Mechanical Ventilators for Dummies

As physical therapists, we will encounter patients using mechanical ventilators. This webpage provides basic information about the in's and out's of mechanical ventilation so we can effectively treat this patient population.

What's goin' on???  Who be usin' dis?  Take a deep breath...  Ride the wave 101  Don't get alarmed!!  Link me up Scottie

Mechanical ventilation is used in patients with acute and chronic respiratory distress.  For a variety of reasons these patients cannot ventilate their lungs sufficiently to satisfy their body's oxygen needs.  The goal of mechanical ventilation is to reproduce the body's normal breathing mechanism.   To achieve this, mechanical ventilators utilize positive intrapulmonary pressure.  Machines based on positive intrapulmonary pressure literally blow air into the lung; they create a positive pressure within the alveoli.
 

Primarily the patients that require mechanical ventilation are suffering from acute respiratory failure. There are two types of acute respiratory failure: hypoxemic respiratory failure and hypercapneic respiratory failure. Hypoxemic respiratory failure (lung failure) is often the result of mismatched V/Q; this is treated with oxygen alone or in combination with PEEP/CPAP. Hypercapneic respiratory failure, or ventilatory pump failure, occurs when the body is unable to maintain a normal PaCO2. Three disorders that commonly lead to hypercapneic respiratory failure include those associated with the CNS, neuromuscular function, and those that increase the work of breathing. Some special patients on ventilators include neonates and people with home ventilators. In the past, negative pressure mechanical ventilators were used extensively for polio patients. Negative pressure mechanical ventilators, like the iron lung, encased the thoracic cavity externally in an air-tight chamber. The chamber was used to create a negative pressure around the thoracic cavity, thereby causing air to rush into the lungs to equalize the pressure. The iron lung is shown below.

Take a Deep Breath...

Mechanical ventilators can be set to deliver three kinds of breaths.

1. Mandatory Breath: started, controlled, and ended by the ventilator. The ventilator does all the work.
2. Assisted Breath: initiated by the patient, but controlled and ended by the ventilator.
3. Spontaneous Breath: initiated, controlled, and ended by the patient; however the volume and/or pressure of the breath delivered by the ventilator is based on patient demand.

1. Beginning of Inspiration: This can be initiated by the ventilator or the patient. If this is ventilator controlled, a breath begins after an elapsed period of time. The amount of time can be programmed into the ventilator. The ventilator can sense changes in pressure, volume, or flow. As a result if the patient breaths spontaneously the ventilator knows this and can deliver a breath.

 
2. Limiting Factors: The pressure, volume, flow, and time can be preset by the ventilator. However, if these end values are reached anytime during inspiration, the inspiration phase does not end.

 
3. Termination of Inspiration (Phase Cycling Mechanism): The way that the ventilator recognizes the end of inspiration is called the cycling mechanism. The cycling mechanism can be preset on the ventilator by using any of the limiting factors. For example, if the ventilator is set to end the inspiration after reaching a set volume, it is called volume-cycled. Also included in the cycling mechanism may be an inspiratory hold. An inspiratory hold aims to maintain air in the lungs at the end of inspiration allowing for more gas exchange to take place.

 
4. Expiratory Phase: This occurs when the expiratory valve opens. Commonly expiration is a passive mechanism generated by the recoil of the diaphragm and lungs. If additional help is need by the patient for expiration NEEP can be used by the ventilator. NEEP generates a negative pressure in the upper respiratory tract; this causes air to rush out of the lungs (ie. expiration).

Ride the Wave 101

Flow rates of a mechanical ventilator are quantified by waves on inspiration. These waves show how the ventilator dispenses air to the patient. The wave flow pattern can be changed to accomodate the patient's lung compliance, elastic resistance, or airway resistance. There are three commonly used wave patterns.

Square Wave: With this setting, the flow accelerates very quickly and reaches a set flow which is maintained throughout inspiration. This wave pattern allows for an adequate I:E ratio with a normal flow rate. This optimization of I:E ratio is beneficial for those patients in which air trapping is a concern. If a patient's peak airway pressures become higher than normal or the patient is uncomfortable, the wave pattern can be changed to decrease this pressure or accomodate a more normal breathing pattern.

Sine Wave: This type of flow slowly accelerates to a peak flow and tapers off towards the end of expiration. Peak airway pressures are minimized here. Sine waves are considered normal flow waves. This wave can be used if high peak airway pressures are encountered using the square wave.

Descending Wave: This setting utilizes a rapid acceleration of flow followed by a gentle tapering. This wave form may require much higher flows to obtain an adequate I:E ratio. It is important to monitor the arterial blood gases of these patients to ensure this adequate ratio is being obtained. This wave can be used if abnormally high peak airway pressures are being encountered with the sine wave form.

Don't get alarmed!!

Mechanical ventilators are equipped with alarms to notify you of any changes in their ventilation status. Here are some of the most common causes of low and high pressure alarm situations.

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If you have comments or suggestions, email us: Nicole Steininger                                                                        Arin Beesley