Marcelo Alcantara Holanda
-Associate Professor of Pulmonary and Critical Care Medicine, Federal University of Ceará (UFC)
-Respiratory Physician in the ICU of the Dr Carlos Alberto Gomes Studart, Messejana Hospital, Ceará
-Founder of the xlung platform for teaching Mechanical Ventilation
By the end of this chapter the reader should be able to:
Understand the physiological spontaneous respiratory cycle;
Understand and classify respiratory cycles during mechanical ventilation ;
Understand the basic modes of mechanical ventilation;
A/C, VCV - Assisted/Controlled, Volume Cycled Ventilation
A/C, PCV - Assisted/Controlled, Pressure Controlled Ventilation (time cycled)
SIMV - Synchronized Intermittent Mandatory Ventilation
PSV - Pressure Support Ventilation
One can define ventilatory mode as the process by which the mechanical ventilator determines, either partially or fully, when the mechanical breaths are to be provided to the patient, thus determining the breathing pattern of the patient during mechanical ventilation. For the purposes of classification, there is still a need for an international consensus or standardization as there remains non-standardized and confusing terminology. This is compounded by the adoption of different commercial brand names by manufacturers of mechanical ventilators, often for modes with similar functionality. In 2010, about 54 names of respiratory "modes" were available in 49 brands of mechanical ventilators. This scenario creates challenges in the adequate training of healthcare professionals, at times leading to the inappropriate management of the most common ventilation modes, and even endangering the lives of patients undergoing mechanical ventilation.
This chapter presents a simple and logical definition of commonly used basic ventilatory modes. It is divided into 4 parts: the concept of the spontaneous physiological respiratory cycle; the respiratory cycle provided by the mechanical ventilator; commonly used ventilation modes, their settings and limitations; and finally, the prospects for new methods that have been recently made available. To facilitate the explanation of the different cycles or modes, figures were created using the xlung simulator. These are based on the equation of the motion of gases in the respiratory system.
- The spontaneous physiological respiratory cycle
- The Respiratory Cycle During Mechanical Ventilation
- Controlled Cycles
Figure 1 shows the physiological, or spontaneous, respiratory cycle without support from a mechanical ventilator.
Figure 1. Physiological respiratory cycles. The intensity and duration of the pressure generated by the inspiratory muscles (Pmus) varies, modifying the flow, volume, and pressure of air and alveolar pathways in a patient with nearly normal lung function (Raw: 3cmH2O.l.s and Cst: 150ml/cmH2O). The dotted line highlights the moment of transition from the inspiratory to the expiratory phase in one of the cycles. Note that the Pmus determine the time, flow and inspired tidal volume to the extent that it can lower alveolar pressure. Refer to the text below for a more detailed explanation.
In this figure, inspiratory muscle effort, represented by Pmus, varies in intensity and duration in each cycle. According to Boyle's law, to expand the volume of the chest cavity the Pmus reduce the pressure of alveolar gas. This is represented by the alveolar pressure in blue, with the values dipping slightly below what is considered the zero reference value of atmospheric pressure. This generates a pressure gradient between the proximal airways (nose and mouth) and the lung parenchyma, resulting in a flow of air from the external environment into the alveoli through the airways generating the inspiratory flow. The shape of the wave and the intensity of this flow is determined by this pressure gradient and the airway resistance. Over time, a certain volume of air is blown into the alveoli, defined as tidal volume (VT), calculated as tje integral of flow x time. As the alveoli are inflated and the pulmonary parenchyma is stretched, the elastic lung tissue pressure rises in direct proportion to the inspired tidal volume divided by compliance of the lungs and chest wall.
The inspiration time is the interval between the beginning of the intake of air to when the maximum value of VT is reached. With the gradual decline of Pmus at the end of inspiration, followed by the complete relaxation of the inspiratory muscles, a priorly negative alveolar pressure progressively rises to the point where it exceeds the pressure of the proximal airways, which remains at zero. At this point, the wave flow is reversed and expiration from the lungs to the external environment begins. Normally, the wave of expiratory flow has a negative value. The exhaled air is passively driven by alveolar pressure that is elevated in the final stage of inspiration due to increased lung elasticity and the relaxation of the inspiratory muscles. Exhalation takes place according to the time constant of the respiratory system, consisting of the product of Raw x Cst until the moment that the alveolar pressure again equilibrates with the airway pressure and the flow ceases.
