Mechanical ventilation is a mixed blessing as its potential good is not always good enough. While offering hope of prolonged life, mechanical ventilation has drastic implications for the quality of life. Whether a particular individual will benefit from mechanical ventilation is initially a medical judgment. Often, however, no clear diagnosis has been established, and even when one has, the individual's prognosis may remain highly uncertain. The patient, family members, physicians, nurses, and other professional caregivers may not agree with each other on the prognosis and thus, the decision-making reverts from the medical expertise to the realms of psychology, ethics, religion, economics, and law. Furthermore, the costs associated with this technology are enormous. Therefore, for severely ill patients, their families, and those required to make health care decisions, the long-term use of this technology can be the source of considerable anguish.
A patient information series published by the American Thoracic Society gives the following description.
Mechanical ventilation is a life support treatment. A mechanical ventilator is a machine that helps people breathe when they are not able to breathe enough on their own. The Mechanical ventilator is also called a Ventilator, Respirator, or Breathing machine. Most patients who need support from a ventilator because of a severe illness are cared for in a hospital's Intensive Care Unit (ICU). People who need a ventilator for a longer time may be in a regular unit of a hospital, a rehabilitation facility, or cared for at home.
WHY ARE VENTILATORS USED?
- To get oxygen into the lungs.
- To get the lungs get rid of carbon dioxide.
- To ease the work of breathing. Some people can breathe, but it is very hard. They feel short of breath and uncomfortable.
- To breathe for a patient who is not breathing because of brain damage or injury (like coma) or high spinal cord injury or very weak muscles.
- If a patient has had a serious injury or illness that causes breathing efforts to stop, a ventilator can be used to help the lungs breathe until the person recovers.
This description appears very simple and easy to understand even for a lay man. In clinical practice, it may not be as simple as it sounds, but is a little more.
Many people including some clinicians look at a ventilator with a sense of distaste. Some feel that it is very difficult to understand and manage. The truth is far from that; it is not anything very special; one must realize that a ventilator, after all is a man made machine used worldwide by clinicians, and there should be no difficulty to understand and manage it.
In the past, Anesthesiologists were expected to know how the ventilators work, as many of them were designed on simple mechanical principles. Now, almost all the modern ventilators are microprocessors controlled (computerised) and the technology is so complex that acquiring this extra knowledge is no longer reasonable or justifiable. Now we have the new generation of specialists in every hospital known as Biomedical Engineers and they have the knowledge and maintain the machines.
At this point, we shall recall the following statement of JS Robinson.
“The user must know what the ventilator can do, not how it does that”
—JS Robinson
- Ventilators come in all sizes and shapes to suit many environments and pockets.
- Some are simple bag squeezers and others are versatile microprocessor controlled machines with monitors and alarms.
Not all the time we may be able to procure the most sophisticated ventilator available in the market costing huge money. Nevertheless, we may have a ventilator which can perform the basic functions of a good ventilator. With a good knowledge of applied basic sciences related to respiratory system, probably the available ventilator can be used on vast majority of patients, giving optimum benefits without any problems.
Most of our patients require only the basic modes of ventilation and only very few of them (such as those with chronic respiratory or cardiovascular illness with structural changes in the lung) need sophisticated modes and settings. This simple fact could be realized by younger colleagues in the course of their clinical practice.
Here we may recall the following famous quotation, by Peter Nightingale and J Denis Edwards.
“Unfortunately many Newer Modes have been introduced merely on the basis of Technical Ability rather than as a result of a defined clinical need or demonstrable advantage to the patient.”
—Peter Nightingale and J Denis Edwards
This statement does not indicate that there is no scope or need for further research in this field and for the development of newer modes of ventilation, but it indicates that available modes can safely be used without grumbling that a better ventilator with the newer modes is not available.5
First of all, a thorough knowledge of applied anatomy and physiology of respiratory system and the mechanics of normal respiration is necessary to understand mechanical ventilation and apply it clinically.
There are certain questions to be answered before instituting ventilator support for a patient.
- What exactly is the requirement of the patient?
- What exactly the machine which we have with us can do?
- How best this machine can be used to meet the requirement of the patient?
- Finally, the most important question is, whether the patient requires ventilatory therapy at all?
Though looking at it superficially, this last question may appear absurd, but long clinical experience has established that many a times, making this decision is very difficult and eventually ventilator therapy is instituted in patients not requiring it. This fact is infrequently realised in clinical practice, because modern ventilator therapy normally does not do any harm.
If the basic metabolism is reduced due to any reason, eventually O2 requirement is reduced, CO2 production is reduced, less metabolite are produced, and the tendency for acidosis is less. Hence minimal reduction in ventilation will not cause hypoxemia and metabolic acidosis as proved by Arterial Blood Gas study.
There are a few more questions which could be answered by an assessment based on clinical evaluation of ventilation along with serial Arterial Blood Gas results.
- When to put a patient on ventilator?
- What mode of ventilation is needed for the patient?
- When to start weaning the patient?
