Overview of High Frequency Ventilation
High-frequency ventilation (HFV) is a breathing help technique used in emergencies when normal breathing support machines don’t work. Instead of imitating normal breaths, HFV rapidly delivers smaller breaths at high speeds. This strategy used to be common for adults with breathing difficulties, but now it’s mostly used in newborn babies.
HFV has a couple of benefits. It’s lower risk compared to other treatments because it reduces the chance of overstretching the lungs, a problem that can cause injury. It also helps to continually keep the small air bags in the lungs called alveoli inflated, making it easier for oxygen to get into the blood.
There are four main types of HFV. They are:
1. High-Frequency Oscillatory Ventilation (HFOV)
2. High-Frequency Positive Pressure Ventilation (HPPV)
3. High-Frequency Jet Ventilation (HJV)
4. High-Frequency Percussive Ventilation (HFPV)
HFOV is often used for people with a severe lung condition called Acute Respiratory Distress Syndrome (ARDS) when traditional breathing support fails. It delivers very small, super-fast breaths that help keep the air bags in the lungs filled and oxygen flowing. This technique also helps to avoid lung injury by keeping the amount of air, also known as tidal volume, delivered to the lungs very low.
HJV is commonly used in newborn babies. It rapidly delivers very small breaths through a tiny tube placed in the breathing tube. This method is also often used with other standard breathing support methods to help fill the lungs with air.
HFPPV is a type of HFV that’s not really used anymore. It used to be delivered through a regular breathing support machine set to its highest setting.
HFPV is a mixture of high-speed breathing support and regular breathing support. This method switches rapidly between two pressure levels and is thought to be safer and better at delivering oxygen than just regular breathing support. It also tends to require less medication to keep the patient relaxed. Plus, it’s more efficient at removing mucus from the lungs.
Anatomy and Physiology of High Frequency Ventilation
During traditional methods of mechanical ventilation, the transfer of gas happens by essentially moving large quantities of gas molecules from the bigger central airways down to the smaller airways that are towards the edges. This method requires a lot of breathing in and out, more so than when just ventilating the areas of the lungs that don’t participate in gas exchange. But, during High Frequency Ventilation (HFV), which is a type of specialized mechanical ventilation, this is not possible, as the amount of air breathed in and out is lower.
One important way gas is transferred during HFV is by a process called “convection”, where gas is moved in bulk from one place to another. This process is mostly helpful in the proximal (near) airways, but doesn’t play a major role in gas exchange in the peripheral (distant) airways.
Turbulence, the chaotic movement of gas, is another way that gas is transferred, especially in the larger airways. In this case, particles with different speeds and directions, result in a net movement or transport of gas. This type of gas exchange is most often seen where airways split. Other important ways how gas exchange happens during HFV include: Taylor dispersion and molecular diffusion (moving from higher to lower concentration). There are also other processes such as pendelluft (air movement within the lungs), cardiogenic mixing (mixing caused by the heart’s movements), and collateral ventilation (alternate air passages within the lung).
Why do People Need High Frequency Ventilation
High-frequency Ventilation (HFV) is a procedure used to help people breathe when they have certain lung conditions. It delivers more breaths per minute at a smaller volume than traditional ventilation methods, helping to prevent lung injury due to over-inflation. This technique is often used in both adults and newborn babies with specific health issues.
For adults, HFV can be used when they have serious cases of a condition called Acute Respiratory Distress Syndrome (ARDS), where the lungs are filled with fluid making it hard to breathe. HFV helps to prevent ventilator-induced lung injury (VILI), which can happen when lungs are subjected to over-pressure or excess oxygen.
It can also be used in scenarios when the individual’s lungs are not staying open consistently due to bronchopleural fistula (a connection between the bronchial tubes and the space around the lungs), pneumothorax (air leak from the lungs), or pulmonary interstitial emphysema (air trapped in the lung tissue). Other situations when HFV could be used include when conventional mechanical ventilation is not successful, or when there’s a life-threatening lack of oxygen in the blood (refractory hypoxemia).
