Monitoring patients receiving mechanical ventilator support has four general goal :1. Assuring that the ventilator is operating as designed and set up by the vlinician2. Providing information to the clinicians to facilitate appropriate adjusments to the ventilator3. Controlling automatic feedback systems ( including alarms ) on the ventilator4. Providing information that might allow prediction of outcome

Common monitoring techniques are the direct clinical examination, assessments of gas exchange, and pressure / flow / volume measurements in the ventilator circuitry, which are reviewed in the first portion of this chapter ( 1-3 ). The second portion of this chapter reviews several new techniques that are either under investigation or have recently been introduced and may address some of the shortcomings of currently available strategies.

Conventional Monitoring of Patients Receiving Mechanical Ventilationa. Clinical EvaluationDuring the course of mechanical ventilator support, eliciting symptoms may be difficult because of the presence of artificial airways and altered mental status from drugs or disease. Neverthcless, symptoms of dyspnea, pain and cough are important indicators of patient status and evidence for these should be sought through both direct and indirect assessments ( table 13-1 )

Table 13 1 Symptoms Evaluation of Petient Requiring Mechanical Ventilation

Careful physicial examination of the respiratory system of the critically ill patient receiving mechanical ventilation is often challenging but remains a vital undertaking. Abnormalities of the respiratory system often present in a nonspecific manner, but patterns of findings may aid in the diagnosis.Careful inspection may reveal important findings ( table 13-2 ). Palpation of the chest may also be a useful examination tool. During palpation of the chest, the examiner may asses for changes in tactile fremitus. Increased fremitus suggest parenchymal consolidation. Decreased fremitus suggests either pleural fluid or hyperinflated parenchyme. Percussion may also be useful to assess for consolidation, pleural disease, or pneumothorax. Although the breath sounds may be obscured by mechanical ventilation, auscultation remains a useful tool, the listener may hear the usual array of adventitious sound and can easily assess for the absence of the absence of breath sounds indicative of a pneumothorax.

Evaluation Of Gas ExchangeOne principal goal of monitoring the mechanically ventilated patient is to ensure adequate oxygen delivery to the tissues. Oxygen delivery ( DO2 ) is related to the cardiac output (Q) and arterial oxygen concentration ( Ca02 ) using Equation 13 1. The Cao2 in turn is related to haemoglobin level ( Hb ), Oxyhemoglobin concentration ( or percent saturation : SaO2 ) , and the partial pressure of oxygen ( PO2 ) Through Equation 13-2 :DO2 = Q x CaO2 = Q x ( 1,34 x Hb x SaO2 ) x 10CaO2 = ( 1.34 x Hb x SaO2 ) + ( 0,003 x PO2 )

