of the bronchioles) and
cystic fibrosis (inherited disease that causes mucus to become thick and sticky
and obstruct the airways, among others). Most of these uses relate to the low
density of Heliox.
Effects of Heliox therapy according to the literature:
- Established positive effects on ventilation
- Reduces work of breathing
- Increases CO2 diffusion
- Potential positive effects on ventilation
- Reduces the peak pressure needed to maintain flow
- Increases CO2 elimination and improves gas exchange
- Increases expiratory flow
- Reduces respiratory rate and auto PEEP
- Increases drug deposition in distal parts of the
airways
Maquet currently offers Heliox capability as an optional upgrade on new as well
as existing Servo-i ventilators.
Clinical Information & User Experiences
For more clinical information and user experiences, please visit www.criticalcarenews.com
|
