Nucleotides Required by rapidly dividing cells, nucleotides improve immune parameters in ex perimental animal models and supplemented neonatal feeds. There is no evidence that nucleot ides alone confer bene \201 t to the critically ill patient except when included within IE N formula. Antioxidant micronutrients Emerging evidence suggests a reduction in 28 day mortality in su rgical ICU patients given continuous EN with supple- mentary infusions of vitamins C and E. Selenium infusions may reduce the mortality rate in patients with septic shock and severe sepsis. Immune-modifying nutrients L-Arginine L-Arginine is a \200\230conditionally essential \200 \231 amino acid during metabolic stress and sepsis. Low circulating L-arginine levels give rise to the views of sepsis as an L-arginine-de \201 cient state or a syndrome of excess n itric oxide, a vasoactive product of L-arginine metabolism. Arginine upregulates macrophage phagocytic activity and neutrophil production of reactive oxygen species (such as peroxyni trite). High level supplementation (above 12g/l) reduces infectious complications and enhan ces wound healing in elective surgery, but conversely can augment rather than subdue the in \202 ammatory response. Patient groups Medical patients Patients with primary lung failure from chest infection or smoke inhalation injury have an increased mortality with IEN. IEN does not appear to bene \201 t this group. Pre-surgical patient Pre-operative IEN given to upper GI cancer patients reduces post-opera tive infectious complications, but not mortality. ITT evidence is currently less than compe lling and there are signi \201 cant \202 aws in the design of recent studies. Glutamine The most abundant circulating amino acid in the extracellular and intracellular c ompartments becomes \200\230conditionally essential \200\231 during critical illness as pl asma glutamine levels fall. Several controlled studies and a meta-analysis have con- \201 rmed glutamine \200\231s bene \201 cial in \202 uence on the in \202 ammatory response, oxidative stress and gut integrity. The preferred energy substrate for rapidly dividing en terocytes and immune cells, it may also improve glucose metabolism by reducing insulin resi stance, and is also a precursor of glutathione. Supplementary enteral glutamine fails to de monstrate clear clinical bene \201 t, except in the trauma or burns patient where it is a recommended addition. Post-surgical patient The incidence of acquired infection and hospital length of stay (LOS) are signi \201 cantly reduced in elective surgery patients given IEN, although reduction in septic complica- tion rates are non-signi \201 cant and overall mortality rates similar . Patients requiring ICU whilst being fed IEN demon- strated a trend for increased mortalit y. Trauma Trauma patients are likely to be the most well nourished of critical care patients. IEN lowers the incidence of bacterae- mias and intra-abdominal infections, but other infect ious complications remain unchanged. 87 CHAPTER 5.3 Immune-enhancing nutrition Further reading Angstwurm MW, Engelmann L, Zimmermann T, et al. Selenium in Intensive Care (SIC): results of a prospective randomized, placebo-controlled, multiple-center study in pa tients with severe systemic in \202 ammatory response syndrome, sepsis, and septic shock. Crit Care Med 2007; 35: 118 \200\22326. Sepsis The potential bene \201 t of L-arginine-enriched IEN is limited to mild sepsis (APA CHE II score 10 \200\22320) rather than those with a pre-existing enhanced in \202 ammator y response present (i.e. severe sepsis or SIRS). Subgroup analysis con \201 rms a signi \201 - cantly higher mortality in septic patients receiving IEN com- pared with control fee d. Comparing IEN with parenteral nutrition, subgroup analysis of severely septic patients d emonstrated a 3-fold increase in mortality compared with controls. Co f \201 er M, D chelotte P. The role of glutamine in intensive care unit patients: mec hanisms of action and clinical outcome. Nutr Rev 2005; 63: 65 \200\2239. Crimi E, Liguori A, Condorelli M, et al. The bene \201 cial effects of anti- oxidant suppl ementation in enteral feeding in critically ill patients: a prospective, randomized, double -blind, placebo-controlled trial. Anesth Analg 2004; 99: 857 \200\22363. ARDS High-fat respiratory feeds reduce PaCO2 and VCO2 in ALI and ARDS by altering the respi ratory quotient and metabolic load of the feed. Use of an IEN respiratory feed (rich in \2113 EPA and \2116 GLA plus antioxidant vitamins) vs a \200\230standard \200\231 respira tory formula to address the underlying in \202 ammatory pathogenesis of ARDS signi \201 c antly reduced ventilator days, incidence of organ failure and ICU LOS, with a trend towards reduced mortality, despite signi \201 cant patient heterogeneity. Kalil AC, Danner RL. L-Arginine supplementation in sepsis: bene \201 - cial or harmful? Curr Opin Crit Care 2006; 12: 303 \200\2238. Kieft H, Roos AN, van Drunen JD, et al. Clinical outcome of immu- nonutrition in a heteroge neous intensive care population. Intens Care Med 2005; 31: 524 \200\22332. Kreymann KG, Berger MM, Deutz NE, et al. ESPEN guidelines on enteral nutrition: intensive care. Clin Nutr 2006; 25: 210 \200\22323. McCowen KC, Bistrian BR. Immunonutrition: problematic or prob- lem solving? Am J Clin Nutr 2003; 77: 764 \200\22370. Montejo JC, Zarazaga A, Lopez-Martinez J, et al. Immunonutrition in the intensive care unit . A systematic review and consensus state- ment. Clin Nutr 2003; 22: 221 \200\22333. Summary Globaly, IEN reduces the risk of infection complications, time on mechanical ventil ation, ICU LOS and hospital LOS in selected patient groups, but has no appreciable effect o n mortality. Variations in the nutrient pro \201 le of IEN feeds (particularly with regard to the level of arginine supple- mentation), errors in trial design, underpowered studies and post hoc interpretation of trial results all create dif \201 cul- ties in recommending IEN to the heterogenous ICU popu- lation. The influence of early and appropriate enteral f eeding per se may be of greater clinical relevance than IEN on positive outcomes in the cri tically ill patient. Sakr Y, Reinhart K, Bloos F, et al. Time course and relationship between plasma selenium co ncentrations, systemic in \202 ammatory response, sepsis, and multiorgan failure. Br J Ana esth 2007; 98: 775-84. This page intentionally left blank 89 Chapter 6 Respiratory monitoring Chapter contents Pulmonary function tests in critical illness 90 End-tidal CO2 monitoring 92 Pulse oximetry 94 90 S SINGH Pulmonary function tests in critical illness Introduction Pulmonary function test results in critically ill patients can be important pr ognostically and guide ventilatory and weaning strategies. However, they are not straightfo rward to measure in mechanically ventilated patients and remain limited to dynamic volumes. Fortunately, most modern mechanical ventilators are able to calculate and display static a nd dynamic lung volumes, together with derived val- ues for airway resistance, compliance a nd \202 ow/volume/ time curves. The ability to monitor these changes after altering venti latory parameters has enabled more sophisti- cated adjustments of ventilation, to prevent p otentially damaging mechanical ventilation. Spirometry measures dynamic lung volumes. This is most useful when assessing the severity of obstructive lung dis- eases (especially asthm a, COPD and obliterative bronchi- olitis). Lung volume measurements by body box plethysmogr aphy (gold standard) or helium dilution (prone to under-repre- sentation in bullous lung di sease or signi \201 cant gas trapping) detect hyperexpansion or restrictive lung diseases (i.e. small lungs, in \202 ammation or \201 brotic lung disease, or abnor- malities of t he muscles or skeleton of the chest wall). underlying lung disease (e.g. atelectasis, consolidation, pulmonary oedema). Monitoring suc h measurements (if practical) can help assess the effectiveness of therapeutic strategies s uch as chest physiotherapy and lung expansion maneuvers (i.e. recruitment and PEEP). Compliance and resistance In a well sedated patient with an indwelling endotracheal tube (E TT), measurements of total respiratory compliance (Crs), airway resistance (Raw) and respir atory resistance (Rrs) can be made during constant \202 ow ventilation. Airway pressure a t the proximal end of the ETT (termed pressure at airway