Some basic concepts of pneumatics, a branch of physics, are required to understand the operating principle, composition, and functions of a ventilator system. These basic concepts are the common foundation of all ventilator systems, simple or sophisticated.
2.2 Pressure (P) and pressure gradient (ΔP)
Let’s assume that two areas, A and B, are connected so that gas can move freely in either direction (Fig. 2.1). Gas moves only when two conditions are satisfied. First, there must be a difference between the pressures in areas A and B. The pressure gradient drives the gas to move from the area of high pressure to the area of low pressure. Second, the connection must be open.
A ventilator system has a defined gas passageway. Along the passageway, the pressure differs at different points, generating several regional pressure gradients. For a ventilator system to function properly, it is critically important to keep the pressures within their specified (normal) ranges, because ultimately these pressure gradients provide the driving forces to move gas in a controlled manner.
Fig. 2.2 shows the gas passageway and the normal pressure ranges in the Hamilton Medical GALILEO ventilator system.
Understandably, if the gas passageway has a hole anywhere, the positive internal pressure pushes the gas out.
Inspiratory pressure, peak pressure, plateau pressure, mean airway pressure, and PEEP (positive end-expiratory pressure) are common terms to describe the pressures encountered in mechanical ventilation.
Gas pressure is expressed in various units. The common ones include bars and pounds per square inch (psi) for high pressures, and millibars (mbar), centimetres of water (cmH2O), kilopascals (kPa), and hectopascals (hPa) for low pressures. Pressure units may be converted back and forth as shown in Table 2.1.
Table 2.1 Conversion of common pressure units
1 bar = 1000 mbar
1 psi = 68.9 mbar
1 mbar = 0.0145 psi and 1 bar = 14.5 psi
1 cmH2O = 0.98 mbar
1 mbar = 1.02 cmH2O*
1 kPa = 10 hPa = 10 mbar
1 hPa = 1 mbar
* For convenience, 1 mbar is rounded to 1 cmH2O.
2.3 Volume (V)
Tidal volume, minute volume, and leak volume are common volume-related terms used in mechanical ventilation. Gas volume is typically expressed in millilitres (ml) or litres (L or l). One litre equals 1000 millilitres.
Gas is compressible. The volume of a given amount of gas decreases when the applied pressure increases, and vice versa. Therefore, a large amount of medical gas at atmospheric pressure (e.g. oxygen) may be compressed into a small gas cylinder at a very high pressure.
The pressure inside the gas passageway of a ventilator system fluctuates regularly during mechanical ventilation. In a passive patient, the circuit, airway, and alveolar pressures typically range between zero and 40 cmH2O. The pressure variation causes corresponding gas compression. For a pressure-based mechanical breath where volume is not directly controlled, the small gas compression is not an issue. However, for a volume-based mechanical breath where the volume of delivered gas is controlled, the compression is the major cause of a phenomenon called ‘lost volume’, which will be described in Chapter 5.
Time is important because mechanical ventilation is essentially the regular alteration of airway pressure over time.
In the vocabulary of mechanical ventilation, you find many terms directly or indirectly related to time, including triggering, cycling, inspiratory or expiratory time, I:E ratio, rate, %Ti, and time constant.
Time is expressed in minutes (min), seconds (s), or milliseconds (ms).
1 hour = 60 minutes
1 minute = 60 seconds
1 second = 1000 milliseconds
Gas moves between two connected areas when there is a pressure gradient between the two areas and the connection is open (Fig 2.3). Flow is the movement of gas over time. The consequence of flow is volume change.
Gas flow has two essential properties: direction and rate. Flow direction is determined by the pressure gradient alone. Flow rate, defined as the amount of gas that flows in a given time, is determined by both the pressure gradient and resistance encountered (refer to the next section). So, gas does not flow if there is a zero pressure gradient or if the resistance is infinite.
In mechanical ventilation, we use positive and negative values to indicate the direction of airway gas flow. Inspiratory flow, expressed in positive values, refers to the gas movement towards the patient. Expiratory flow, expressed in negative values, refers to the gas movement away from the patient.
The rate of gas flow is usually expressed in litres per minute (L/min) or millilitres per second (ml/s). One litre per minute is equal to 16.7 millilitres per second.
At a low flow rate, gas tends to move smoothly. The resultant pattern is called laminar flow. At a high flow rate, gas tends to move unevenly. The resultant pattern is called turbulent flow.
2.6 Resistance (R)
Resistance is flow dependent. Whenever a gas flows through a tube, a given resistance is generated. It is usually expressed in millibars per litre per minute (mbar/L/min) or centimetres of water per litre per minute (cmH2O/L/min).
The following factors determine the magnitude of resistance:
◆ Flow rate. The higher the flow rate, the greater the resistance, and vice versa. If the flow rate drops to zero, resistance disappears.
◆ Physical properties of the tube, such as length, internal diameter, inner surface, and curvature.
◆ Physical properties of the gas, such as density and viscosity. Heliox is a mixture of helium and oxygen. It is sometimes used as a supply gas in mechanical ventilation. The density of heliox is lower than that of air, which is a mixture of nitrogen and oxygen. At the same flow rate, heliox generates less resistance than air. This may be clinically beneficial in patients with abnormally high airway resistance. This fact is the foundation of heliox therapy.
