INTRODUCTION

A ventilator is a machine that performs some or all of the work of breathing. In veterinary medicine conventional positive-pressure ventilation is used most commonly. These machines utilize an increase in airway pressure to move gas into the lungs, in contrast to spontaneous breathing where airway pressure decreases below atmospheric pressure in order to generate the inspiratory phase of a breath.¹

The respiratory function of the lungs is to oxygenate the arterial blood and remove carbon dioxide from the venous blood. Oxygenation refers to the movement of oxygen from the alveoli into the pulmonary capillaries and is primarily dependent on the surface area available for gas exchange and preservation of the delicate structure of the gas-exchange barrier.² Removal of carbon dioxide is primarily dependent on fresh gas movement into the alveoli; this process is ventilation. When managing patients on mechanical ventilation it is useful to think of oxygenation and ventilation as two separate processes.

VENTILATOR BREATH

Ventilator breaths can be spontaneous, assisted, or controlled. Spontaneous breaths occur when the patient determines the respiratory rate and tidal volume. Assisted breaths occur when the patient determines the respiratory rate but the tidal volume is generated by the machine. During controlled ventilation the machine determines both the respiratory rate and the tidal volume.³

The ventilator can generate a breath in one of two basic ways. It can deliver a preset tidal volume over a given inspiratory time (volume control), or the machine can provide and maintain a preset airway pressure for a given inspiratory time (pressure control). When delivering a volume-controlled breath, the peak airway pressure generated will be dependent on the preset tidal volume chosen and the compliance of the respiratory system. When a pressure-controlled breath is delivered, the tidal volume will depend on the preset airway pressure chosen and the compliance of the respiratory system.^1,3 The more basic machines tend to be either volume-control ventilators or pressure-control ventilators. More modern, advanced machines have the capability to generate several different breath types.

COMPLIANCE

Compliance is a measure of the distensibility of the lung and is defined as the change in lung volume for a given change in pressure.² A lung with high compliance will have a large increase in volume for a small pressure change, whereas low compliance would be characterized as requiring a large pressure change to create a small increase in volume. The normal, healthy lung is very compliant and, as a result, should require small airway pressures for adequate mechanical ventilation. In contrast, most pulmonary disease processes common to veterinary medicine will reduce pulmonary compliance and require higher airway pressures to adequately oxygenate and ventilate the patient.¹

VENTILATOR SETTINGS

Every model has a different range of settings. The more modern and advanced the machine, the more options it will provide for the operator to manipulate the ventilator breath. Despite the apparent complexity of modern ventilators, only a few important ventilator settings, available on almost all machines, allow an effective ventilation protocol to be determined for a patient. These include respiratory rate, tidal volume, peak inspiratory pressure, inspiratory time, inspiratory-to-expiratory ratio, trigger sensitivity, and positive end-expiratory pressure (PEEP) (Table 213-1). The parameters that can be preset will depend on the type of ventilation being used. With volume-controlled ventilation the tidal volume or minute ventilation is preset by the operator and peak airway pressure is a dependent variable. Rather a peak airway pressure alarm limit is set to alert the operator of excessive airway pressures. If pressure control is used, the peak airway pressure is preset and tidal volume is a dependent variable. In some cases the parameters can be set directly or are indirectly determined by other settings. For example, the inspiratory-to-expiratory ratio can be preset directly on some ventilators, but with many machines it is the consequence of the inspiratory time and respiratory rate that is chosen by the operator.^1,3

Table 213-1 Important Characteristics of a Ventilator Breath

The trigger variable is the parameter that initiates inspiration, that is, how the ventilator determines when to deliver a breath. In animals that are not making respiratory efforts of their own, the trigger variable is time and is determined from the set respiratory rate. If the animal is making respiratory efforts, the trigger variable may be a change in airway pressure or gas flow in the circuit resulting from the patient attempting to initiate inspiration.^3,5 The trigger variable or sensitivity of the machine is set by the operator. An airway pressure drop of 2 cm H₂O or gas flow change of 2 L/min is an appropriate trigger sensitivity in most patients. It is important to always set the trigger sensitivity so that any genuine respiratory efforts made by the patient are detected by the machine and thus obvious to the operator. This is because an increase in respiratory rate may be the only mechanism by which a ventilated patient can indicate that there is a problem. The trigger variable can be too sensitive, such that nonrespiratory movement such as patient handling may initiate breaths. This should be avoided.

PEEP is available on many ventilators. If not provided by the machine, PEEP can be added by attaching a tube to the exhalation port of the ventilator. This can then be attached to a PEEP valve or the end of the tube can be submerged in the desired depth of water. PEEP, as the name suggests, maintains positive pressure during exhalation that prevents the lung from emptying completely. As a result the lung is “held” at a higher volume and pressure during exhalation.^1,3,4 PEEP is thought to increase the oxygenating efficiency of diseased lungs by recruiting previously collapsed alveoli, preventing further alveolar collapse, and reducing ventilator-induced lung injury.^1,3,5 The appropriate magnitude of PEEP depends on the severity of the lung disease and the clinical response of the patient. Initially it may be set between 2 and 5 cm H₂O and then titrated appropriately.