◆ Modern ventilators display real time curves, which can help the physician to understand the interactions between the patient and the ventilator.
◆ In patients undergoing mechanical ventilation, measurements of respiratory mechanics can be performed at the bedside in dynamic or static conditions.
◆ In flow-limited chronic obstructive pulmonary disease (COPD) patients, the expiration is interrupted by the next breath and, therefore, the end-expiration volume remains trapped into the alveoli. This phenomenon is called dynamic hyperinflation and also depends on the tidal volume, the expiratory time, the resistance and the compliance of the respiratory system.
◆ Patient–ventilator synchrony represents a main goal in the management of the mechanically-ventilated patient. The correct interpretation of the waveforms provided by the monitor can help the physician to set properly the ventilator.
◆ The variables that should be monitored during mechanical ventilation are airway pressure, flow, tidal volume, and minute ventilation, whereas positive end-expiratory pressure (PEEP) and mean airway pressure gain significance in acute respiratory distress syndrome.
Modern ventilators employed in intensive care units (ICUs) display in real time and breath by breath flow (V̇), volume (V), and pressure (Paw) curves, both as a function of time and as a loop. Data obtained from curve analysis can help the physician to understand the interactions between the patient and the ventilator. The right interpretation of information provided from modern ventilators allows real time monitoring of the actual needs of the patient, ensuring a custom ventilatory support and reducing the risk of complications that can increase the mortality and prolong the ICU length of stay. In patients undergoing mechanical ventilation, measurements of respiratory mechanics can be performed at the bedside in dynamic (no flow interruption) or static (occlusion techniques) conditions. From these, it’s also possible to derive the values of pulmonary compliance and airway resistance .
In dynamic conditions, without flow interruption, the values of resistance (Rrs) and compliance (Crs) of the respiratory system are obtained by inserting in the equation of motion, the values of airway pressure (Paw), flow, and volume, provided by the ventilator time after time, and applying a multiple linear regression (least square fitting, LSF) . This allows the derivation of the mean values of compliance and resistance of the respiratory system using the numerical values derived from repeated statistical operations performed on a single breath. If there is a limitation of expiratory flow, such as in chronic obstructive pulmonary disease (COPD) patients, it is appropriate to restrict the analysis at the inspiratory phase alone, since it was shown that this condition significantly undermines the accuracy of the measurements of resistance and compliance, compared with traditional (end-inspiratory occlusion). Since in volume-controlled ventilation (VCV) the flow remains constant during the whole inspiratory tract, the pressure–time curve can be considered as a pressure–volume or elastance curve. From the analysis of his shape it is possible to recognize hyperinflation of the lung, which can lead to barotrauma, and the alveolar recruitment phenomenon—both requires an adjustment of the ventilatory parameters .
Instead, in static conditions, the monitoring is provide by the multiple occlusion technique and requires a square flow waveform. If a pause is inserted at the end of the inspiratory phase, it can be possible to distinguish, analysing the pressure–time curve of the ventilator, the drop in pressure due to the resistive forces from that due to the elastic component of the respiratory system. The pressure–time curve takes a characteristic shape, with an end-inspiratory peak (Ppeak) followed by a rapid drop (P1), which precede a slow decay until the achievement of the plateau pressure (Pplat). If the end-inspiratory pause is long enough to permit both lungs to reach equilibrium, the plateau pressure can be assimilated to the alveolar pressure (Palv). The pressure difference between Ppeak and P1 depends on the flow and resistance variations of the endotracheal tube and the airways, whereas P1–Pplat depends on the pendelluft phenomenon, i.e. the shift of air from alveoli with short time constant to alveoli with fast time constant, and on the visco-elastic properties of the respiratory system (Fig. 84.1). The analysis of the pressure–time curve can give some useful information about the respiratory system: the Ppeak can rise during bronchospasm episodes or presence of bronchial secretions and if a too small endotracheal tube size is used. A slow achievement of the Pplat suggests the presence of a flow limitation disease, whereas the missed reaching of it can tell on leaks of the respiratory circuit .
