Intensive care ventilators, basic principles of equipment operation.

Introduction

This document is a short concise detail to a medical ventilators function and basic operation. (well depending on your mechanical and engineering understanding may appear to be very detailed 5 pages LOL). It is designed to be easy reading to understand the operation of the medical device in a medical environment to all. Please feel free to copy and distribute as you feel relevant in order to educate all to its operation.  Effort has been made to keep it simple but also not to remove information that may be of interest or value to whoever reads the document.

Intensive care ventilators are defined as advanced mechanical ventilators that can be configured to provide basic and advanced ventilation modes for newborns, children and / or adults. Many of these devices systems may also offer non-invasive ventilation, special patient weaning modes, user interface graphic packages and interface functions for alarm and data packages.

Mechanical/intensive care ventilators provide temporary or permanent ventilation support for patients who have a compromised respiratory system and cannot breathe on their own. These patients need assistance maintaining an adequate ventilation, oxygen and air flow process due to illness, trauma, respiratory system defects in the body, or the effects of medications (e.g., anesthetics). In most cases, mechanical ventilators are used for a short period of time (up to weeks) to pressurize medical gases for delivery to the patient’s lungs. This delivery is to promote gas exchange and allow the respiratory muscles to rest until the patient is able to is to maintain effective ventilation without mechanical support. However, some patients may require permanent ventilation.

Operating principles

Positive pressure simulated breaths from the ventilator are usually delivered through a tube mechanism called an endotracheal tube or a tracheotomy tube. With each positive pressure breath, the lungs expand in proportion to the delivered gas volume until a preset pressure, volume, or time limit is reached. A valve then opens to relieve pressure in the lungs, allowing the patient to exhale passively.

An intensive care ventilator typically consists of a flexible breathing circuit, a control system, a gas supply, and monitors and alarms. Heating and humidifying (adding moisture) equipment are available as additional components. Most ventilators have advanced microprocessor circuitry to accurately control the pressure, volume and flow of supplied positive pressure breathing. In addition, the amount of inhaled oxygen based on the control settings is also monitored. Electronic communication interfaces and measuring circuitry are usually included in modern devices, so information about control settings, monitored variables and alarm status can be recorded. The critical patient information from the ventilator can sent to a patient monitoring system at the bedside or a central monitoring system, patient information system or other interface device.

Electrical power is supplied from a wall outlet with internal battery support. A rechargeable battery is used for short-term ventilation in the event of a power failure or in the case of a transport unit to transport patients within a hospital.
Intensive care ventilators are designed to use air and oxygen (medical gases) that come from a central supply via a bedhead  or surgical theater outlets and deliver gas at a pressure of about 50  PSI (pounds per square inch). Air and O2 cylinders equipped with a 50-psi pressure regulator can also be used during patient transport. Gas flow to the patient is controlled by flow control valves in the ventilator. To obtain the desired the fraction of inspired oxygen for delivery to the patient, most intensive care ventilation devices include an air / oxygen blender (oxygen mixer).

The gas is delivered to the patient through a reusable or disposable flexible breathing circuit. These ventilators use a two-limb breathing circuit made of a corrugated plastic tubing to transport the gas from the device through one tube (the inhalation circuit) to the patient and return the exhaled gas through another limb (the exhalation circuit). During the gas supply (the inspiration phase), an exhalation valve in the ventilator or in the breathing circuit is closed to force air into the patent hose and lungs. After the inspiration phase, the exhalation valve is opened, to allow the patient to exhale passively into the ambient air via the ventilator (exhalation phase).

The breathing circuit can also provide connections and fittings where the supplied gas can be heated, humidified, checked for airway pressure, and mixed with medications of a gaseous nature. Some breathing circuits also contain reservoirs for collecting condensation from the hoses. Many models have sensors for within the breathing circuit line that measure airway pressure or flow and can provide feedback to the device via microprocessor control and integrated sensors to automatically record and adjust performance.

From here on the explanation gets a little more detailed but it is still interesting and easy reading.

Control Circuitry

Regulators are used to select the patient breath setting requirement and ventilation parameters (e.g., breath rate, fraction of inspired oxygen and tidal volume). In order for the ventilator to generate a prescribed breathing pattern, different parameters can be set independently. These would be the length of the inspiration or expiration phase, the shape of the waveform, the tidal volume, the flow rate, the peak pressure and the positive end-expiratory pressure (PEEP)and the speed of the mechanical breaths.

The PEEP and Continuous Positive Airway Pressure (CPAP) controls, restrict the flow from the exhalation valve to maintain a predetermined (user-defined) positive pressure in the breathing circuit at the end of the exhalation phase. This increased base pressure helps to improve oxygenation in the lungs when inflated to increase lung volume and improve oxygenation. (through small airways and alveolar inflation). It helps maintain the diffusion of oxygen through the alveolar capillary membrane.
The ratio of inspiration to expiration (I: E ratio) is a measured indication of a breath’s distribution in inspiration and expiration times. Normally, the expiration time is set longer than the inspiration time (for example, ratio 1: 2 or 1: 3) to allow full exhalation before the start of the next breath. However, an equal or inverse ratio can also be used as needed for certain types of patients (for example I: E ratio 1: 1 or 1: 0.5). Because inverse I: E ratio settings are not often used, some units will signal when an inverse I: E ratio is set, while others do not provide this function.

The flow waveform that determines the machine’s breath delivery pattern is possible via controls for adjustment. Volume-controlled ventilation flow patterns generally include sinusoidal, accelerating, decelerating, or square waveform. Pressure boost settings are also available in pressure-controlled modes. Such adjustments allow the user to maximize flow and pressure levels while maintaining flow delivery, reducing breath-work and increasing patient comfort and reducing lung stress and patient stress.