opening, Pao), during a brief post -inspiratory pause at varying volumes (between FRC and total lung capacity (TLC)) through t he inspiratory cycle, enables plots of volume against Pao, and pressure change against \202 ow to calculate Crs, Raw and Rrs, respectively. Normal values for Crs are >100ml/cm H2 O. In ALI and ARDS, Crs is often <25ml/H2O. Monitoring these measure- ments may allow optim ization of ventilatory maneuvers such as recruitment, and applying PEEP. For example, at th e onset of inspiration, as airway pressure is increased, the slope on the V \200\223P curve suddenly increases at the lower in \202 ection point. This represents the improved compli ance at closing volume. As pressure is increased, an upper in \202 ec- tion point is encou ntered where compliance tends to fall again, re \202 ecting overin \202 ation. The optima l airway pressures are between these two points, and so Crs measures and V \200\223P loops may be used to adjust mechanically imposed vol- umes and pressures between these points, so improving gas exchange and reducing ventilator-induced lung injury. Airway resistance (Raw ) measurements are useful for con- sidering the causes of increased Raw. Normal values in s pontaneously breathing patients are <2.5 l/s cmH2O/l/s. In a normally ventilated patient, t his may rise to 4. In cardio- genic pulmonary oedema, this may be>10, ARDS >15 and obstruc tive airways disease >20 cmH2O/l/s. Intrinsic PEEP (PEEPi) Intrinsic or auto-PEEP describes the increase in end-tidal alveolar pressure whenever the expiratory time is inade- quate t o allow the lung to de \202 ate to its relaxation volume. PEEPi occurs if expiratory resis tance is increased (i.e. air- \202 ow obstruction) or if expiratory time is too short. It is measured with the proximal pressure transducer during an end-expiratory occlusion maneu ver. At no \202 ow, a positive Pao re \202 ects static PEEPi. A dynamic measurement of P EEPi can be recorded by the airway pressure at which inspira- tory flow commences in sponta neously breathing or assisted modes. Dynamic hyperin \202 ation can arise, leading to incr eased work of breathing (in spontaneous ventila- tion), reduced ability to trigger the vent ilator in assisted modes, increased intrathoracic pressures and conse- quences, and risks o f volu-barotrauma. If the reduction of PEEPi is deemed bene \201 cial, treating the underl ying cause, adjusting ventilation or the application of external PEEP (with cautious monito ring of PEEPi and other parameters of dynamic hyperinflation) are warranted. Lung function tests on mechanically ventilated patients A number of factors should be consi dered when perform- ing lung function on the ICU. Such patients are invariably sedated, exc luding volitional tests. They are not easily moved, and may not be in an ideal upright posi tion. Their high oxygen requirements may also affect accurate gas exchange measurements. Ho wever, the presence of an arti \201 cial airway simpli \201 es the measurement of lung vo lumes and pressure \200\223volume relationships. Those on mandatory modes of mechanical ven tilation may be unable to perform a vital capacity (VC) maneuver, although a passive com- p lete pressure \200\223volume (P \200\223V) loop will be possible. Measuring pulmonary mechanics Lung volumes and VC Portable spirometers allow bedside measur es of VC and forced expiratory volume (FEV) in spontaneously breathing extubated patients. For intubated patients, a pneumotacho- graph (which measures VC through an accurate flow se nsor) is preferred. These values usually will be effort dependent, and may be underestimate s possibly due to inadequate effort, underlying lung disease or muscle weak- ness. A VC of >10ml/kg is a reasonable indicator of the need for assisted ventilatory support in those wi th neuromuscu- lar disease such as Guillian \200\223Barre syndrome. However, trend monitori ng of declining VC, with symptomatic hyper- capnic acidosis, is often a more practical mark er of the need for intervention in these situations. VC monitoring is less useful for predi cting need for ventilatory assistance or suc- cessful weaning in other conditions. Function al residual capacity (FRC) has been measured using a