For newborns, HFV can be used for several conditions, including persistent pulmonary hypertension (high blood pressure in the lungs), ARDS, pulmonary interstitial emphysema, meconium aspiration (inhaling of meconium, a newborn’s first stool, into the lungs), and pulmonary hypoplasia (when the lungs do not develop fully).
However, it’s important to note that there is currently no standard guidance recommending the use of HFOV in adults. It’s typically used as a last-resort means to help with severe ARDS, or when other health conditions make it hard to ventilate the lungs or keep them open.
When a Person Should Avoid High Frequency Ventilation
High-frequency oscillatory ventilation (HFOV), a type of breathing support, doesn’t have any conditions where it absolutely can’t be used. However, it might not work as well for some diseases that increase pressure and resistance in the chest area, like severe asthma. This is because it can cause air to get trapped in the lungs and over-expand them, something known as hyperinflation.
Using HFOV can sometimes lead to complications such as barotrauma (injury caused by increased air pressure) and air leak syndromes. Examples of these are pneumothorax (air trapped in the space outside the lungs), pulmonary interstitial emphysema (air pockets in the lung tissues), and pneumomediastinum (air in the middle part of the chest).
We might also avoid using HFOV in patients who have bleeding inside the skull (intracranial hemorrhage), or severe sepsis. Sepsis is a life-threatening infection that spreads throughout the body and can cause failure of multiple organs.
Equipment used for High Frequency Ventilation
There are different manufacturers that make High Frequency Ventilation (HFV) devices. HFV devices are special types of mechanical ventilators that are used to help patients who are having trouble breathing on their own.
Who is needed to perform High Frequency Ventilation?
It’s important to have trained professionals who can properly set up the medical equipment and deal with any issues or alerts that may arise during its operation.
Preparing for High Frequency Ventilation
Before starting, it’s important to ensure that the medical equipment is working correctly. This includes checking the machine for any issues, setting any necessary alarms, and noting the patient’s Mean Arterial Pressure (MAP) – a measure of the average pressure in a person’s arteries during one heartbeat cycle – if they’re using a traditional breathing machine.
How is High Frequency Ventilation performed
In high-frequency oscillatory ventilation (HFOV) – a method of providing breathing support when you aren’t able to breathe effectively by yourself – there are six main settings which are adjusted according to your needs:
- Breathing Rate/Frequency
- Strength/Power/Delta P
- Average Airway Pressure
- Bias Flow
- Breathing in Duration
- Amount of Oxygen (FiO2)
In HFOV, the frequency refers to how many times the machine breathes for you in a minute. Usually, this is set between 3 to 6 times per second (which would be 180 to 360 times per minute) depending on the individual’s condition.
The amplitude, also known as Delta P, is the primary determinant of how large each breath is (in other words: the tidal volume). A higher amplitude results in larger breaths. This setting is primarily adjusted according to your CO2 levels in your blood.
The mean airway pressure can be adjusted to improve your levels of oxygen. If you’re not getting enough oxygen, the machine can increase the average airway pressure to help. But this adjustment must take place slowly and carefully, usually in small increases of 2 cm H2O.
The bias flow is the rate at which the air flows through the ventilator. In high-frequency ventilation, this is usually around 40 to 60 liters per minute and is adjusted according to your needs.
The duration of the time in which you are breathing in is set to be less than half of the total time of the respiratory cycle. This helps to increase the airway pressure and thereby improves oxygenation. However, it should be kept constant to avoid any risks of lung injury.
The initial settings of HFOV are the starting point and can be adjusted as per your needs. Typically, the bias flow starts with lower values, between 20 to 35 liters per minute and the inspiratory time is set at 33%, resulting in a breathing out to breathing in ratio of 1 to 2. The breathing rate usually starts around 5 to 6 times a second (or 300 per minute).