Operating Modes

Intensive care ventilators have several different modes of operation that are carefully controlled through microprocessor and computer controls. A mode of operation defines the formula or algorithm that will be used to trigger and end a machine breathing cycle. Different modes can provide either partial or full cycle support.  Modes of operation depend on the patients’ medical condition and on the individual patient’s clinical requirements.

Some modes are listed as follows:

Assist / Control (commonly referred to as A/C mode is a full support mode by providing mandatory breaths at preset time intervals and assisted breaths when the ventilator feels the patient’s inspirational effort increasing. This mode is intended for patients who have difficulty breathing, but still have the ability to inspire. When a patient’s respiratory effort is detected, either by a drop in pressure in the respiratory circuit (generating the pressure) or as a flow difference between the inspiratory and exhaled limbs of the circuit (flow sensing and triggering) a breath is triggered.

Synchronized Intermittent Mandatory Ventilation (SIMV), provides controlled, fixed-speed breaths and allows the patient to breathe spontaneously through the ventilator without mechanical support. Required measured breaths in this mode are synchronized with the patient’s spontaneous breathing effort if that effort is sufficiently close to the point where mandatory breath would have been generated. This reduces the chance of excessive inflation of the lungs, which may result from the stacking of mandatory breath with spontaneous breathing

Pressure Support (PS) is a mode that reduces the work of spontaneous breathing on the ventilator by applying a positive pressure preset to the patient’s airways with each natural inspiration action. This short “burst of pressure” reduces the work of the patient’s respiratory muscles and minimizes the effort required to draw air into the lungs. It as a result compensates for the extra effort to breath due to the breathing hose, the machine valves and the artificial airways possible restrictions in gas/air flow. Pressure support can be used with any breathing mode that is spontaneous.

Noninvasive ventilation mode (NIVM/NIV) . Many intensive care ventilators now also have a non-invasive ventilation mode available. This mode is intended to facilitate short-term use of the ventilator in patients without an artificial airway tubes such as endotracheal tubes. By effectively compensating for leakage and pressure and pressure gradient changes the non-invasive ventilation mode allows the use of a full or partial face mask to connect the patient to the ventilator. While in theory any mechanical ventilator can be used for non-invasive ventilation, large air leaks due to difficulty in creating an effective seal (Mask to Patient) around the mask can make maintenance of this type of patient interface extremely difficult.

Since the NIVM is specially designed to deal with these issues, it can significantly improve ventilation effectiveness. NIVM is commonly used in patients with acute severe respiratory problems likely to require ventilation for a short period of time, and in intubated patients who may require gradual weaning from invasive ventilation.

Additional information re modes

Intensive care ventilators can provide both volume and pressure-controlled breaths that can be used for full or partial patient ventilation support. A control system is used for volume-controlled breaths to ensure that the tidal volume set is delivered during each inspiration cycle. If the pressure generated during a breath exceeds high pressure set point limit or relief valve setting, the breath will be interrupted and the set volume may not be fully delivered. Pressure-controlled breaths control flow delivery is designed to achieve a user-defined peak inspiratory pressure with each breath.

Pressure-controlled breaths are affected by changes in airway resistance and lung compliance, creating a variation in the volume of air delivered during each breath. Pressure-controlled mode is used in infants based on the reasoning that pressure-controlled breaths may be advantageous for patients with acute lung injury or shortness of breath syndrome. This is based on the hypothesis that pressure breathing promotes lung recruitment recovery.

There are now combination modes available for most models and these modes provide controlled pressure respiration corresponding to a “target” respiration volume. In combined mode, the ventilator starts at a low inspiratory pressure, which gradually increases from breath to breath until the desired breath volume is reached. The ventilator then uses this inspiratory pressure for each breath, unless adjustments in the patient’s airway configuration require adjustment. As a result, the combination modes offer the benefits of pressure control while ensuring adequate tidal volume when patient airway parameters change. .
Some ventilators can provide high frequency ventilation which uses positive pressure to deliver extremely small breaths at a rate much higher than the normal respiratory rate. High frequency ventilators have been developed to reduce barotrauma and some of the harmful hemodynamic effects associated with the high volumes and tidal pressures provided by some conventional ventilator designs.

In addition, depending of clinical requirement and manufacturer design there may be additional modes and adjustable pre-set modes to accommodate for a patient ventilation process in the most accommodating means while reducing any form of trauma that may be introduced by the device.

Ventilator Alarms and interfacing.

There are a few basic types of alarm systems to ensure the safety of patients that are receiving ventilation support. These are designed and take into consideration the noise in the environment and the business of a department. The systems are designed to be of a nature to bring attention to medical staff any irregularities that may negatively impact on the patients.

The systems may interface to a central station system which is fed into an all comprehensive electronic patient record, a system interfacing to a nurse call system, interfacing the systems to a physiological monitoring system and linking to remote audible and visual alarm systems.

Within the ventilator itself there are a variety of alarms to detect equipment related problems and changes in a patient respiratory status.

Graphic monitors provide real time displays of flow pressure and volume over time periods. These assist medical staff to track a patient’s progress. Many systems record a patient’s graphs relating to all measures parameters and contain input menus to attach these to specific patient details. Systems include entry of patient fields such as name, age, ID, patient number, dates and many facts related to the patient’s medical condition. All of this is also able to be interfaced to most generic and vendor neutral patient information and monitoring systems. With a patient information system that is fully integrated the patient details can be pulled through on a work-list and then the manual errors of entry of patient information is reduced.

While this is a fairly short introduction for some it may have been a tedious read for others.

End (Stuart Gray Clinical Engineering Project Consultant)