Introduction

Ventilators (support and therapy)

When the patients are unable to breathe themselves they must have artificial (assisted) respiration. As this can go on for many hours and days (intensive care units) it must be taken care of by a machine, the ventilator⁵. We are talking about lightening the bagging burden for medical personnel, giving them a “third hand”. In addition to this support function the ventilator is used therapeutically by controlling pressure and flow to the optimum condition for lung healing.
As we have seen earlier in this chapter the natural lungs initiate inspiration by lowering the diaphragm thus increasing the lung sack volume and creating a negative pressure in the lung alveoli so that external gas is inhaled. The ventilator functions the opposite way, during inspiration the ventilator creates a higher pressure pushing fresh gas into the lungs. It is only the “iron lung”⁶ with the whole patient (except head) enclosed in a chamber that reproduce natural physiological conditions: chamber reduced pressure initiates inspiration.
An advanced ventilator must, like a pacemaker, have a demand modus. If there are no efforts from the patient, the ventilator should be in control. But as the patient recovers or wakens, the patient starts to breathe spontaneously, and the ventilator can let the patient gradually take over. The respiration cycle time parameters are important: inspiration time, pause, expiration time. An advanced respirator may be set either to volume or pressure controlled mode. In the volume controlled mode the inspired volume is measured continuously, and when it reaches a preset level the inspiration phase is terminated. Pressure is also measured continuously, and the user has chosen values that shall not be exceeded or shall result in an alarm. For instance a selected PEEP (Peak End Expiratory Pressure) modus decides when to leave the expiration phase so that the lung pressure does not drop below a critical point where the lungs might collapse. In the pressure controlled mode it is the pressure reaching a preset value which triggers the ventilator to end the inspiration phase. The choice is made from what is considered to be best for the patient. And remember that the flow and volumes which we just have referred to are different in different parts of the airways inside and outside the patient, the sampling position is important as already stated in the subchapter on gas sampling.

Technology: piston, bag-in-bottle, servo

Small ventilators for use outside hospitals may be purely gas driven, but all advanced ventilators are electrically driven. The main components are:

1. 
   A pressure generating and controlling device (gas under pressure or electric pump)
   
2. 
   A cycling device with timer, changing the modus between inspiration, pause and expiration
   
3. 
   Sensing elements and displays
   

The most direct version is to put the bag of a manually operated breathing system (Fig.10) system into a bottle, and control the pressure in the bottle from an external ventilator supply. Another way is to let a motordriven piston supply the pressure cycle. Fig.23 illustrates a servocontrolled system, shown during the inspiration cycle.

Figure 23 Servocontrolled ventilator shown in the inspiration cycle
The fresh gas enters a pressure-controlled static reservoir. The pressure setting there determines the maximum pressure which can be supplied to the patient, and is therefore also a safety feature. The gas then enters the inspiration servo-controlled part. The flow in this example is measured by the deflection of a vane and compared with the set-point coming from the electronic control box. The minute volume is a basic parameter set by the operator and used in the control system. Deviations result in a servo correction signal which is sent to the control valve. The set point can represent different wave forms, not just a square wave on-off function. The inspiration pressure is measured and the information sent to the control box. Dependent on the mode chosen by physician the pressure can control the servo e.g. during CPAP operation (Continuous Positive Airway Pressure). During long-term patient treatment in an intensive care unit the gas must be humidified. In series with the humidifier a vaporizer for anaesthetic volatile drugs may be inserted for use in the operating room. When the control box so determines the inspiration phase is ended, the inspiration valve closes and the expiration valve opens. Also here the expiration flow is measured continuously and the control box can set up any flow curve preferred. The expiration pressure can also control the valve if the ventilator is set in such a modus (e.g. PEEP Positive End-Expiratory Pressure). Both pressure measuring systems may be coupled to alarm circuits if a preset pressure level is exceeded.

The ventilator shown on Fig.23 has no rebreathing system.

Compression losses

Figure 24 Compression loss model

The pressure P in the patient system increases during the inspiration cycle when a piston is pushed in as shown on Fig.24. The lungs are gradually filled until the machine ends the inspiration cycle. Let us simplify and only consider the inspiration and expiration tubings and with zero compliance. With a certain ventilation volume ΔV_resp, the pressure build up is dependent on the volume of the two tubes V₁ = V_i + V_ex. According to the universal gas law P₂=P₁V₁/V₂ in a closed system. If V₁= 1L, P₁=100kPa and ΔV_resp is 0.2L, then P₂=1·100/0.8 = 125 kPa. If the lung compliance C_L = ΔV_L/ΔP is low and the airway resistance R is high, the rise in pressure ΔP = 25 kPa may result in a very low lung ventilation ΔV_L. The ventilation of the lungs may be much smaller than the ventilator setting. Therefore: the larger the tube volume with respect to the ventilation volume, the less part of the ventilator gas enters the lungs of the patient. This is called ventilation loss, and must be taken into consideration especially with small and stiff lungs (children) or long tubes.

Risk considerations Ventilators

Difference between inspiratory system and expiratory system with respect to humidity, mucus (slime) and need of disinfection and sterility, especially in one-way breathing systems.

Transducer breakdown. Gas leakage, wrong tube connection. System control before a new patient is connected to the ventilator. Stops, electric power failure, gas delivery failure. Accidental change of settings.