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Friday, March 18, 2011

Mechanical ventilation

This may involve a machine called a ventilator or the breathing may be assisted by a physician or other suitable person compressing a bag or set of bellows. Traditionally divided into negative-pressure ventilation, where air is essentially sucked into the lungs, or positive pressure ventilation, where air (or another gas mix) is pushed into the trachea.

It can be used as a short term measure, for example during an operation or critical illness (often in the setting of an intensive care unit). It may be used at home or in a nursing or rehabilitation institution if patients have chronic illnesses that require long-term ventilatory assistance.

Owing to the anatomy of the human pharynx, larynx, and esophagus and the circumstances for which ventilation is required then additional measures are often required to "secure" the airway during positive pressure ventilation to allow unimpeded passage of air into the trachea and avoid air passing into the esophagus and stomach. Commonly this is by insertion of a tube into the trachea which provides a clear route for the air. This can be either an endotracheal tube, inserted through the natural openings of mouth or nose or a tracheostomy inserted through an artificial opening in the neck. In other circumstances simple airway maneuvres, an oropharyngeal airway or laryngeal mask airway may be employed. If the patient is able to protect their own airway such as in non-invasive ventilation or negative-pressure ventilation then no airway adjunct may be needed.

Mechanical ventilation is often a life-saving intervention, but carries many potential complications including pneumothorax, airway injury, alveolar damage, and ventilator-associated pneumonia.

In many healthcare systems prolonged ventilation as part of intensive care is a limited resource (in that there are only so many patients that can receive care at any given moment). It is used to support a single failing organ system (the lungs) and cannot reverse any underlying disease process (such as terminal cancer). For this reason there can be (occasionally difficult) decisions to be made about whether it is suitable to commence someone on mechanical ventilation. Equally many ethical issues surround the decision to discontinue mechanical ventilation.

Negative pressure machines

The iron lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative-pressure machines used for long-term ventilation. It was refined and used in the 20th century largely as a result of the polio epidemic that struck the world in the 1940s. The machine is effectively a large elongated tank, which encases the patient up to the neck. The neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air.

While the exchange of oxygen and carbon dioxide between the bloodstream and the pulmonary airspace works by diffusion and requires no external work, air must be moved into and out of the lungs to make it available to the gas exchange process. In spontaneous breathing, a negative pressure is created in the pleural cavity by the muscles of respiration, and the resulting gradient between the atmospheric pressure and the pressure inside the thorax generates a flow of air.

In the iron lung by means of a pump, the air is withdrawn mechanically to produce a vacuum inside the tank, thus creating negative pressure. This negative pressure leads to expansion of the chest, which causes a decrease in intrapulmonary pressure, and increases flow of ambient air into the lungs. As the vacuum is released, the pressure inside the tank equalizes to that of the ambient pressure, and the elastic coil of the chest and lungs leads to passive exhalation. However, when the vacuum is created, the abdomen also expands along with the lung, cutting off venous flow back to the heart, leading to pooling of venous blood in the lower extremities. There are large portholes for nurse or home assistant access. The patients can talk and eat normally, and can see the world through a well-placed series of mirrors. Some could remain in these iron lungs for years at a time quite successfully.

Today, negative pressure mechanical ventilators are still in use, notably with the Polio Wing Hospitals in England such as St. Thomas' (by Westminster in London) and the John Radcliffe in Oxford. The prominent device used is a smaller device known as the cuirass. The cuirass is a shell-like unit, creating negative pressure only to the chest using a combination of a fitting shell and a soft bladder. Its main use is in patients with neuromuscular disorders who have some residual muscular function. However, it was prone to falling off and caused severe chafing and skin damage and was not used as a long term device. In recent years this device has re-surfaced as a modern polycarbonate shell with multiple seals and a high pressure oscillation pump in order to carry out biphasic cuirass ventilation.

