Mechanical Ventilation

Mechanical Ventilators

In light of the coronavirus and the concern for mechanical ventilation, I wanted to share some information some of you may be interested. I am a Registered Respiratory Therapist of forty-six years at the University of Kentucky Medical Center. I am not writing this post to impress anyone, just to inform those who may want to know a little more about mechanical ventilation. The basic goals of mechanical ventilation are to: support ventilation, increase efficiency of respiration, and decrease cardiac and ventilatory workload.

Normal spontaneous ventilation includes the following: tidal volume 400-600 ML; minute ventilation 5-7 LPM; alveolar ventilation 3-5 LPM; respiratory rate 12-18 BPM; inspiratory to expiratory time 1 to 1.5 seconds; and inspiratory flowrate 25-30 LPM. In other words, when you take a breath, you breathe in 400 milliliters of volume, and if your respiratory rate is 12 breaths per minute, then your minute volume after 1 minute of breathing would be 5 liters. As you take a breath, your inspiratory time to get the 400 ML of volume into your lung would be 1 second, and to exhale the 400 ML, it takes 1.5 seconds for the lung to exhale the volume out. When we breathe, we breathe 21% oxygen from the air around us.

There two important factors that affects how well we ventilate. First, is airway resistance, which involves any obstruction to flow in the airway. The greater the obstruction to flow, the greater the airway pressure. This can be an increase in pulmonary sections for example. Second, is lung-chest (thorax) compliance, which involves the elasticity of the lungs and chest cavity. As our lungs and the muscles and lingings of our lungs loose their elasticity to expand, the compliance value of ventilation decrease, and as the lungs improve, compliance increases. Normal RAW is 0.6-2.4 cmH2O/L/sec. Normal CLT is 100 ML/cmH2O.

Mechanical ventilators are volume or pressure operated, depending on how the manufacture decides. Both have their place in the field of ventilation. The simplest mechanical ventilator is the “bag and mask”, which works well in short-term emergencies’. However, once the patient gets to the emergency room or the ICU, it is good to have a ventilator with the accessory modes of ventilation to meet the patient’s ventilatory needs. Ventilators have come a long way from the simple piston-cylinder type to the sophisticated microprocessor ventilators of today. Today, ventilators have between 10 and 15 different modes of ventilation, all of which can be tailored to individual needs of each patient. Ventilator settings, such as volume, pressure, respiratory rate, flowrate, I:E ratio, and FIO2, also can be adjusted to the needs of the patients respiratory demands.

When a person is put on a ventilator, the initial settings are a lot different from spontaneous breathing. Their tidal volume is 10-15 ML/Kg of body weight, minute ventilation is 7-16 LPM, respiratory rate is 10-20 BPM, I:E ratio is 1:1 to 1:3, inspiratory time depends on I:E ratio set, flowrate is 40-60 LPM, and the oxygen % (FIO2) is 0.3-1.0. Normal airway resistance on a ventilator is 6-8 cmH2O/L/sec because the patient has been intubated, and as soon as air is “forced” into the lungs, airway resistance increases from higher volumes and increased secretions. Normal compliance on a ventilator is 40-50 ml/cmH20 because of increased secretions and other lung tissue changes.

What I want you to see, mechanical ventilation is not a normal way of breathing. Nothing can take the place of our God given bodies to spontaneously breathe. Patients should not be on a ventilator any longer than necessary. Sedation should be lessened as soon as possible so the patient can begin to breathe on his/her own. As soon as they go on one, you start looking how to get them off. Ventilators are only used to borrow time for the body to heal.

Next, I want to talk a about lung and thoracic pressures. During normal breathing or resting breathing, the constant pull between the collapsing lung and the expanding thoracic cavity generates a subatmospheric intrapleural pressure of approximately –5 cmH20 between the parietal and visceral pleura. The visceral pleura is the delicate membrane that covers the surface of each lung, and dips into the fissures between the lobes of the lung. The parietal pleura is the outer membrane which is attached to the inner surface of the thoracic cavity. So there is a space between chest cavity and the lungs.

During normal breathing, the pressure inside the alveoli (air scacs), or intrapulmonary pressure, drops from -2 to -4 cmH2O, and the intrapleural pressure between the chest cavity and lungs drops from –5 to –7 cmH20, allowing air to flow into the lungs.
The majority of the respiratory work is to overcome the elasticity of the lungs. The difference between intrapleural and intrapulmonary pressures created by the respiratory muscles of the chest is that pressure which overcomes the elasticity of the lungs. The airway pressure difference between the mouth and the alveoli is -1 to -2 cmH20 and is the pressure that overcomes airway resistance. During mechanical ventilation, pressures to ventilate are no longer negative, which facilitate normal blood flow through the lungs and heart, but now are positive. Positive pressure to ventilate can affect pulmonary perfusion if not monitored. With "stiff", non-compliant lungs, clinicians may find themselves walking a "tight-rope" between adequate oxygenation and perfusion due to high pressure to ventilate.

So in light of the coronavirus, patients that exhibit pneumonia-like pathology will need mechanical ventilation if and when their pulmonary status worsens. When this occurs, pulmonary resistance and compliance changes will occur. Some of the conditions that affect pulmonary compliance are fibrotic tissue formation, fluid overload, pulmonary edema and lung parenchyma changes such as the loss of elasticity. Placing the proper size endotracheal tube for maximum flow and volume, along with endotracheal suctioning, can lessen airway resistance problems.

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