Pressure Controlled Ventilation

     
       

 

         
       

Remember: pressure control only refers to the flow target mechanism: pressure control can be controlled (CMV), assist-controlled or SIMV.

Pressure Control refers to the type of breath delivered, not the mode of ventilation. Many different modes are pressure controlled (1). Conventionally, the term “pressure control” refers to an assist control mode (there is also a SIMV pressure control mode on some ventilators). In pressure control, a pressure limited breath is delivered at a set rate. The tidal volume is determined by the preset pressure limit. This is a peak pressure rather than a plateau pressure limit (easier to measure). The inspiratory time is also set by the operator. Again this is a trade off between short times with rapid inflow and outflow of gas, and long times with gas trapping. The flow waveform is always decelerating in pressure control: this relates to the mechanics of targeting airway  pressure: flow slows as it reaches the pressure limit.

Gas flows into the chest along the pressure gradient. As the airway pressure rises with increasing alveolar volume the rate of flow drops off (as the pressure gradient narrows) until a point is reached when the delivered pressure equals the airway pressure: flow stops. The pressure is maintained for the duration of inspiration (2). Obviously, longer inspiratory times lead to higher mean airway pressures (the “i” time (Ti) is a pressure holding time after flow has stopped). The combination of decelerating flow and maintenance of airway pressure over time means that stiff, noncompliant lung units (long time constants) which are difficult to aerate are more likely to be inflated. Gas distribution in pressure control is like dropping a glass of water on the floor: the water trickles into every nook and cranny. (3).

Pressure Assist Control Ventilation, 1 second inspiratory time: note that the breaths are identical in duration, whether controlled or assisted.

It is known that decelerating flow patterns improve the distribution of ventilation in a lung with heterogeneous mechanical properties (as in acute lung injury) (4). Pressure control is also useful in patients whose airway cannot be fully sealed – children, patient with bronchopleural fistulae etc. The reason for this is that, although volume is lost through the leak, the ventilator will continue to attempt to pressurize the airway for the duration of the Ti: a constant flow pattern will be measured if the leak is large enough.

Patients can breath spontaneously on pressure control as long a the inspiratory time has not be unduly prolonged. The trigger mechanism is the same as in volume control. The key advantage of pressure targeted ventilation is unlimited flow in inspiration to satisfy the patient’s demands. The harder the patient draws in, the greater the pressure gradient, and the higher the flow. 

What are the drawbacks of pressure control?

Pressure control does not guarantee minute ventilation, and therefore requires more monitoring by the operator. The minute ventilation is a complex mix of the peak pressure, the Ti, the lung and chest wall compliance, resistance in the airway and from other thoracic structures. If there is a rapid change in the compliance, then the patient may hypoventilate and become hypoxic.

 

This is a volume-pressure relationship in the same patient, on volume control ventilation (darker line) and pressure control ventilation (lighter line). Note that both modes achieve the same tidal volume, but the peak pressure is considerably lower in pressure control. This was achieved by using different flow patterns – constant flow for volume breaths, decelerating for pressure breaths. In addition, the pressure breath used a slightly longer inspiratory time.

To initiate pressure control is slightly more difficult than volume control. Again, the PEEP and FiO2 are determined by lung mechanics and oxygenation targets. The inspiratory pressure is determined by looking for a tidal volume of 5-6ml/kg. The respiratory rate is determined by the minute volume requirement. The inspiratory time is usually set at 1sec, but can be increased if 1. target tidal volume is not achieved with, 2. The patient remains hypoxic in spite of a plateau pressure >30cmH2O. In this way the mean airway pressure is used to increase overall lung volumes, and improve V/Q matching. Unfortunately, there is a limit to this process, auto-PEEP. Longer inspiratory times and faster respiratory rates predispose to alveolar gas trapping. This intrinsic PEEP is present in addition to applied PEEP at the beginning of inspiration, placing the patient on a less compliant (overdistended) part of the volume-pressure curve. Thus tidal volumes fall and airway pressures rise.

What is pressure regulated volume control (PRVC)
or volume assured pressure control?

This is a clever mode that combines a pressure limit with volume assurance, by mechanically manipulating Ti and flow. It is thus a duel control mode of ventilation (5). The advantage is that minute ventilation can be guaranteed, the disadvantage is that weaning is very tricky.

What is pressure-assist ventilation (PAV)?

Pressure assist ventilation is pressure control without a set rate. Patients take pressure controlled breaths at the rate of their choosing, and the volumes derived are determined by the pressure preset level, the Ti and the flow demanded. This is a very comfortable mode, and is used in weaning from pressure control (the pressure limit is weaned). The Ti assures the duration of the breath, and prevents the patient from hyperventilating, which sometimes occurs in pressure support.

SEE ALSO
CLICK HERE TO READ ABOUT PRESSURE CONTROLLED VENTILATION IN ACUTE LUNG INJURY
CLICK HERE TO LEARN PRESSURE CONTROL VENTILATION STRATEGY

References

  (1)   Cinnella G, Conti G, Lofaso F, Lorino H, Harf A, Lemaire F et al. Effects of assisted ventilation on the work of breathing: volume- controlled versus pressure-controlled ventilation. Am J Respir Crit Care Med 1996; 153(3):1025-1033.

  (2)   Tharratt RS, Allen RP, Albertson TE. Pressure controlled inverse ratio ventilation in severe adult respiratory failure. Chest 1988; 94(4):755-762.

  (3)   Burke WC, Crooke PS, III, Marcy TW, Adams AB, Marini JJ. Comparison of mathematical and mechanical models of pressure-controlled ventilation. J Appl Physiol 1993; 74(2):922-933.

  (4)   Munoz J, Guerrero JE, Escalante JL, Palomino R, De La CB. Pressure-controlled ventilation versus controlled mechanical ventilation with decelerating inspiratory flow. Crit Care Med 1993; 21(8):1143-1148.

  (5)   MacIntyre NR, Gropper C, Westfall T. Combining pressure-limiting and volume-cycling features in a patient- interactive mechanical breath. Crit Care Med 1994; 22(2):353-357.

         
                   
       

         
     

       
       

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