Positive pressure machines
The design of the modern positive-pressure ventilators were mainly based on technical developments by the military during World War II to supply oxygen to fighter pilots in high altitude. Such ventilators replaced the iron lungs as safe endotracheal tubes with high volume/low pressure cuffs were developed. The popularity of positive-pressure ventilators rose during the polio epidemic in the 1950s in Scandinavia and the United States and was the beginning of modern ventilation therapy. Positive pressure through manual supply of 50% oxygen through a tracheostomy tube led to a reduced mortality rate among patients with polio and respiratory paralysis. However, because of the sheer amount of man-power required for such manual intervention, mechanical positive-pressure ventilators became increasingly popular.

Positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube. The positive pressure allows air to flow into the airway until the ventilator breath is terminated. Subsequently, the airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume -- the breath—out through passive exhalation.

Indications for use

Mechanical ventilation is indicated when the patient's spontaneous ventilation is inadequate to maintain life. It is also indicated as prophylaxis for imminent collapse of other physiologic functions, or ineffective gas exchange in the lungs. Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease, the patient's underlying condition should be correctable and should resolve over time. In addition, other factors must be taken into consideration because mechanical ventilation is not without its complications (see below)

Common medical indications for use include:

Acute lung injury (including ARDS, trauma)
Apnea with respiratory arrest, including cases from intoxication
Chronic obstructive pulmonary disease (COPD)
Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) > 50 mmHg and pH <>
Increased work of breathing as evidenced by significant tachypnea, retractions, and other physical signs of respiratory distress
Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) <>
Hypotension including sepsis, shock, congestive heart failure
Neurological diseases such as Muscular Dystrophy and Amyotrophic Lateral Sclerosis

Types of ventilators


Ventilation can be delivered via:

  • Continuous-flow or Anaesthesia (or T-piece) bag
  • A mechanical ventilator. Types of mechanical ventilators include:
    • Transport ventilators. These ventilators are small, more rugged, and can be powered pneumatically or via AC or DC power sources.
    • ICU ventilators. These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure). This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time). Many ICU ventilators also incorporate graphics to provide visual feedback of each breath.
      • NICU ventilators. Designed with the preterm neonate in mind, these are a specialized subset of ICU ventilators which are designed to deliver the smaller, more precise volumes and pressures required to ventilate these patients.
    • PAP ventilators. these ventilators are specifically designed for non-invasive ventilation. this includes ventilators for use at home, in order to treat sleep apnea.





Introduction to Electrical Mechanical Drafting

Mechanical drafting courses are available as early as the high school level. While called electrical mechanical drafting, the emphasis is on the mechanical aspect. The basic principles are the same: to make and read an engineering drawing.


Mechanical Drafting
Mechanical drafting involves drawing a part or assembly for manufacturing purposes. It requires learning basic drafting standards, such as paper size and equipment, and training in drafting terminology for the fundamentals, such as orthographic view projection, sectioning, auxiliary views, line weights and line usage. These techniques apply to other forms of drafting, including architectural and electrical.

Electrical Drafting
Electrical drafting is a separate training module, spent learning the different electrical symbols for making electrical schematics, printed circuit design and other drawings for the electrical industry.

Computer Aided Drafting
Computer Aided Design (CAD) uses a computer drafting program to make the drawings. There are several different drafting programs, but all use the basic mechanical techniques for the drawing. Some have optional add-on modules with electrical symbols.




Basics of Mechanical Drawing

The basics of mechanical drafting begin with understanding the concept of orthographic projection. Learning mechanical drawing is similar to learning another language. With a few simple tools and knowledge of views, drawings can be understood and made.

  1. Tools

    • The first item needed is access to a mechanical drawing book. A good technical drawing book will be kept as the reference for a lifetime, and will be referred to as long as a person is in the industry.

      The basic drawing tools are simple. An eraser, lead holder, mechanical pencils, or even wooden pencils of the proper lead weights are acceptable. Drawing paper, a tee square and two triangles, one 45 degrees, one 30/60 degrees, a compass and some plastic circle templates are the items a person needs to get started.

