When is mechanical ventilation required

Careful measures taken by intensive care providers help to reduce this risk. In some cases, tracheostomy a surgically placed breathing tube through an incision in the neck may be offered to improve a person's care when intubation is required for a longer time period. Ventilators, like all other mechanical devices, can malfunction. Sophisticated alarms and system checks are built into the machines to prevent harm. Routine, short-term use during general anesthesia for surgical procedures.

Respiratory failure from pneumonia, chronic obstructive pulmonary disease COPD—chronic bronchitis, emphysema , acute asthma attack, acute respiratory distress syndrome, or severe viral infections such as West Nile virus or influenza.

Many are available in English and Spanish. A Patient Page on intensive care units was published in the March 25, , issue; one on ventilator-associated pneumonia was published in the August 20, , issue; one on lung complications after surgery was published in the October 14, , issue; one on COPD was published in the November 26, , issue; and one on sepsis was published in the February 24, , issue.

The information and recommendations appearing on this page are appropriate in most instances, but they are not a substitute for medical diagnosis. For specific information concerning your personal medical condition, JAMA suggests that you consult your physician. This page may be photocopied noncommercially by physicians and other health care professionals to share with patients. Mechanical Ventilation. Coronavirus Resource Center. Our website uses cookies to enhance your experience.

By continuing to use our site, or clicking "Continue," you are agreeing to our Cookie Policy Continue. Save Preferences. Symptoms include flu-like illness…. A pulmonologist is a doctor who focuses on the respiratory system. Discover the conditions they treat such as COPD , exams they conduct, and much…. Health Conditions Discover Plan Connect. The Times a Ventilator Is Needed.

What is a ventilator? When a ventilator is used. How a ventilator works. Risks of being on a ventilator. What to expect on a ventilator. How to prepare if a loved one is put on a ventilator. What to expect when taken off a ventilator. The takeaway. Read this next.

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What Does a Pulmonologist Do? This corresponds to a 15 percent outside air OA fraction in the office HVAC unit, and 50 percent in the classroom unit.

The actual outdoor air ventilation rate for each space is:. While this simplistic analysis suggests that the actual OA cfm to the classroom is less than design cfm vs. The office is over-ventilated cfm vs. The Standards allow this design provided that the system always delivers at least cfm to the classroom including transfer or recirculated air , and that any transfer air is free of unusual contaminants.

This method can be used for any space, but is particularly applicable to conference rooms, toilet rooms, and other rooms that have high ventilation requirements. Transfer air must be free from any unusual contaminants, and as such should not be taken directly from rooms where such sources of contaminants are anticipated.

It is typically taken from the return plenum or directly from an adjacent space. Air may be transferred using any method that ensures a positive airflow. Examples include dedicated transfer fans, exhaust fans and fan-powered VAV boxes. A system having a ducted return may be balanced so that air naturally transfers into the space.

Exhaust fans serving the space may discharge directly outdoors, or into a return plenum. Transfer systems should be designed to minimize recirculation of transfer air back into the space; duct work should be arranged to separate the transfer air intake and return points. When each space in a two-space building is served by a separate constant volume system, the calculation and application of ventilation rate is straightforward, and each space will always receive its design outdoor air quantity.

However, a central system serving both spaces does not deliver the design outdoor air quantity to each space. Instead, one space receives more than its allotted share, and the other less. When a return plenum is used to distribute outside air to a zonal heating or cooling unit, the outside air supply must be connected either:. Within 5 ft. Within 15 ft. Water source heat pumps and fan coils are the most common application of this configuration.

The unit fans should be controlled to run continuously during occupancy in order for the ventilation air to be circulated to the occupied space. A central space-conditioning system s augmented by a few zonal units for spot conditioning may use transfer air from spaces served by the central system. A direct source of outdoor air is not required for each zonal unit. Similarly, transfer air may be used in buildings having central interior space-conditioning systems with outdoor air, and zonal units on the perimeter without outdoor air.

While not required, the Standards recommend that sources of unusual contaminants be controlled through the use of containment systems that capture the contaminants and discharge them directly outdoors. Such systems may include exhaust hoods, fume hoods, small space exhausts and differential pressure control between spaces. For spaces served by variable air volume VAV systems, this means that the minimum supply setting of each VAV box should be no less than the design outdoor ventilation rate calculated for the space, unless transfer air is used.

If transfer air is used, the minimum box position, plus the transfer air, must meet the minimum ventilation rate.

If transfer air is not used, the box must be controlled so that the minimum required airflow is maintained at all times unless demand controlled ventilation or occupant sensor are employed.

Section 4. In these tests, the minimum outside air in VAV systems will be measured both at full flow and with all boxes at minimum position. Figure shows a typical VAV system. In standard practice, the testing and balancing TAB contractor sets the minimum position setting for the outdoor air damper during construction. It is set under the conditions of design airflow for the system, and remains in the same position throughout the full range of system operation.