The PO2 and SaO2 are usually measured from direct analysis of arterial blood. Arterial blood gas analysis for acid base status ( Paco2 and pH ) is also criticial to monitor in mechanically ventilated petients. The pH affects many biologic functions, and Psco2 is inversely proportional to the minute ventilation provided by the mechanical ventilator.Because analysis of the arterial blood gases is a cornerstone of respiratory monitoring in the mechanically ventilated patient, it is essential to use a consistent, straightforward method of analysis. Modern blood gas analyzers use various electrodes and light absorption techniques to provides and light absorption techniques to provide values for Po2, Pco2, and pH. It is important to correct these measured values for the patients temperature. Modern analyzer also can measure various forms of haemoglobin such as carbonhemoglobin and methemoglobin. Carbonmeglobin is useful for diagnosis of carbon monoxide intoxication. Methemoglobin is an alternate form of hemoglobin with reduced oxygen binding capacity that is typically or the use of oxidant drugsOlder analyzer did not measure SaO2 directly and instead plotted Po2 against an ideal oxygen-hemoglobin dissociation curve to generate an expected SaO2. Caution must be used in interpreting calculated values for SaO2 because many factors in critically ill patients may create a difference between true SaO2 and this epected SaO2 ( e.g pH, temperature, abnormal hemoglobin binding, and alternate forms of hemoglobinIt is important to remember that sampling of aterial blood reflect only one point in time and is not reflection of gas exchange over time. Moreover, intermittent blood gas sampling is complicated by several potential sampling and measurement errors ( e.g. air bubbles mixed with the sample during transportations or temporary storage, and anylyzer calibration factor ). In addition to these errors, frequent blood gas sampling exposes health care personnel to blood and it increses the considerable amount of blood used for testing in critically ill patient.In an effort to avoid many of these problems, technology has been developed for continuous intra-arterial blood gas monitoring. Continuos blood gas monitoring is currently achieved by placement of a catheter with a sensor in the intravascular space. The two main sensing modalities used are the fiberoptic ( optode ) and electrochemical ( electrode ) systems. Both systems have shown promise in laboratory and animal testing, but several problems have limited widespread clinical application. For example, during periods of circulatory failure, peripheral blood flow is stagnant, thus likely leading to lower Po2, Higher Pco2, and lower pH than expected. In addition, the catheter may touvh the arterial and tissue values. Similar to other intravascular devices, these catheters may be thrombogenic and the waveform may be altered by patient and user interactions. Finally, these indwelling devices are costly,An alternative to actual blood sampling for SaO2 is the pulse oximeter. Using the differential absorption of red and infrared light of oxyhemoglobin and deoxyhemoglobin, the pulse oximeter generates ad estimate of arterial oxygen saturation ( SpO2 ) from light beams applied to the surface of the skin. In general, the SpO2 correlates reasonably well with SaO2. However, because of the sigmoid shape of the oxyhemoglobin dissociation shpe of the oxyhemglobin dissociatioin curve, a 4 % error in an SpO2 reading of 95% could leave the range of underlying Po2 as large as 100mmHg. Furthermore, the use of pulse oximetry traditionlally calibrates with healthy volunteers, which may not translate accurately in the setting of critical illness. Second, because of the inappropriate nature of exposing healthy volunteers to more severe hypoxemia, calibration is only done to an SpO2 of 70 % , thus leaving pulse oximeters less reliable at lower SpO2. In addition to severe hypoxia, pulse oximetry is also uneable to detect hyperoxia because the oxyhemoglobin desaturation curve is flat at high Po2 and further increases in Po2 lead to minimal, if any, increses in SaO2 or SpO2.

Mechanical Ventilator Device MonitorModern electronic and microprocessor based mechanical ventilator systems have considerable internal monitoring of electronic and pneumatic function that are designed to assure safe operation. Modern mechanical ventilators also routinely have mean airway pressure ( Paw) and flow (V) sensor in the circuitry that continuously monitor and display data for clinical use. In addition to these direct measurements, many derived values (e.g, inspired and exhaled volumes, minute ventilation, inspiratory to expiratory ratio ( I/E ) usully are available also. Most modern ventilators also have oxygen sensors in the circuitry to assure delivery of desired FIO2, ande some ventilators may also have analyzers for exhaled CO2 and inhaled therapeutic gases such as NO or heliox. The exhaled CO2 analyzer may have particular value as a back up disconnect alarm.In addition to routinely monitored values, modern ventilators can also make calculations after various maneuvers. The most common maneuvers are the vital capacity, SBT, maximal inspiratory mouth pressure (PI max ), measurements of respiratory system mechanics, and estimates of intrinsic positive end expiratory pressure ( PEEPi)The vital capacity requires a maximal voluntary expiratory effort from total lung capacity by the patient, and the exhaled volume is measured by the ventilator. The SBT is done with either minimal or no ventilator assistance and the spontaneous ventilation along with the frequency to tidal volumes (f/Vt) measured by th ventilator. The negative inspiratory force is measured by occluding the inspiratory circuit for at least 20 second and rhen measuring the negative pressure the patient can generate.Commonly measured respiratory system mechanics are airway resistance and respiratory system compliance. These are measured using a constant flow controlled breath with an end inspiratory pause ( i.e, no patient effort ). The distending pressure ( peak airway pressure ) ( Ppeak ), expiratory pressure (PEEP), V, compliance of the respiratory system (CRS ) in Equation (13-3) and tidal volume (Vt) measurements obtained during such a breath are used to calculate total airway resistance (Raw) in equation 13-4CRS =Vt/Pplat PEEP ( 13-3 )Raw = ( Ppeak Pplat ) / V ( 13-4 )A more sophisticated way to assess respiratory system mechanics using these signals is to generates a quasistic pressure volume (PV) plot. However, this requires a heavily sedated or paralyzed patient to allow multiple Pplat determinations to be plotted over the entire range of lung volumes. A modification on this approach is the use of a very slow inspiratory flow (