      Other tools can be acquired as needed. A small drawing board for home use would be useful. A triangular drafting scale with different scales on each face is needed. It is not to be used to draw lines, but for measuring.

    Orthographic Views

    • Understanding orthographic views is the basic knowledge of engineering drawing. If a person imagines an "L" with thickness, the L as printed would be called the front view. If looked at from the top, it is a bar with a line across the width to identify the upper portion of the L. This is called the top or plan view. From the right side, it is a bar with a line near the bottom to identify the lower portion of the L. This is called the right-side view.

    Drawing Paper

    • Drawing paper for mechanical drawings is vellum. It is a thin paper so that it can be used to make blueprints. It is tough, and stands up to repeated erasures. In the corner is the title block for such information as the title, scale of the drawing, drafter, checker and date.

    Drawing Scale

    • With the concept of the "L" layout in mind, the idea is to put it on paper. If the length of the upper leg of the L is 6 inches, the lower leg is 3 inches, and the thickness is 2 inches and the paper is 8.5 x 11, the drawing won't fit.

      If it were drawn half size, it would fit, but wouldn't leave extra room. A quarter size would work. Draw the upper leg 1.5 inches, the lower leg .75 inches and the thickness at .25 inches. Enter the scale in the title block and draw.

    Dimensions

    • Engineering drawing needs to have dimensions. Extension lines extend to identify features, and then lines with arrows at the end perpendicular to them give the number of the feature. On our "L" the upper leg would be 6, the lower leg 3, and 2 would identify the thickness of the legs and width.



Mechanical Drawing Exercises

Mechanical drawing generally refers to technical drawings of objects. San Diego State University points out that the idea behind mechanical drawing is to represent the object as mathematically accurate as possible. In creating a mechanical drawing, you are attempting to communicate with engineers and machinists who can reproduce your ideas in the real world. Examples of mechanical drawings can include dimensional representations of buildings, images of specific tools or diagrams of how pieces are intended to be put together.

Immediate Exercises

Learning mechanical drawing requires learning many basic drawing techniques such as perspective and proportion. Learn to Draw offers some basic lessons on how to use space, how to measure distances, how to capture proportion and how to work with perspective. An example exercise is to measure proportion by holding your pencil out at arm's length and look at the object you're drawing. Use the pencil to determine how large the shape should be on your paper. Although this is a good introductory program into general drawing techniques, mechanical drawing requires more precision. To practice this precision, you may wish to attempt some of the detailed perspective exercises offered by Kevin Hulsey. These lessons are specifically oriented toward learning technical drawing and thus discuss practice and techniques more fully.

Self-Directed Study Online
Another option for the aspiring mechanical artist is to learn how to replicate designs that are featured online. Look for basic mechanical diagrams that will help you understand how the patterns and appearances were created. The University of South Florida, for example, offers 180 mechanical images that you can work to duplicate. These exercises provide you with a finished pattern, such as a series of nested circles, and hints regarding how to re-create the pattern at home. An important tip to keep in mind when using tools such as triangles or curves is to keep your pencil in the same vertical orientation to the tool as you trace its edge. As you work to get elements of perspective and shading more refined, you can either make multiple copies of the shapes you've made on a machine copier or you can print them from the Internet.

Invest in Books
If you feel you have progressed as far as you can trying to work from these types of exercises, you can invest in a self-help book. Several books on the market are designed to help you achieve stronger mechanical drawing skills. Be sure to pick out one such as "Mechanical Drawing Self-Taught" by Joshua Rose. These kinds of books focus exclusively on mechanical drawing, discuss the tools you will need to succeed and provide detailed instructions on how to achieve the effects discussed. One of the advantages of older books like this is that they don't presume you have a computer handy with CAD software installed. Instead, the book attempts to teach you to create these designs by relying on nothing but the mechanical tools listed in the introduction.