Does this meet code? The answer is no. As the system airflow drops, so will the pressure in the mixed air plenum. A fixed position on the minimum outdoor air damper will produce a varying outdoor airflow. As depicted in Figure , this effect will be approximately linear in other words, outdoor air airflow will drop directly in proportion to the supply airflow.

The following paragraphs present several methods used to dynamically control the minimum outdoor air in VAV systems, which are described in detail below.

Regardless of how the minimum ventilation is controlled, care should be taken to reduce the amount of outdoor air provided when the system is operating during the weekend or after hours with only a fraction of the zones active. This can be provided by having the VAV boxes return to fully closed when their associated zone is in unoccupied mode. When a space or group of spaces is returned to occupied mode e. During this partial occupancy the ventilation air can be reduced to the requirements of those zones that are active.

If all zones are of the same occupancy type e. This method does not comply with the Standards; the airflow at a fixed minimum damper position will vary with the pressure in the mixed air plenum see Figure It is explicitly prohibited in This method complies with the Standards. An inexpensive enhancement to the fixed damper setpoint design is the dual minimum setpoint design, commonly used on some packaged AC units. The minimum damper position is set proportionally based on fan speed or airflow between a setpoint determined when the fan is at full speed or airflow and minimum speed or airflow.

This method complies with the letter of the Standards but is not accurate over the entire range of airflow rates and when there are wind or stack effect pressure fluctuations.

But with DDC , this design has very low costs. The energy balance method Figure uses temperature sensors in the outside, as well as return and mixed air plenums to determine the percentage of outdoor air in the supply air stream. The outdoor airflow is then calculated using the equations shown in Figure This method requires an airflow monitoring station on the supply fan. While technically feasible, it may be difficult to meet the outside air acceptance requirements with this approach because:.

It is difficult to accurately measure the mixed air temperature, which is critical to the success of this strategy. Even with an averaging type bulb, most mixing plenums have some stratification or horizontal separation between the outside and mixed airstreams. Even with the best installation, high accuracy sensors, and field calibration of the sensors, the equation for percent outdoor air will become inaccurate as the return air temperature approaches the outdoor air temperature.

When they are equal, this equation predicts an infinite percentage outdoor air. The accuracy of the airflow monitoring station is likely to be low at low supply airflows. The denominator of the calculation amplifies sensor inaccuracy as the return air temperature approaches the outdoor air temperature.

It only works theoretically when the minimum outdoor air rate equals the rate of air required to maintain building pressurization the difference between supply air and return air rates. Return fan tracking Figure uses airflow monitoring stations on both the supply and return fans. The theory behind this is that the difference between the supply and return fans has to be made up by outdoor air, and controlling the flow of return air forces more ventilation into the building.

Several problems occur with this method:. The relative accuracy of airflow monitoring stations is poor, particularly at low airflows;. The cost of airflow monitoring stations;. It will cause building pressurization problems unless the ventilation air is equal to the desired building exfiltration plus the building exhaust. ASHRAE research has also demonstrated that in some cases this arrangement can cause outdoor air to be drawn into the system through the exhaust dampers due to negative pressures at the return fan discharge.

Again, this method is technically feasible but will likely not meet the acceptance requirements depending on the airflow measurement technology. Most airflow sensors will not be accurate to a percent turndown the normal commercial ventilation range. Controlling the outdoor air damper by direct measurement with an airflow monitoring station Figure can be an unreliable method. Its success relies on the turndown accuracy of the airflow monitoring station.

Depending on the loads in a building , the ventilation airflow can be between 5 and 15 percent of the design airflow. If the outdoor airflow sensor is sized for the design flow for the airside economizer, this method has to have an airflow monitoring station that can turn down to the minimum ventilation flow between 5 and 15 percent.

Of the different types available, only a hot-wire anemometer array is likely to have this low-flow accuracy while traditional pitot arrays will not. One advantage of this approach is that it provides outdoor airflow readings under all operating conditions, not just when on minimum outdoor air.

For highest accuracy, provide a damper and outdoor air sensor for the minimum ventilation air that is separate from the economizer outdoor air intake. This method complies with the Standards, but it is expensive and may require additional space. Note that an airflow sensor and damper are required since fan airflow rate will vary as mixed air plenum pressure varies.

The injection fan method Figure uses a separate outdoor air inlet and fan sized for the minimum ventilation airflow. This inlet contains an airflow monitoring station, and a fan with capacity control e. The discharge damper is recommended since a damper must be provided anyway to shut off the intake when the AHU is off, and also to prevent excess outdoor air intake when the mixed air plenum is very negative under peak conditions.

The fan is operating against a negative differential pressure and thus cannot stop flow just by slowing or stopping the fan.

