Frank P. Primiano, Jr., PhD
Amethyst Research, LLC
San Diego, CA
Robert L. Chatburn, BS, RRT, FAARC
University Hospital Health Systems and Case Western Reserve University
Background on Ventilation
A ventilator is an automatic mechanical device designed to provide all or part of the work the body must produce to move gas into and out of the lungs. The act of moving air into and out of the lungs is called breathing, or, more formally, ventilation.
Background on Ventilation
During breathing, a volume of air is inhaled through the airways (mouth and/or nose, pharynx, larynx, trachea, and bronchial tree) into millions of tiny gas exchange sacs (the alveoli) deep within the lungs. There it mixes with the carbon dioxide-rich gas coming from the blood. It is then exhaled back through the same airways to the atmosphere. Normally this cyclic pattern repeats at a breathing rate, or frequency, of about 12 breaths a minute (breaths/min) when we are at rest (a higher resting rate for infants and children). The breathing rate increases when we exercise or become excited.
Gas exchange is the function of the lungs that is required to supply oxygen to the blood for distribution to the cells of the body, and to remove carbon dioxide from the blood that the blood has collected from the cells of the body. Gas exchange in the lungs occurs only in the smallest airways and the alveoli. It does not take place in the airways (conducting airways) that carry the gas from the atmosphere to these terminal regions. The size (volume) of these conducting airways is called the anatomical “dead space” because it does not participate directly in gas exchange between the gas space in the lungs and the blood. Gas is carried through the conducting airways by a process called “convection”. Gas is exchanged between the pulmonary gas space and the blood by a process called “diffusion”.
One of the major factors determining whether breathing is producing enough gas exchange to keep a person alive is the ‘ventilation’ the breathing is producing. Ventilation is expressed as the volume of gas entering, or leaving, the lungs in a given amount of time. It can be calculated by multiplying the volume of gas, either inhaled or exhaled during a breath (called the tidal volume), times the breathing rate (e.g., 0.5 Liters x 12 breaths/min = 6 L/min).
Therefore, if we were to develop a machine to help a person breathe, or to take over his or her breathing altogether, it would have to be able to produce a tidal volume and a breathing rate which, when multiplied together, produce enough ventilation, but not too much ventilation, to supply the gas exchange needs of the body. During normal breathing the body selects a combination of a tidal volume that is large enough to clear the dead space and add fresh gas to the alveoli, and a breathing rate that assures the correct amount of ventilation is produced. However, as it turns out, it is possible, using specialized equipment, to keep a person alive with breathing rates that range from zero (steady flow into and out of the lungs) up to frequencies in the 100’s of breaths per minute. Over this frequency range, convection and diffusion take part to a greater or lesser extent in distributing the inhaled gas within the lungs. As the frequency rates are increased, the tidal volumes that produce the required ventilation get smaller and smaller.
We will consider two classes of ventilators here: those that produce breathing patterns that mimic the way we normally breathe (i.e., at rates our bodies produce during our usual living activities: 12 – 25 breaths/min for children and adults; 30 – 40 breaths/min for infants) – these are called conventional ventilators; and those that produce breathing patterns at frequencies much higher than we would or could voluntarily produce for breathing – called high frequency ventilators.
There are two sets of forces that can cause the lungs and chest wall to expand: the forces produced when the muscles of respiration (diaphragm, inspiratory intercostal, and accessory muscles) contract, and the force produced by the difference between the pressure at the airway opening (mouth and nose) and the pressure on the outer surface of the chest wall. Normally, the respiratory muscles do the work needed to expand the chest wall, decreasing the pressure on the outside of the lungs so that they expand, which in turn enlarges the air space within the lungs, and draws air into the lungs. The difference between the pressure at the airway opening and the pressure on the chest wall surface usually does not play a role in this activity because, both of these locations being exposed to the same pressure (atmospheric), this difference is zero. However, when the respiratory muscles are unable to do the work required for ventilation, either or both of these two pressures can be manipulated to produce breathing movements.
It is not difficult to visualize that, if the pressure at the mouth and nose of an individual were increased while the pressure surrounding the rest of the person’s body remained at atmospheric, the person’s chest would expand as air is literally forced into the lungs. Likewise, if the pressure on the person’s body surface were lowered as the pressure at the person’s open mouth and nose remained at atmospheric, then again the pressure at the mouth would be greater than that on the body surface and air would be forced into the lungs. Thus, we have two approaches that can be used to mechanically ventilate the lungs: apply positive pressure (relative to atmospheric) to the airway opening – devices that do this are called positive pressure ventilators; or, apply negative pressure (relative to atmospheric) to the body surface (at least the rib cage and abdomen) – such devices are called negative pressure ventilators.
The simplest mechanical device we could devise to assist a person’s breathing would be a hand-driven, syringe-type pump that is fitted to the person’s mouth and nose using a mask. A variation of this is the self-inflating, elastic breathing bag. Both of these require one-way valve arrangements to cause air to flow from the device into the lungs when the device is compressed, and out from the lungs to the atmosphere as the device is expanded. Also, it can be appreciated that such arrangements are not automatic, requiring an operator to supply the energy to push the gas into the lungs through the mouth and nose.
Automating the ventilator so that continual operator intervention is not needed for safe, desired operation requires 1) a stable attachment (interface) of the device to the patient, 2) a source of energy to drive the device, 3) a control system to make it perform appropriately, and 4) a means of monitoring the performance of the device and the condition of the patient.