In physiological conditions, at the end of expiration, the lungs reach the functional residual capacity, but if a flow limitation subsists, i.e. COPD patients, the expiration is interrupted by the next breath and, therefore, the end-expiration volume do not reach the functional residual capacity (FRC) and remain trapped into the alveoli, whereas it can be seen as a continuous expiratory flow. This phenomenon is called dynamic hyperinflation (also note as auto-PEEP or PEEPi) and also depends on the tidal volume, the expiratory time, and the resistance and the compliance of the respiratory system . This phenomenon could also present in the absence of conditions limiting the expiratory flow, such as high volume ventilation or an increase in the resistance of the ventilation circuit (i.e. the presence of secretions in the endotracheal tube with narrowing of the lumen) with the consequence that the lungs do not have the time to reach the functional residual capacity, which means that at the end of expiration it is still possible to detect the presence of an expiratory flow, supported by the pressure gradient between the alveoli and the atmosphere.
In paralysed patients or patients who have become well adapted to the ventilator and undergoing mechanical ventilation, it is possible to recognize the existence of dynamic hyperinflation of the lungs observing the flow and pressure curves at the end of expiration (dynamic autoPEEP). Whenever the end-expiratory flow does not reach zero, the respiratory system is considered to be hyperinflated, i.e. the alveolar pressure is greater than the atmospheric pressure or the PEEP applied. The autoPEEP can be also measured by inserting an end-expiratory occlusion, and is called static autoPEEP. This is because the dynamic autoPEEP reflects the instantaneous pressure value of short time constant lung units, while the longer time constant pulmonary units are still being emptied. The occlusion manoeuvre allows the lung units to equilibrate. Furthermore, to avoid underestimation, the value of static compliance should be measured correctly according to the presence of autoPEEP. The increase in volume due to the application of a PEEP or a condition of dynamic hyperinflation can be measured disconnect the patient from the ventilator or prolonging the expiratory time to allow the patient to reach the FRC. Once they reach the equilibrium, the patient is reconnected to the ventilator and it can be seen that the initial volume inspired will be greater than the volume expired. In the absence of extrinsic PEEP, the difference between the volume inspired and expired corresponds to the volume of air trapped in the lungs at the end of expiration. If an extrinsic PEEP is applied, the end-expiration volume observed will be the result of the sum of two components—the PEEP applied and the PEEP due to dynamic hyperinflation.
Patient–ventilator synchrony represents one of the main goals in the management of the mechanically-ventilated patient in the ICU . It has been shown that up to 25% of ventilated patients exhibit problems of interaction with the ventilator and that this may be associated with increased duration of ventilatory support. The correct interpretation of the waveforms provided by the monitor can help the physician to correctly set the ventilator. Two variables determine the breath delivery in a modern positive pressure ventilator—the trigger and the cycling-off variable. The first one determines the start of the mechanical breath and can be pressure- or flow-regulated. With pressure triggering, the ventilator is able to detect the drop in airway pressure generated by the inspiratory effort of the patient—if the effort is effective, i.e. the reduction of pressure is equal to or above the threshold value set by the machine (usually between –0.5 and –1 cmH2O), the ventilator delivers a breath. The erroneous application of a pressure trigger (e.g. a too high value in relation to the patient’s muscle strength) increases the work of breathing and promotes patient–ventilator asynchrony. The consequence of this phenomenon is the need for patient sedation, lengthening the weaning from mechanical ventilation. When the physician reduces the sensitivity of the pressure trigger (i.e. the patient must generate more negative pressure to trigger the ventilator), that inevitably increases the number of ineffective breaths. They appear on the monitor like negative deflections on the pressure curve, which would not be followed by a positive deflection in the volume curve, in other words, the effort of the patient is not followed by the provision of a mechanical breath.
With a flow trigger system, the ventilator provides the ventilation circuit with a constant flow (bias flow), measured continuously at the inlet and at the outlet. When the value of the bias flow in output is lower than that in entry (and in the absence of leak in the system or when that is compensated), it means that the patient has performed an inspiratory effort and the ventilator then provides a breath even before any change of pressure in the system. The flow trigger decrease the inspiratory work, but on the other hand, promotes the phenomenon of self-triggering, i.e. incorrect triggering of breaths due to registration of changes in flow, which are not attributable to a patient’s inspiratory effort (e.g. leak in the circuit, water in the circuit, and cardiogenic oscillations). Understanding the mechanisms that regulate the ventilation with flow triggering is crucial during non-invasive ventilation through a helmet or mask. The presence of leak in the system means that the bias flow needs to be set higher in order to avoid the phenomenon of self-triggering and promote patient–ventilator synchrony, ensuring adherence to the treatment.