This method works, but the cost is high and often requires additional space for the injection fan assembly. This approach is low cost and takes little space. It can be accurate if the differential setpoint corresponding to the minimum outdoor air rate is properly set in the field.

An inexpensive but effective design uses a minimum ventilation damper with differential pressure control Figure In this method, the economizer damper is broken into two pieces: a small two position damper controlled for minimum ventilation air and a larger, modulating, maximum outdoor air damper that is used in economizer mode.

A differential pressure transducer is placed across the minimum outdoor air damper. During start-up, the air balancer opens the minimum outside air OA damper and return air damper, closes the economizer OA damper, runs the supply fan at design airflow, measures the OA airflow using a hand-held velometer and adjusts the minimum OA damper position until the OA airflow equals the design minimum OA airflow.

At this point the design pressure DP across the minimum OA damper is measured. This value becomes the DP setpoint. The principle used here is that airflow is constant across a fixed orifice the open damper at fixed DP. As the supply fan modulates when the economizer is off, the return air damper is controlled to maintain the DP setpoint across the minimum ventilation damper. The main downside to this method is the complexity of controls and the potential problems determining the DP setpoint in the field.

It is often difficult to measure the outdoor air rate due to turbulence and space constraints. If the minimum required ventilation rate for a space is cfm, what is the minimum allowed airflow for its VAV box when the design percentage of outdoor air in the supply is 20 percent?

Immediately prior to occupancy, outdoor ventilation must be provided in an amount equal to the lesser of:. The minimum required ventilation rate for 1 hour; or.

Either criteria can be used to comply with the Standards. This air may be introduced at any rate provided for and allowed by the system, so that the actual purge period may be less than an hour. A pre-occupancy purge is not required for buildings or spaces that are not occupied on a scheduled basis, such as storage rooms.

Also, a purge is not required for spaces provided with natural ventilation. Where pre-occupancy purge is required, it does not have to be coincident with morning warm-up or cool-down. The simplest means to integrate the two controls is to simply schedule the system to be occupied one hour prior to the actual time of anticipated occupancy. This allows the optimal start, warm-up or pull-down routines to bring the spaces up to or down to desired temperatures before opening the outdoor air damper for ventilation.

This will reduce the required system heating capacity and ensure that the spaces will be at the desired temperatures and fully purged at the start of occupancy. What is the length of time required to purge a space 10 ft. In a building with natural ventilation, do the windows need to be left open all night to accomplish a building purge? A building purge is required only for buildings with mechanical ventilation systems.

How is a purge accomplished in a building without a regularly scheduled occupancy whose system operation is controlled by an occupancy sensor? There is no purge requirement for this building. Note that occupancy sensors and manual timers can only be used to control ventilation systems in buildings that are intermittently occupied without a predictable schedule. Demand controlled ventilation DCV systems reduce the amount of ventilation supply air in response to a measured level of carbon dioxide CO 2 in the breathing zone.

The purpose of CO 2 sensors is to track occupancy in a space; however, there are many factors that must be considered when designing a DCV system. There is often a lag time in the detection of occupancy through the build-up of CO 2. This lag time may be increased by any factors that affect mixing, such as short circuiting of supply air or inadequate air circulation, as well as sensor placement and sensor accuracy. Build-up of odors, bioeffluents, and other health concerns may also lag changes in occupancy; therefore, the designers must be careful to specify CO 2 based DCV systems that are designed to provide adequate ventilation to the space by ensuring proper mixing, avoiding short circuiting, and proper placement and calibration of the sensors.

The Standards requires the use of DVC systems for spaces with all of the following characteristics:. Served by single zone units with any controls or multiple zone systems with Direct Digital Controls DDC to the zone level, and. Has an air economizer. Where the space exhaust is greater than the required ventilation rate minus 0. DCV devices are not allowed in the following spaces: Spaces that have processes or operations that generate dusts, fumes, mists, vapors, or gases and are not provided with local exhaust ventilation, such as indoor operation of internal combustion engines or areas designated for unvented food service preparation, or beauty salons.

The spaces ' listed in Exception 1 are exempted either due to concerns about equipment maintenance practices schools and public buildings or concerns about high levels of pathogens social service buildings, medical buildings, healthcare facilities and to some extent classrooms. An example of this is a restaurant seating area where the seating area air is used as make-up air for the kitchen hood exhaust. The third exception recognizes that some spaces may need additional ventilation due to contaminants that are not occupant borne.

It addresses spaces like theater stages where theatrical fog may be used or movie theater lobbies where unvented popcorn machines may be emitting odors and vapors into the space in either case justifying the need for higher ventilation rates. DCV devices shall not be installed in spaces included in Exception 3.

The fourth exception recognizes the fact that DCV devices may not be cost effective in small spaces such as a 15 ft x 10 ft conference room or spaces with only a few occupants at design conditions. The fifth exception allows an occupant sensor to reduce the amount of ventilation supply air in a vacant room.