Bedside Assessment of the Respiratory System During Invasive Mechanical Ventilation

Abstract

Assessing the respiratory system of a patient receiving mechanical ventilation is complex. We provide an overview of an approach at the bedside underpinned by physiology. We discuss the importance of distinguishing between extensive and intensive ventilatory variables. We outline methods to evaluate both passive patients and those making spontaneous respiratory efforts during assisted ventilation. We believe a comprehensive assessment can influence setting mechanical ventilatory support to achieve lung and diaphragm protective ventilation.

Keywords: acute respiratory distress syndrome; mechanical ventilation; respiratory physiology.

Figure 1

Assessment of the inspiratory phase. Volume control with constant (square) flow: the first breath describes the static conditions of an inspiratory hold required to assess driving and resistive pressures. Notably, the conductive pressure equals the airway resistive pressure, indicating the absence of airway closure; the second and third breaths describe dynamic conditions with negative and positive stress indexes, respectively. Pressure control with decelerating flow: both breaths describe the static conditions of an inspiratory hold. However, in the first one the flow reaches zero before the hold, therefore peak and plateau pressures are synonymous, while a resistive decay is appreciated in the second breath as the flow is still positive when the hold is performed. The green “Partitioning” box describes how to convert airway into transpulmonary pressures with the “elastance derived” method using oesophageal pressures. P_aw: airway pressure, P_peak: peak inspiratory pressure, P_res(A): airway resistive pressure, P_res(T): tissue resistive pressure, ∆P: driving pressure, P_plat: plateau pressure, P_cond: conductive pressure, PEEP_set: set positive end-expiratory pressure, Pes: oesophageal pressure, Pes_peak: peak inspiratory oesophageal pressure, Pes_plat: plateau oeasophageal pressure, ∆Pes: driving oesophageal pressure, E_rs: respiratory system elastance, C_stat: static respiratory system compliance; Vt: tidal volume; E_cw: chest wall elastance, E_L: lung elastance, E_R: elastance ratio, P_L: inspiratory transpulmonary pressure, ∆P_L: driving transpulmonary pressure.

Figure 2

Assessment of the expiratory phase. The first two breaths describe a decremental PEEP trial (from 15 to 5 cm H₂O) to calculate the recruitment to inflation ratio in volume control ventilation with constant (square) flow. Note that the second breath shows the presence of airway opening pressure and of intrinsic PEEP, the latter visualised during an expiratory hold. The driving pressure is calculated from the airway opening pressure, being higher than the set and total PEEP. Airway opening pressure can be assessed with three methods: (1) AOP is detected when P_cond is significantly higher than P_res. The AOP value is defined as: AOP = PEEP + (P_cond − P_res); (2) as the change in slope in a low-flow (<10 L/min) pressure–time curve (third breath); (3) as the beginning of inflation during the corresponding low-flow pressure–volume curve. P_aw: airway pressure, Vol: volume, ∆P_rec: change in PEEP during a decremental PEEP trial; P_res(A): airway resistive pressure, P_cond: conductive pressure, AOP: airway opening pressure, ∆P_low: driving pressure at the lower PEEP, PEEP_set: set positive end-expiratory pressure, PEEP_i: intrinsic positive end-expiratory pressure, PEEP_tot: total positive end-expiratory pressure, V_rel: release volume during a decremental PEEP trial, Vt: tidal volume, V_infl: PEEP-induced inflation volume, V_rec: PEEP-induced recruited volume; FRC: functional residual capacity, C_stat(low): static respiratory system compliance at the lower PEEP, C_rec: compliance of the recruited volume, R/I: recruitment to inflation ratio, LIP: lower inflection point, UIP: upper inflection point; Max_D: maximal distance between inspiratory and expiratory limb of a low-flow pressure–volume loop, V_max: maximum volume inflated during a low-flow manoeuvre.

Figure 3

Mechanical power in volume and pressure control ventilation. Note the different shapes of the dynamic pressure–volume curves under volume control with constant (square) flow and pressure control with decelerating flow. The greater resistive work (pink area) in pressure control ventilation is due to the higher initial flows. To highlight this phenomenon, we have assumed here that tissue resistances play a significant role only in pressure control ventilation, although they certainly (but at a lower level) exist in volume control ventilation as well. The green and yellow areas represent the elastic work due to PEEP and driving pressure and are not different in the two modes of ventilation. The sum of the yellow, green, and pink areas describes the inspiratory work, from which the mechanical power is currently calculated. Conversely, the area (pink + green) enclosed by the inspiratory and expiratory limbs (solid black lines) represents the hysteresis area, indicating the dissipated energy that remains in the parenchyma after a whole breath. Please note that the pressure–volume curve describing mechanical power is dynamic (i.e., at clinical inspiratory flow), as opposed to the low-flow pressure–volume curve described in Figure 2 to assess recruitability. Paw: airway pressure, Vol: volume, P_res(A): airway resistive pressure, ∆P: driving pressure, PEEP: positive end-expiratory pressure, P_plat: plateau pressure, P_peak: peak inspiratory pressure, P_res(T): tissue resistive pressure.

Figure 4

Respiratory Drive and Effort. Panel (A): Control of breathing. 1: The metabolic hyperbola describes changes in minute ventilation and PaCO₂ depending on its total metabolic production (V’CO₂) and the dead space (Vd/Vt). 2: The ventilatory response to PaCO₂ (in L/min/mmHg of PaCO₂) relative to the eupnoeic PaCO₂. This slope is affected by hypoxia, distress, inflammation. 3: The eupnoeic PaCO₂ is the intercept between the CO₂ response curve and the metabolic hyperbola. It may be shifted by hypoxia. 4: In a wakeful state apnoea does not normally occur whilst during sleep or sedation apnoea occurs when the actual PaCO₂ < eupnoeic CO₂. 5: The ventilatory response to PaO₂ is minimal, >60 mmHg (with normal PaCO₂), below which it exponentiates. This curve resembles an inverted oxygen–haemoglobin dissociation curve. Panel (B): Pressures related to respiration and their relations. A positive transpulmonary pressure (P_L) is required to generate inspiratory flow. This can occur through P_mus (leading to ↓ P_pleura) or through Pvent (↑ P_aw) or a combination. Pressures measured during inspiration against a closed valve (P0.1, P_occ) or during an end-inspiratory hold (P_plat, PMI) can be used to estimate invasive measurements. Panel (C): Assessments of respiratory drive and effort. Panel (D): Breath 1 is pressure controlled: P_mus is easiest to see on the flow waveform where it exceeds the exponential decay seen during a passive inspiration or expiration (black line). Breath 2 is volume controlled, whereby inspiratory P_mus is more obvious on the pressure scalar.

Figure 5

Aims for lung and diaphragm protective ventilation. (A): Either non-invasive intermittent techniques (above) or continuous invasive monitoring of P_es and P_L can be used (below). Non-invasive techniques during inspiration either against a closed valve (P0.1 and Pocc) or at end inspiration (PMI). When valves are closed and there is no flow, pressures measured at the airway approximate pleural pressures during active efforts (P0.1, Pocc). Alternatively, an end-inspiratory hold allows the relaxation pressure of the inspiratory muscles to be measured (PMI). (B): Either invasively measured or estimated measures of respiratory effort and transpulmonary pressure can be used. Relevant calculations together with suggested targets are listed. (C): Selected strategies to achieve a sustainable degree of respiratory effort alongside a protective level of lung stress are shown, depending on whether the total stress applied to the lung is high or acceptable and on the degree of respiratory effort.