number of different techniques such as simple helium dilution (rebreathing 6 breaths from a 1liter \200\230bag-in bottle \200 \231 system), nitrogen washout or prolonged inert gas dilution. FRC (normally 2 \200\2233l) is frequently reduced in ventilated patients, for such reasons as supine position, sedativ es, neuromuscular blocking agents, abdominal distension and Respiratory muscle testing Bedside assessment of respiratory muscle testing in the ICU may allow prediction of prolonged weaning. The ability 91 CHAPTER 6.1 Pulmonary function tests in critical illness The oxygenation index is another measure that is the product of mean airway pressure, FiO2 divided by PaO2. The oxygenation index takes into account ventilatory adjustments and is sa id to predict outcomes better in pediatric acute respiratory failure than PaO2:FiO2. Venous admixture and shunt In a lung with abnormal gas exchange, regions with little or poor gas exchange act (e.g. consolidation or collapse) as shunts, and the oxygen tension of mixed ve nous blood, which is added essentially unchanged to arterial blood, governs PaO2 to a large extent. Mixed venous PaO2 re \202 ects peripheral oxygen delivery and consumption. An est imate of venous admixture and shunt is possible with simultane- ous sampling of mixed venou s, pulmonary arterial and arterial blood. However, practical uses are limited. to sustain spontaneous breathing is governed by respira- tory drive, respiratory muscle str ength and endurance, and respiratory muscle loading (combination of resistance, compliance and PEEPi). Respiratory muscle weakness is common during prolonged mechanical ventilation. It is commonly due to the consequences of critical illness such as muscle wasting, critical illness myoneuropathy, malnutri- tion, hypercatabolism, iatrogenic causes (e.g non-depolar izing blocking agents and steroids, or prolonged neuromuscular blockade) and, less commonly , due to primary neuromuscular disorders. Strategies Respiratory drive can be assessed by measuring airway pressure 0.1s after occlud ing the airway against an inspira- tory effort (P0.1). Most modern ventilators have this op tion. This represents neuromuscular activation of the respiratory system and correlates wit h the work of breathing. A high value P0.1 >6cm H2O may imply an unsustainable work of brea thing, and is used as one of a number of indicators of failure to wean to extubation. Very low values of P0.1 re \202 ect inadequate drive and probaby weaning failure. Non-volitiona l techniques for measuring diaphragm strength are now available on the ICU. They utilize a combi- nation of external magnetic stimulators of the phrenic nerves, with oesopheageal and gastric pressure transduc- ers, thus allowing transdiaphragmatic pressure change (PDi) mea surements. Pulmonary function as an outcome predictor in the critically ill Pulmonary function testing is useful for evaluation of different forms of lung disease or for assessing the presence of dis- ease in a patient with known risk factors, such as smoking. Other indications inclu de: \200 Evaluation of symptoms such as chronic persistent cough Objective assessment of bronchodilator therapy \200 Addressing occupational exposures \200 Pre-operative risk assessment prior to thoracic or upper abdominal surgery Weaning predictors Tests of respiratory muscle function have greatest application in the pr ediction of weaning outcome. Predictors, such as maximum inspiratory pressure, VC and minut e ventilation, are frequently falsely positive and negative. Although patients failing a we aning trial may have an elevated pressure \200\223time index and P0.1, these tests have not gained popularity in everyday ICU practice. The ratio of respiratory frequency to tidal vo lume (f/V) is the most reliable simple predictor of weaning outcome. Thus, a value <80 prov ides a high likelihood ratio (>7.5) of success- ful weaning. In contrast, values >105 may s uggest a further period of weaning. Objective assessment of impairment or disability. Poorer FEV1 predicts an increased likelih ood of post-operative pulmonary complications, particularly in those with COPD. A lower gas transfer coef \201 cient (TLCO), and greater rate of decline preceding critical