The cycling-off variable determines how the ventilator terminates the inspiration. Usually, the criterion used in patients without inspiratory effort is time, whereas during assisted ventilation flow or pressure was preferred. When pressure is used, the ventilator opens the expiratory valve and begins the expiration when the airway pressure increases above a predetermined threshold (usually 1–3 cmH2O), due to expiratory muscle contraction or sudden relaxation of inspiratory muscles. On the contrary, flow cycling-off occurs when inspiratory flow decreases to a preset flow value (usually a percentage of the peak inspiratory flow, 25–50%). Regardless of the type of cycling-off criterion used, the end of mechanical inspiration should coincide with the end of neural inspiration, but this synchrony is as yet not obtainable. In fact, expiratory asynchrony is common in ICU and occurs in two ways—premature or delayed termination of mechanical inspiration. The first one occurs when the exhalation valve is open, while the neural inspiration is still ongoing. The flow wave did not reverse from inspiratory to expiratory due to elastic recoil of the respiratory system, but remained around the zero line, indicating that the inspiratory effort should continue. In addition, if the remaining effort is sufficient to meet the trigger set, it generates another mechanical breath, leading to the phenomenon of so-called double trigger or breath stacking. In other words, the delayed opening of the exhalation valve can be observed on the ventilator when there is a rapid decrease of the inspiratory flow toward the end of mechanical inspiration. It should be noted, that, as mentioned previously, if the premature expiratory effort increases the pressure of the system above a predetermined threshold, the exhalation valve opens, and the expiration can start.
Capnography and CO2 clearance
The advanced technology combination of airway flow monitoring and mainstream capnography enables a non-invasive breath-by-breath bedside calculation of CO2 elimination per breath, independent of set ventilatory parameters. Carbon dioxide kinetics depend on three factors—peripheral production, cardiac output, and alveolar ventilation. If catabolism is assumed to be constant, haemodynamic or alveolar ventilation modifications produce typical volumetric capnographic curves. It is generally known that all situations producing a decrease in lung flow (pulmonary embolism and severe haemorrhage) affect the capnographic wave, which decreases in width. This phenomenon is due to the decrease in pulmonary blood flow, alveolar ventilation being equal. In this situation, the shape of phase III on volumetric capnograms (CO2/VT curve) does not vary except in width, and VCO2 elimination decreases. Lung pathologies affect CO2 washout, altering both convective and diffusive processes, as well as the time, available cross-section, and intra-airway concentration gradient. Bronchial obstruction makes regional alveolar ventilation inhomogeneous, altering the normal V/Q ratio. This determines different readings of CO2 alveolar pressure that are asynchronously exhaled, changing the shape of the CO2/VT curve, with a prevalent increase in the slope of phase III .
Recently, a new index, the fraction between alveolar ejection volume and tidal volume (VAE/VT), was introduced by Lucangelo et al. . Briefly, VCO2 plotted as a function of expired volume originates a VCO2/VT curve (Fig. 84.2). From this curve, the last 50 points of every cycle were back-extrapolated by least-square linear regression analysis representing the ideal lung behaviour. Assuming a fixed amount of dead space contamination of 5% (dead space allowance), a straight line having its maximal value at end-expiration and slope of 0.95 (1-dead space allowance) times that of the ideal line is plotted. Alveolar ejection begins at the intersection between the VCO2/VT curve and the linear regression. The volume between this point and end-expiration is VAE, and this is expressed as a fraction of expired tidal volume. Lucangelo et al.  demonstrated that VAE/VT represents a valid prognostic value in acute respiratory distress syndrome patients and is less dependent on haemodynamic variations.
In conclusion, the only variables of crucial significance to the vast majority of patients are airway pressure, flow, tidal volume, and minute ventilation, which also provides useful data regarding the synchrony of patient–ventilator interactions.
The authors would like to thank Lluis Blanch MD, PhD, for intellectual content and revision of the chapter.
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