The expiratory time is calculated as the interval from the beginning of the expiratory flow to the beginning of the next inspiration. This process is controlled by the brain’s respiratory or pneumotoxic center located in the medulla. This is determined by a complex set of mechanisms involving, among other elements, afferent neural impulses from peripheral and central chemoreceptors, mechanoreceptors in the lungs and chest wall, the cerebral cortex and other regions of the central nervous system. It is this intricate mechanism of the respiratory cycle that operates the human being's "natural ventilator". Not surprisingly, ventilation support still presents major limitations despite great technological advances in recent decades.
Mechanical ventilation is essentially a process that replaces all or part of the action of the inspiratory muscles as well as the neural control of breathing. Two basic types of respiratory cycles can be defined. In the first type, the ventilator "controls" the entire inspiratory phase, or totally replaces the respiratory muscle effort and neural control of the patient. This cycle is referred to as a “controlled” cycle. In the second type, the venitlator only assists the inspiratory muscles that are active and is referred to as an “assisted” cycle. Some authors use the term "spontaneous cycle" to refer to that which occurs during supply of pressure support (PS). Instead, the term “assisted” is used here to designate the second type of cycle in order to be consistent with the above definition. The term “spontaneous cycle” is used here only for physiological breathing.
In addition to these two major divisions, the cycle of the mechanical ventilator can also be classified according to variables that are controlled during inspiration: these include time, flow, volume or pressure. On the other hand, it could be a combination of two or more of these. For example, a “controlled cycle” could be called VCV (Volume Cycled Ventilation) if it is programmed to end or "cycle" when it reaches a predetermined value of tidal volume (VT) -- or it could be a timed cycle, called PCV for Pressure Controlled Ventilation.
Figures 2 and 3 show Volume Controlled Cycles
Figure 2. Volume Cycled Ventilation (VCV) mechanical respiratory cycles. The inspiratory flow was modified in three cycles, resulting in different times and inspiratory airway pressures (in red). The alveolar pressure (in blue) did not vary because it was determined by a fixed VT (dashed line). See the text below for more details.
Figure 2 presents three cycles of VCV. In this situation, the respiratory muscle effort, represented by the Pmus, is zero. The VT was set to 500ml (0.5l). In addition to the VT, the intensity and the wave pattern of flow is determined by the operator of the ventilator. Thus, the inspiratory time is predefined, based on the ratio of VT/flow. In the first cycle, with a constant flow, or square type, set to 60l/min (1 l/s), the inspiratory time corresponds to the division of 0.5l by 1l/ s, or 0.5s. In the second cycle, the flow was reduced by half or 30l/min (0.5l/s) doubling the Ti to 1s. In the third cycle, not only was the maximum flow reduced by half, but the wave pattern of flow was adjusted, on a descending slope, reducing it to 50% of its initial value. This adjustment resulted in an average flow of 22.5l/min or 0.375l/s and thus an even longer Ti, 0.5l/0.375l/s or 1.33s. Note that the airway pressure (in red) but not the alveolar pressure (in blue) varies according to adjustments of flow, since this influences the resistive pressure of the airways. The alveolar pressure remains the same in the three cycles since the VT, its principal determinant, is the same in all three cycles. As in the spontaneous cycle, the air is passively exhaled by the increase of the pulmonary elasticity (alveolar pressure) by simply having the ventilator prevent the entry of air, and opening the expiration valve. Note that the expiration proceeds until the alveolar pressure returns to a predetermined value, in this case, above zero as determined by the setting of a positive end expiratory Pressure or PEEP.
Figure 3 shows the impact of the VT settings on the Ti and the airway pressure in controlled VCV cycles.
Figure 3. Volume Cycled Ventilation (VCV) mechanical breathing cycles. The VT was modified in three cycles, resulting in different times and inspiratory airway pressures and alveoli. The flow rate was held constant (dotted line). See the text below for more details.
In Figure 3 the operator of the ventilator modifies the VT and maintains a constant flow. The pressures of the airway opening and lung volume vary in direct proportion to changes in the VT. Note that the Ti also varies (Ti = VT/flow). In practice, the VCV cycling is very easily adjusted by simply defining the target VT. For example, 8ml/kg of predicted body weight, and adjusting the flow to guarantee a Ti of approximately 0.6s to 1.2s, depending, of course, on the recommended specific ventilation strategy for a particular patient.