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Clinical assessment of the mechanically ventilated patient: a pragmatists' guide.

Mechanical ventilation is the most common intervention in Critical Care. Indeed, the clinical utilization of mechanical ventilation has grown exponentially in the past decade. This can be attributed in large part to advances in medical therapeutics, the expansion of Critical Care therapeutics and expertise as well as the evolutionary progression in the discipline of Critical Care.

Clearly the management of the patient undergoing mechanical ventilation calls for an experienced and practiced hand. With that end in mind, this column will review current assessment theory. It is essential that every patient being mechanically ventilated be assessed on a continuing basis. It is also essential that the information gleaned from patient assessments be clinically applied in the form of treatment plans.

The ongoing assessment process is eminently valuable for the most proficient care of our mechanically ventilated patient population. Through vigilant assessment we can expect enhanced weaning potential, prevention of Ventilator Induced Lung Injury and avoidance of Ventilator Associated Pneumonia.

My purpose here is to briefly comment on aspects of the assessment of the mechanically ventilated patient that are most imperative in their clinical application and significance. The notion being that an effective assessment strategy should yield an efficient and judicious weaning plan.

Assessment should begin with an observation and inspection of the patient-ventilator system. Begin your observations at the patient and work your way back to the ventilator. Use your hands to inspect the airway for signs of compromise or excessive pressure both on the airways and the surrounding tissue. Use your eyes and your hands to palpate and observe air movement. As much as possible palpate for equal and bilateral movement in a descending bilateral fashion. Look for any compromise in the mechanical aspect of the patient-ventilator system and act to correct it.

Auscultation is one of the most important direct physical assessment practices the bedside clinician can perform. The act of listening to the patient's chest accomplishes many clinically relevant goals. It makes the clinician aware on a very visceral and sensory level what is going on with the patient and how she is responding to the physical reality of mechanical ventilation. Auscultation connects the therapist and patient; and patients who experience more contact have more positive outcomes. Auscultation gives the therapists a real time experience of the patient ventilator system and the tidal changes experienced by the patient. Auscultation should be performed in a bilateral descending fashion. Attention to the phase of the breath, breath type and auditory character of any sound must be noted in some detail. The interesting aspect of auscultation is in the reporting; one must create a narrative depiction of auditory phenomena that may change on a breath to breath basis.

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The measurement of Plateau Pressure (PPLAT) is one of the most indispensable values we can determine. PPLAT is the pressure measured at the end of inspiration during an inflation hold. This inflation hold allows inspired gas to equilibrate in regions of the lung with incongruous time constants.

PPLAT is the pressure required to counterbalance end inspiratory forces and is related to the static end inspiratory elastic recoil pressure of the total respiratory system. Airway pressure measured during an end inspiratory occlusion replicates the elastic threshold stress to the pulmonary system sans the inevitable resistive forces present during active inspiration. PPLAT faithfully approximates alveolar pressure and as such is a very useful clinical assessment tool.

Elevated PPLAT will alert the clinician to increased alveolar pressure. Incremental changes in PPLAT are inversely related to lung compliance. An increase in the plateau pressure signals a fall in the global lung compliance. Indeed, a PPLAT of 35 cmH2O represents the normal peak alveolar pressure necessary to reach TLC. It has been suggested that PPLAT equal to, or in excess of that needed to reach TLC would facilitate lung injury or impede efforts to ventilate the already hyper-inflated lung. Plateau pressure is needed to calculate total lung compliance as the relationship between PPLAT and delivered volume. Compliance is derived in the following manner. CI = [V.sub.t] / (PPLAT-PEE[P.sub.tot])

Common causes of decreased compliance in the mechanically ventilated patient include; mainstem intubation, pneumothorax, CHF, ARDS, pleural effusion, and chest wall deformity.

The difference between Peak Airway Pressure and PPLAT is a function of resistive forces in the patient ventilator system. Raw is calculated by looking at the pressure gradient between the peak airway pressure and the plateau divided by the flow.

Raw = PAP - PPLAT / Flow (L/sec.)