The relationship between pressure gradient, flow, and resistance is described by Ohm’s law. Flow equals the pressure gradient divided by resistance:
So, flow increases when the pressure gradient increases and/or resistance decreases, and vice versa.
To better understand this concept, let’s take a look at some examples we see routinely in clinical practice:
◆ Intubation decreases the effective cross section of the airway, resulting in a higher airway resistance. A larger pressure gradient is required to achieve the same airway flow. This is particularly important for a ventilated patient who is actively breathing.
◆ A partial occlusion of the gas passageway results in higher resistance to gas flow. Typical examples include a kinked endotracheal tube (ETT), a bent or compressed patient circuit tube, and a blocked air filter.
◆ An arbitrary change in length or inner diameter of a tube causes a corresponding change in resistance.
◆ Asthma and bronchial spasm are well known clinical examples of excessive airway resistance.
2.7 Compliance (C)
A balloon and human lungs have the following in common: they are elastic, hollow, and gas-tight structures. Their volumes change when their internal pressures vary. Compliance is a parameter used to describe the pressure–volume relationship of such structures. A simple physical model can help our understanding (Fig 2.4).
Imagine that we have two balloons made of the same elastic material. The wall of one balloon is slightly thicker than the other. Using a T-piece equipped with a valve, we connect the two balloons to a source of high-pressure gas. In this arrangement, the pressure in both balloons always remains the same.
Now we gradually open the valve, causing gas to flow into both balloons and increasing their internal pressure. As a result, both balloons begin to expand. Not surprisingly, the balloon with the thin wall ‘grows’ faster than that with the thick wall.
If we measure the internal pressure and balloon volumes simultaneously and then plot the readings on a two-dimensional chart, we get two pressure–volume curves with different angles to the horizontal. The angles represent the compliances of the two balloons.
By nature, compliance is a static property, and should be measured under conditions of zero flow.
Compliance is usually expressed in millilitres per centimetre of water (ml/cmH2O).
Normal respiratory compliance is approximately 150–200 ml/cmH2O in adults and 6–8 ml/cmH2O in newborns. This means that an increase in alveolar pressure of 1 cmH2O causes an increase in the lung volume by 150–200 ml in an adult or 6–8 ml in a newborn. Respiratory compliance is the sum of lung compliance and chest wall compliance.
Both overly high and overly low compliances are abnormal. ‘Soft lung’ refers to the situation where lung compliance is abnormally high. Even a small pressure gradient results in a sizeable lung volume change. Typical clinical examples are chronic obstructive pulmonary disease (COPD) and emphysema patients. ‘Stiff lung’ refers to the situation where the lung compliance is abnormally low. Even a large pressure gradient results in a small lung volume change. Typical clinical examples are acute respiratory distress syndrome (ARDS), acute lung injury (ALI), and respiratory distress syndrome (RDS) in newborns.
2.8 Time constant (RC)
Flow is defined as the change in volume over time. It takes time to complete lung inflation or lung deflation. If the defined time is shorter than required, the process is incomplete.
If the permitted inspiratory time (Ti) in a pressure-based breath is shorter than required, the resultant tidal volume may be substantially reduced. Perhaps surprisingly, you can increase tidal volume by increasing Ti.
If the permitted expiratory time (Te) is shorter than required, a part of the gas volume that should be expired is trapped in the lungs. This results in higher alveolar pressure at the end of expiration than would otherwise be the case. This abnormal phenomenon is called autoPEEP or intrinsic PEEP.
The length of time required to complete a volume change varies in individual patients, depending on their clinical condition. Is there any way to objectively estimate the time required to complete a volume change for a specific patient, you may wonder. Yes—the tool is the time constant. (Fig 2.5).
The time constant is the product of the monitored compliance (C) and resistance (R) of the respiratory system:
Therefore, the time constant increases when resistance and/or compliance increases, and vice versa. In a passive lung model, the volume change is exponential over the course of inspiration or expiration, being highest at the beginning and then slowing down.
The time constant is expressed in seconds. In theory:
◆ After one time constant (1 × RC), 63% of the volume change is complete;
◆ After two time constants (2 × RC), 86.5% is complete;
◆ After three time constants (3 × RC), 95% is complete;
◆ After four time constants (4 × RC), 98% is complete; and
◆ After five time constants (5 × RC), 99% is complete.
It is generally agreed that, to be adequate, inspiratory time should be at least three inspiratory time constants long, while expiratory time should be at least three expiratory time constants long. For instance, if the estimated expiratory RC is 1 second, Te should be at least 3 seconds.
The time constant is clinically significant, as it is how we can estimate the minimal Ti and Te required for a given patient.
In most clinical cases, there may be sufficient room for Ti and Te to complete lung inflation and deflation. In COPD patients, characterized by high R and high C, the time constant is often very long. To ensure complete lung inflation and deflation, we may need a low rate for a very long Te and a corresponding large tidal volume. In this case, the acceptable ranges for the Ti and Te settings are very limited.