Chapter 22: Monitoring Mechanical Ventilation

INTRODUCTION

The goal of ventilation is to generate adequate flow and volume to provide sufficient alveolar ventilation while minimizing the work of breathing (WOB). In mechanical ventilation, it is vital to closely monitor the function of the ventilator system regularly, including the settings, alarms, circuitry, and patient’s clinical status.

MONITORING QUALITATIVE CLINICAL SIGNS

• Respiratory rate—To detect apnea, set or measured respiratory rate can be measured by airflow sampling, capnography, inductive plethysmography, oscillometry frequency-based changes, ECG, or ventilation acoustics.

• Physical examination—Vigilant physical assessment of chest excursion is a necessity for accurate monitoring of mechanical ventilation. Asymmetric chest motion or unilateral breath sounds may indicate pneumothorax, endobronchial intubation, or atelectasis. Paradoxical chest motion can signify flail chest or respiratory muscle dysfunction. Poor synchrony of a patient’s breathing pattern with the ventilator’s drive may indicate that the ventilator settings are inappropriate or that the patient’s depth of anesthesia is too light. Tympanic percussion or tracheal deviation could help diagnose a pneumothorax. Audible endotracheal leaks around the airway cuff indicate insufficient air or a potential cuff rupture.

• Movement of reservoir bag—Free and unencumbered movement of reservoir bag during spontaneous ventilation assures a patent airway or early detection of circuit obstruction.

• Breath sounds—Continuous auscultation with a precordial or esophageal stethoscope is extremely valuable in detecting disconnects, leaks, airway obstruction by secretions or bronchospasm, and apnea.

MONITORING GAS EXCHANGE

Adequacy of mechanical ventilation can be determined by the ability of the patient to maintain ventilation and oxygenation. Pulse oximetry and capnography are two standard American Society of Anesthesiologists (ASA) monitors that are utilized for this purpose. Furthermore, arterial blood gas analysis provides significant insight into ventilatory status. A low Pao₂ on an arterial blood gas (ABG) indicates hypoxemia—a dysfunction of the ability to oxygenate arterial blood. A number of ventilator factors can directly affect the Pao₂: chiefly, the Fio₂, positive end-expiratory pressure (PEEP) level, and the patient’s lung function. It is important to interpret the Pao₂ as a function of these dependent variables, as a “normal” Pao₂ does not necessarily indicate ideal physiologic pulmonary function.

MONITORING VENTILATORY DRIVE AND BREATHING PATTERN

Dependent upon clinical scenario, mechanical ventilation can be adjusted to provide as much or as little support as necessary. Positive pressure breathing can be categorized by three variables: the trigger variable, which initiates the breath; the limit variable, which governs the gas delivery; and the cycle variable, which terminates the breath. The dependent variable for triggering is time in controlled mechanical ventilation modes. Each of these breaths will provide a preset volume or pressure at regular intervals. Cycle time can, therefore, be adjusted based on volume, pressure, or flow.

Partial ventilator support can also be utilized through pressure support and synchronized ...