Common causes of increased Raw include; bronchospasm, bronchoconstriction, secretions, airway obstruction, narrow endotracheal tube and mucosal edema. Bear in mind that both inspiratory and expiratory Raw may vary in different pathologies. The PaO2/FiO2 index (P/F ratio), quantifies the ratio of arterial oxygen tension to available gen concentration. It is a very useful formula in evaluating the degree of intrapulmonary shunt and subsequent compromise of cardiopulmonary function. The Pa02/F102 index is also valuable indicie of diffusion capability and a primary tool in assessing the degree of injury to the lung. Low diffusion states will have a low ratio of arterial oxygen in relation to a given Fi02. The Pa02/F102 index acts to identify the severity of lung injury. lithe Pa02/Fi02 index is < 300 strongly suspect Acute Lung Injury. (ALI) If the Pa02/Fi02 index is < 200 strongly suspect Acute Respiratory Distress Syndrome. (ARDS)

The Pa02/Fi02 Ratio quantifies the ratio of oxygen available vs. arterial oxygen required. Is an excellent predictor of mortality and is an essential tool in assessing lung injury.

Ventilator Induced Lung Injury, (VILI), is a spectrum of lung injury facilitated by the mechanical ventilator. VILI is a complex collection of lung deconstuctive elements that manifests itself in a number of forms including but is not limited to the following categories of ventilator induced lung injury; Volutrama, Barotrauma, Biotrauma, RACE and Atelectrauma.

The calculation of the pulmonary time constant is essential in determining the tidal cycle indispensably important for avoiding dynamic hyperinflation and ventilator induced or associated injury. The Pulmonary Time Constant (TC, Kt), is the product of the airways resistance times the static lung compliance in seconds that will allow for 63% of transferred volume to equilibrate. The time constant is the natural consequence of the lungs inherent capacity for expansion and resistance to airflow. The time constant will fluctuate in response to changes in resistance and compliance. Time constants may vary regionally. Gas moves into and out of the lung at a rate and in a fashion that is conditioned by the collective impedance of individual lung units. Lung units with high compliance will have a longer time constant; as will lung units with high airways resistance. Conversely, lung units with low compliance and low airways resistance will have a shorter time constant. The time constant of any individual lung unit may alter with the phase of ventilation and will vary with changes in the patient-ventilator system. (Think locally but treat globally.) The time constant is calculated by multiplying the RAW times the CL. This is perhaps the most important calculated value in the management of the ventilated patient.

The Rapid Shallow Breathing Index (RSBI), is the most reliable predictor of a patient's potential for success in the weaning process. The clinician evaluates the patient's breathing pattern by analyzing the relationship linking breathing frequency and average tidal volume. The RSBI is an accurate forecaster of the patient's ability to perform endurance related work and assume the work of breathing when extubated. The RSBI is calculated as the spontaneous frequency divided by the average spontaneous tidal volume in Liters, (f/VT). The patient is evaluated while breathing spontaneously without inspiratory adjuncts such as PSV. An index of <100 is a predictor of weaning success while an index of >100 suggests probable weaning failure.

Peak Airway Pressure, (PAP/PIP), is the maximum airway pressure recorded during an inspiratory cycle. This maximum or extreme pressure is usually actualized at the end of inspiration. This pressure reflects the collective result of machine and patient variables and is dynamic in character. The extreme pressure at the end of volume delivery.

Mean Airway Pressure, (MAP), is the average airway pressure during one complete ventilatory cycle. Also, the area under the pressure-time curve for one breathing cycle divided by cycle time. MAP is directly related to PEEP and influenced by PAP, inspiratory time and inspiratory flow. NOTE: Frequently expiratory airways resistance is greater than inspiratory airways resistance. This will cause machines to underestimate true MAP.

Auto-PEEP is the PEEP not set by the clinician. This PEEP represents a dynamic hyperinflation of the lung. PEEP that is present but not reflected by the monitoring systems of the mechanical ventilator. The pressure gradient between alveolar pressure as set by the clinician, (PEEP or CPAP), and actual end expiratory alveolar pressure. Frequently a result of asynchronous ventilation, inadequate expiratory time, long pulmonary time constants or inadequate inspiratory flow.

The prudent clinician will incorporate these various elements: inspection, auscultation, time constant, plateau pressure, mean airway pressure, airways resistance, lung compliance, RSBI and P/F ratio into a comprehensive assessment dashboard from which a clear plan of care will emerge. I will devote future columns to the clinical application of these assessment tools.

by Dave Wheeler, RRT
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Title Annotation:CLINICAL RESPIRATORY CARE
Author:Wheeler, Dave
Publication:FOCUS: Journal for Respiratory Care & Sleep Medicine
Date:Jan 1, 2012
Words:1537
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