The changes to these HFOV settings are done gradually based on your body’s response, primarily guided by your blood gas levels and clinical evaluation. A key guide for adjusting the strength or amplitude is to look for visible chest movements. If your CO2 levels are too high and the oxygenation is not sufficient, the settings can be increased.
Before removing HFOV, the settings are gradually reduced until you are able to tolerate ordinary ventilation. Once the goals for oxygenation are met, the amount of oxygen (FiO2) you’re receiving is reduced to less than 60% and the average airway pressure is decreased in increments of 2 cm H2O until it reaches 30 cm H2O. After that, the aim is to further reduce the FiO2 to 0.4 and the average airway pressure to 20 to 25 cm H2O before transitioning you to conventional, lung-protective ventilation. If you’re not able to maintain oxygen levels above 88% in the initial 48 hours after transitioning to conventional ventilation, that is considered as a failure of weaning from HFOV.
Possible Complications of High Frequency Ventilation
High-frequency ventilation (HFV), a kind of breathing support for patients with severe lung problems, does come with some challenges. It isn’t always effective in patients with high airway resistance, meaning their airways are narrow or blocked, which can cause air to get trapped in the lungs. This trapping can cause a variety of lung injuries, including a collapsed lung (pneumothorax), air in the space around the heart (pneumopericardium), or air in the space surrounding the lungs (pneumomediastinum), and a condition where air gets into the tissue of the lungs (pulmonary interstitial emphysema).
Another issue is that HFV can affect the interaction between the heart and lungs, leading to high pressures within the chest. This in turn can decrease the amount of blood returning to the heart and being pumped out to the rest of the body.
Since HFV isn’t a common way to help patients breathe, it can also make it more difficult for the body to clear mucus and other secretions from the lungs. This can increase the risks of developing an infection in the bloodstream (sepsis).
Also, HFV can come with practical problems. It can be difficult to move patients with this type of breathing support, and the machines can be noisy, which can make it hard for doctors to perform examinations. These issues may also delay doctors from identifying any complications that might come up.
What Else Should I Know About High Frequency Ventilation?
Mechanical ventilation, while necessary, can sometimes harm the lungs. This is commonly due to issues like barotrauma (injury caused by increased air pressure), volutrauma (damage from too much air volume), and atelectrauma (injury from the repeated expansion and collapse of small air sacs in the lungs, known as alveoli). High volumes of air (tidal volumes) and high pressure can make this lung injury worse.
The most reliable way to prevent this type of injury is by using a low volume of air. This method is also known as low tidal volume ventilation. In high-frequency ventilation (HFV), a different kind of breathing support, air operates in a safe zone that avoids overinflating the lungs and prevents injury.
HFV allows the lungs to fill up with air uniformly and stops the constant opening and closing of small air sacs in the lungs. This helps prevent lung injury and helps make sure the body gets enough oxygen. Less volume of air than the volume of the airways that are not involved in gas exchange (dead space volume) is used in HFV which again prevents this harmful opening and closing. It maintains a consistent ventialtion pressure, limiting the risk of lung injury, and assists in the opening of collapsed areas of the lungs through high pressure at the end of expiration.
Despite this, the benefits and risks of HFV in Acute Respiratory Distress Syndrome (ARDS – a life-threatening lung condition where the lungs get filled with fluid) haven’t been extensively studied. Some studies, like the OSCILLATE trial, found a higher death rate among those receiving HFV compared with conventional ventilation. Others, like the OSCAR trial, found that HFV had no significant impact on death rates in ARDS patients. The RESTORE study found that pediatric ARDS patients receiving HFV required more sedation and had longer stays in the intensive care unit (ICU).
A 2017 research review found that HFV may not only be ineffective in mild to moderate ARDS but could potentially be harmful. Still, it might be beneficial as rescue therapy in severe ARDS cases with uncontrolled low oxygen levels, especially when other treatment options aren’t available.
