Mechanical Ventilation and Post-Intubation Sedation, Hemodynamics, and Bundle

Sydney J. Hansen

Matthew E. Prekker

INTRODUCTION

The transition from spontaneous breathing to positive pressure ventilation is abrupt and contributes to the risk of severe peri-intubation adverse events, including hypotension, hypoxemia, or cardiac arrest. Approximately 45% of critically ill patients who undergo intubation worldwide will experience at least one of these severe complications.¹

Many clinicians performing airway management in the emergency department (ED) or intensive care unit (ICU) are also responsible for supervising the initiation of mechanical ventilation. A general understanding of ventilator terminology, settings, and alarms is mandatory to work effectively with respiratory therapists and to troubleshoot the alarming ventilator or the patient who is dyssynchronous or progressively hypoxemic.

These same clinicians are also tasked with other consequential decisions in managing the newly intubated and ventilated patient including the choice and dose of sedative or analgesic drugs and the provision of prophylactic measures to avoid nosocomial adverse events such as venous thromboembolism, upper gastrointestinal ulceration, or skin breakdown. Early clinical decisions in the care of critically ill patients receiving invasive mechanical ventilation impact both short-term stability and the risk of delayed complications such as delirium, ventilator-associated pneumonia, and pulmonary barotrauma. To lessen the cognitive burden on clinicians and improve adherence to evidence-based practices, routine postintubation interventions are increasingly “bundled” into a single order set or checklist that can be delegated to other team members such as nurses or respiratory therapists.²

This chapter will outline the key concepts and potential pitfalls encountered when providing invasive mechanical ventilation, prescribing and titrating sedation, analgesia, and sometimes neuromuscular blockade for intubated patients, and managing hemodynamic perturbations that frequently occur in the peri-intubation period.

MECHANICAL VENTILATION

See Table 30.1 and Figure 30.1.

Figure 30.1: Pressure-time waveform schematic from ventilator tracing demonstrates PIP, P_plat, driving pressure, and total PEEP in graphic form.