- Patient Interface. Positive Pressure Ventilators: The ventilator delivers gas to the patient through a set of flexible tubes called a patient circuit. Depending on the design of the ventilator, this circuit can have one or two tubes. The circuit connects the ventilator to either an endotracheal or tracheostomy tube that extends into the patient’s throat (causing this arrangement to be called invasive ventilation), or a mask covering the mouth and nose or just the nose (referred to as noninvasive ventilation). Each of these connections to the patient may have a balloon cuff associated with it to provide a seal – either inside the trachea for the tracheal tubes or around the mouth and nose for the masks. Negative Pressure Ventilators:The patient is placed inside a chamber with his or her head extending outside the chamber. The chamber may encase the entire body except for the head (e.g., iron lung), or it may enclose just the rib cage and abdomen (cuirass). It is sealed to the body where the body extends outside the chamber. Although it is not generally necessary, the patient may have an endotracheal or tracheostomy tube in place.
- Power Sources. Positive Pressure Ventilators are typically powered by electricity or compressed gas. Electricity is used to run compressors of various types. These provide compressed air both for motive power as well as air for breathing. More commonly, however, the power to expand the lungs is supplied by compressed gas from tanks, or from wall outlets in the hospital. The ventilator is generally connected to separate sources of compressed air and compressed oxygen. This permits the delivery of a range of oxygen concentrations to support the needs of sick patients. Because compressed gas has all moisture removed, the gas delivered to the patient must be warmed and humidified in order to avoid drying out the lung tissue. A humidifier placed in the patient circuit does this. A humidifier is especially needed when an endotracheal or tracheostomy tube is used since these cover or bypass, respectively, the warm, moist tissues inside of the nose and mouth and prevent the natural heating and humidification of the inspired gas. Negative Pressure Ventilators are usually powered by electricity used to run a vacuum pump that periodically evacuates the chamber to produce the required negative pressure. Humidification is not needed if an endotracheal tube is not used. Oxygen enriched inspired air can be provided as needed via a breathing mask.
- Control System. A control system assures that the breathing pattern produced by the ventilator is the one intended by the patient’s caregiver. This requires the setting of control parameters such as the size of the breath, how fast and how often it is brought in and let out, and how much effort, if any, the patient must exert to signal the ventilator to start a breath. If the patient can control the timing and size of the breath, it is called a spontaneous breath. Otherwise, it is called a mandatory breath. A particular pattern of spontaneous and mandatory breaths is referred to as a mode of ventilation. Numerous modes, with a variety of names, have been developed to make ventilators produce breathing patterns that coordinate the machine’s activity with the needs of the patient.
- Monitors. Most ventilators have at least a pressure monitor (measuring airway pressure for positive pressure ventilators, or chamber pressure for negative pressure ventilators) to gauge the size of the breath and whether or not the patient is properly connected to the ventilator. Many positive pressure ventilators have sophisticated pressure, volume and flow sensors that produce signals both to control the ventilator’s output (via feedback in the ventilator’s control system) and to provide displays (with alarms) of how the ventilator and patient are interacting. Clinicians use such displays to follow the patient’s condition and to adjust the ventilator settings.
The vast majority of ventilators used in the world provide conventional ventilation. This employs breathing patterns that approximate those produced by a normal spontaneously breathing person. Tidal volumes are large enough to clear the anatomical dead space during inspiration and the breathing rates are in the range of normal rates. Gas transport in the airways is dominated by convective flow and mixing in the alveoli occurs by molecular diffusion. This class of ventilator is used in the ICU, for patient transport, for home care and in the operating room. It is used on patients of all ages from neonate to adult.
High Frequency Ventilators
It has been known for several decades that it is possible to adequately ventilate the lungs with tidal volumes smaller than the anatomic dead space using breathing frequencies much higher than those at which a person normally breathes. This is actually a common occurrence of which we may not be fully aware. Dogs do not sweat. They regulate their temperature when they are hot by panting as you probably know. When a dog pants he takes very shallow, very fast, quickly repeated breaths. The size of these panting breaths is much smaller that the animal’s anatomical dead space, especially in dogs with long necks. Yet, the dog feels no worse for this type of breathing (at least all the dogs interviewed for this article).
Devices have been developed to produce high frequency, low amplitude breaths. These are generally used on patients with respiratory distress syndrome (lungs will not expand properly). These are most often neonates whose lungs have not fully developed, but can also be older patients whose lungs have been injured. High frequency ventilators are also used on patients that have lungs that leak air. The very low tidal volumes produced put less stress on fragile lungs that may not be able to withstand the stretch required for a normal tidal volume.
There are two main types of high frequency ventilator: high frequency jet ventilators (HFJV) and high frequency oscillatory ventilators (HFOV). The HFJV directs a high frequency pulsed jet of gas into the trachea from a thin tube within an endotracheal or tracheostomy tube. This pulsed flow entrains air from inside the tube and directs it toward the bronchi. The HFOV uses a piston arrangement that moves back and forth rapidly to oscillate (vibrate lengthwise) the gas in the patient’s breathing circuit and airways. Both of these techniques cause air to reach the alveoli and carbon dioxide to leave the lungs by enhancing mixing and diffusion in the airways. Convection plays a minor role in gas transport with these ventilators while various forms of enhanced diffusion predominate.
Although high frequency devices that drive the pressure on the chest wall have been developed, most high frequency ventilators in use today are applied to the airway opening.
In future articles, the authors will explore topics such as how ventilators work, the controls and monitors that can be available on a ventilator, interpretation of graphical displays of ventilatory variables, as well as various clinical aspects of ventilator use.