When someone has inadequate ventilation, some form of support to sustain near normal ventilation without 6causing additional damage is quiet sufficient for sustaining and saving the life. If ventilation is maintained for some time even with the crudest method available at hand, time can be bought to get the right type of ventilator needed for that patient.
An event in the history has clearly proved us that even an ordinary resuscitator bag such as Ambu Bag or other such resuscitator bags can be used to maintain ventilation for many days and thus sustain life.
In 1952 in Denmark there was a severe epidemic of paralyzing poliomyelitis. At Blegdam Hospital in Copenhagen, the apparatus available were only one tank ventilator and six cuirass ventilators, but the number of patients requiring respiratory assistance was very high. By this time “controlled respiration” was well established in anesthesia and as a last resort this technique was extended to patients needing long-term ventilation. At one time 70 patients were receiving respiratory assistance from “ventilators which consisted of medical students squeezing the bag of a to and fro system with carbon dioxide absorption”. Medical students were made to do this in a shift of 8 hours duty and were paid for each shift. More than 30 patients survived.
Mostly, improper management in ventilator therapy is likely to be caused by any of the following reasons.
- Improper assessment of the patient's condition.
- Inability to decide about the patient's requirement for ventilatory support.
- Inadequate knowledge about the ventilator settings (Not the mechanism by which it ventilates the lung).
- Not knowing the limitations of the ventilator available, to do the job which we expect.
At this point, there may be a need for a question from the reader; “Will I be able to operate all ventilators by reading this book?” The straight forward, simple and honest answer is— “It may not be possible immediately”.
However, the descriptions in this book will give a very clear idea as to how the basic sciences could be usefully applied to a patient on ventilator. That is, with the orientation of the mechanism of normal respiration and the mechanism of artificial respiration in mind, applying artificial ventilation for the patient. Then make the necessary modulations in that, to achieve the best form of ventilation (the best suited mode) for the particular patient. Certainly everything else can be built on that basis.
The principle involved in their use must be understood. If an unfamiliar ventilator is encountered, for the first time, we will certainly be worried how to operate it? It can be done by any one of the following methods.
- The manufacturer's “User hand book” must be used. Carefully read the operating manual fully. Then connect a dummy lung (a rubber bag meant for that purpose) to the ventilator and try all the modes and settings in that to understand it well. It is always helpful to use a “dummy lung” and understand the “Capabilities” and “Limitations” of a particular ventilator.
- Getting the relevant information directly from some one who is using the particular ventilator routinely.He can explain briefly the operating modalities; modes and settings, and the method of operating it. He can even operate it and explain all about it. It will be an easier short cut method of knowing about it.
It is potentially hazardous to connect the patient to an unfamiliar ventilator and attempting to set the mode and other settings. As the patient who needs ventilator support 8is usually a critically ill person, he may not stand even minimal insults in this process.
In such a situation, the patient can be manually ventilated by connecting him to an Ambu Bag or to an anesthetic machine. In the meanwhile a dummy lung can be connected to the ventilator and operated to know clearly about the settings and then the ventilator with proper settings needed may be connected to the patient.
Whatever way it is done, we have to be concerned about four main aspects.
1. | Volume of ventilation: | It must be adequate. Not more. |
2. | Mean airway pressure: | It must be optimal. |
3. | Distribution of gases in lung: | It must be uniform to all areas. |
4. | Diffusion of gases: | It must be adequate. If not adequate, FRC has to be slightly raised so that more alveoli are recruited to take part in the diffusion |
For that purpose, the knowledge of applied aspects of the following is essential and is discussed in the preliminary chapters.
- Respiratory Anatomy
- Respiratory Physiology
- Respiratory Mechanics
- Mechanics of Artificial Respiration (Mechanical ventilation).
The sophistications if needed can be added one by one in the ventilatory support after carefully studying the actual requirement of the patient and also based on his improvement after starting the therapy.
In a critically ill person, once ventilatory therapy is instituted, the normal physiological range of pressures, both “intrapulmonary” and “intrapleural” may be modified causing significant hemodynamic compromise 9and in turn may result in serious cardiovascular collapse. This is likely to be more severe, if the patient has depressed cardiovascular reflexes either due to autonomic imbalance or due to severe depression of central nervous system.
When the ventilator therapy is instituted, the means for managing such an eventuality if occurs must be at hand.
The most important aspect of respiration is that in normal conditions of life it happens by the control of the respiratory center, modulated by various reflexes related to the regulation of respiration. It is so effortless that obviously no one normally realizes that breathing is going on. Only when there is a problem, respiration is realized as a work and that increased work of breathing is unpleasant and distressing. It is abnormal (pathological), and there we may need a ventilator to support respiration.
With this information of introduction we may have further discussions about the ventilator therapy to understand it in the simplest way possible.
BIBLIOGRAPHY
- Peter Nightingale and J. Denis Edwards. Critical Care. In: Wylie and Churchill-Davidson's A Practice of Anaesthesia. Sixth Ed: Edward Arnold. London 1995.
- Robinson JS. Respiratory care. In: Medical Management of the Critically Ill. Ed. Hanson GC and Wright PL Academic Press London: 1978.