TABLE 30.1 Terminology of Mechanical Ventilation

Tidal volume (V_t)	The volume of a breath, usually reported in milliliters of gas (mL). The current standard is to target a V_t of between 6 and 8 mL/kg predicted body weight (PBW). PBW can be calculated from the patient’s sex and height. Unlike the actual (measured) body weight, sex and height are closely correlated with lung volume so it makes sense to index V_t to PBW. The height can be easily measured in a supine intubated patient using a disposable measuring tape. As an example, a woman who is 66 inches tall has a PBW of approximately 60 kg; the initial V_t delivered by the ventilator should be around 420 mL (60 kg × 7 mL/kg PBW) and subsequently adjusted, if necessary, to avoid injury due to excess volume or pressure in the lungs.
Respiratory rate or frequency (RR or f)	The number of breaths per minute. RR or f is normally set between 12 and 20 breaths/min in adults, and higher in children, when using an assist-control mode on the ventilator. In an assist-control mode, additional breaths that the patient takes above the set RR are assisted with the same volume or pressure as “control” or set breaths.
Minute ventilation (V˙_E)	The total volume of gas that is expired by the lungs each minute, usually represented as liters/min. The V˙_E is calculated as the product of V_t and the actual RR. Remember that V˙_E is made up of both alveolar ventilation (V_A) and a fixed amount of dead space ventilation (V_D). The conducting airways do not participate in gas exchange and, therefore, represent anatomic dead space, while areas of the lungs that are well ventilated but poorly perfused (e.g., the lung apices) represent additional physiologic dead space. When V_t is reduced to achieve lung-protective ventilation (6-8 mL/kg PBW) after intubation, dead space (a fixed quantity) will make up a proportionally larger portion of each V_t. To avoid hypercarbia, it is important to increase V˙_E (and thereby the absolute V_A) by increasing RR during intentional V_t reduction. This increase in RR will balance the decrease in effective alveolar ventilation due to dead space during intentional V_t reduction. Increasing the V˙_E by RR increases on the ventilator is also an effective way to compensate for a metabolic acidosis (via an induced respiratory alkalosis).
Fractional concentration of inspired oxygen (F_IO₂)	The ventilator, connected to medical oxygen supply, can provide any concentration of oxygen to the patient between that found in ambient air (F_IO₂ 0.21 or 21%) and pure oxygen (F_IO₂ 1.0 or 100%) by using a gas blender. Initially, F_IO₂ is set at 1.0 and weaned over time to the lowest F_IO₂ required to maintain adequate oxygenation (an oxygen saturation ≥95% by pulse oximetry in most conditions).
Inspiratory flow rate (IFR)	The maximal rate of gas flow during V_t delivery, typically set between 60 and 80 L/min. An IFR at the higher end of that range may facilitate ventilator synchrony in a patient with a high flow demand (“air hunger”). Higher IFR will shorten the inspiratory phase of the respiratory cycle, allowing more time for expiration, at the cost of an increase in peak airway pressure, especially when high airways resistance is already present (e.g., obstructive lung disease).
Positive end-expiratory pressure (PEEP)	A static pressure applied to the airways by the ventilator during expiration, typically set initially at 5 cm H₂O. Through an increase in mean airway pressure and end-expiratory lung volume (alveolar recruitment), increasing the PEEP can improve oxygenation in patients with diffuse lung diseases like pulmonary edema. On the other hand, excess PEEP risks over-distending aerated alveoli which can lead to barotrauma (e.g., ventilator-induced lung injury), elevated pulmonary vascular resistance (e.g., RV afterload), or compromised venous return via an increase in intrathoracic (pleural) pressure. In certain circumstances, the total PEEP in the lungs may be greater than the set PEEP on the ventilator. This occurs when there is still end-expiratory airflow as the next breath is delivered, as in a patient with exacerbated COPD (i.e., prolonged expiratory phase of the respiratory cycle), a patient with a very rapid RR (i.e., inadequate time for full expiration of the V_t), or RR set >30 bpm with an inspiratory time of 1 s or more. The difference between total PEEP, as measured by the ventilator during an end-expiratory pause maneuver, and set PEEP is termed intrinsic PEEP or “auto-PEEP.”
Peak inspiratory pressure (PIP)	The maximum pressure reached during inspiration. PIP represents the sum of the total PEEP, driving pressure, and the pressure required to overcome airways resistance to gas flow (a function of endotracheal tube diameter, IFR, and intrinsic airways resistance). The PIP is valuable because it is routinely measured and reported by the ventilator for each breath. While an elevated PIP is context-dependent and does not always indicate a patient or equipment problem, a significant increase in PIP is often the first indicator of a problem and should trigger a prompt evaluation as to its cause.
Plateau Pressure (P_plat)	The plateau pressure is measured by the ventilator during an end-inspiratory hold maneuver and requires a sufficiently passive patient for accurate measurement. P_plat is less than or equal to PIP because during the end-inspiratory hold, airflow and therefore resistive pressure is zero, and the pressure required to distend the lungs and thorax (the distending pressure, or maximal alveolar pressure) is isolated. P_plat measurement is necessary to calculate the compliance of the respiratory system (see below). Elevated P_plat may increase the risk of barotrauma and ventilator-induced lung injury, and current recommendations are to target a P_plat <30 cm H₂O in ARDS.
Driving pressure (DP)	DP is defined as the difference between P_plat and total PEEP. In patients with ARDS managed with low tidal volume ventilation, the DP goal is generally <15 cm H₂O.
Respiratory system compliance (CRS)	A measure of distensibility of the respiratory system defined as the change in volume (V_t) divided by the driving pressure. Graphically, CRS can also be determined from the slope of the pressure-volume relationship of the respiratory system. The more pressure required to inflate the respiratory system by a given volume, the lower the CRS.

Ventilation Modes

Practical experience and a lack of robust comparative effectiveness data highlight that there is unlikely to be one “best” ventilator mode for all patients. Pairing an understanding of the ventilator brand used in your ED and ICU with the therapeutic goals for your critically ill patient (e.g., rescue

severe hypoxemia, mitigate work of breathing to avoid fatigue, assess readiness to liberate from the ventilator, etc.) must be used to optimize the ventilator prescription. With many modes to choose from on full-feature ventilators, we adhere to the simple rule that the safest mode is the one that all members of the critical care team (e.g., nurses, respiratory therapists, trainees) understand and can troubleshoot in the event of an emergency.

Two common modes of ventilation are assist control (AC) and pressure support (PS):

Assist Control (AC) is the preferred initial mode for patients with acute respiratory failure. A backup respiratory rate and mandatory breath type (volume cycled, pressure controlled, or pressure regulated volume controlled) are set. Inspiration is triggered by either an elapsed time based on the set respiratory rate (RR) (controlled breath) or a patient inspiratory effort sensed by the ventilator (assisted breath). In either case, the ventilator responds by delivering the same mandatory breath type on each inspiration. Table 30.2 provides a comparison of three different mandatory breath types used with the AC mode: volume AC, pressure AC, and a hybrid mode termed pressure-regulated volume control (PRVC).
Pressure Support (PS) is a “spontaneous” mode of ventilation which is especially useful when assessing patient readiness to liberate from the ventilator. PS delivers a set level of inspiratory pressure above positive end-expiratory pressure (PEEP) in response to each spontaneous inspiratory effort of the patient. Unlike in the volume AC mode, the V_t and RR (and therefore the minute ventilation) are not set by the clinician, rather they are
determined by the patient’s respiratory drive, effort, and lung mechanics. The ventilator cycles from inspiration to expiration when the patient’s inspiratory airflow decays to a certain percentage of its maximum (e.g., 25%). If a patient ventilated with PS becomes apneic, the ventilator will revert to a backup AC mode based on alarm settings.