illness, predict an increased mortality in patients with diffuse parenchymal lung disease. Summary Pulmonary function testing serves as an important tool in critical illness. Prior i nformation and serial decline can pre- dict worse outcomes from intercurrent critical illne ss, par- ticularly if associated with the underlying chronic lung condition. Measurements a nd interpretation of static and dynamic lung function in the critically ill can also provid e opportunities to recognize deterioration, optimize ventila- tory assistance and predict w eaning. Assessment of gas exchange A wide range of indices of gas exchange ef \201 ciency have bee n proposed, such that arterial blood gas tension (PaO2) can be assessed at differing concen trations of inspired oxygen(FiO2). The PaO2:FiO2 ratio is one criterion used to de \201 ne ALI (<40kpa) and ARDS (<27kPa). Further reading Overview of pulmonary function testing. www.uptodate.com http://www.thoraci c.org/sections/publications/ statements/ 92 A WAGSTAFF End-tidal CO2 monitoring End-tidal carbon dioxide monitoring is now advised for all ventilated patients on a critica l care unit and is essential monitoring for the transfer of such patients both within and b etween hospitals. Whilst useful for monitoring of the presence of ventilation, capnography can also offer other important and valuable information to the critical care practitioner. There are two types of commonly used gaseous carbon dioxide monitoring used in clinical pra ctice. produce falsely elevated readings. Care is required as the heat can cause facial burns. The additional weight of the apparatus on the breathing system can cause traction on the ETT. Unlike the side-stream method, the main-stream technique provides immediate readings with r espiration. The capnogram waveform The normal capnogram is usually displayed at 7mm/s. It has a wavefor m that is divided in to three phases. I Carbon dioxide-free phase. Anatomical dead space an d that of the breathing system, HME, catheter mount. II An S-shaped upswing which denotes m ixed dead space Capnography The instantaneous graphical record of carbon dioxide (CO2) concentration in the respired gasses during respiration. and alveolar gasses. III A plateau phase due to alveolar emptying. The normal plateau often has a slight upward slope; the highest point of this slope is the end-tidal CO2 partial pressure (PetCO2) and, if the slope has achieved a plateau, is the closest approximation of the partial pressure o f arterial CO2 (PaCO2). Capnometry The measurement and display of CO2 concentrations on a digital or analog indicat or. Both methods can be employed in the critical care environ- ment. Analysis of the CO2 co ncentration and waveform can help in the management of both the respiratory and cardiovascu lar systems. Volumetric capnography Plotting the partial pressure of expired CO2 (PECO2) against time do es not account for expiratory \202 ow. Whilst giving useful information about the presenc e of ventilation and an estimate of the PaCO2, the inclusion of \202 ow meas- urement all ows for a greater amount of pulmonary physiol- ogy to be detected and applied in management . This leads to the use of volumetric capnography based on the single breath test for CO2 ( SBT-CO2). It differs from standard capnography by the integration of gas \202 ow to produ ce a graph of PECO2 against volume rather than time. The SBT-CO2 gives more information on the ventilation/ perfusion (V/Q) status of the lung. Measurement techniques Capnography Four main methods are used to measure CO2 concentra- tio n in respiratory gasses: mass spectrography, Raman spec- trography, photoacoustic spectrogr aphy and, most commonly, infrared spectrography. This \201 nal method uses the physical p rinciple that polyatomic gasses absorb infrared light. CO2 selectively absorbs light at a s peci \201 c wavelength of 4.3 m. The absorption of this wavelength of light is propor- ti onal to the concentration of CO2 molecules and, by com- paring them with a known standard, the concentration can be displayed \200\224usually as a partial pressure (mmHg or kPa). Clinical applications of capnography The use of capnography has been a standard requirement for monitoring at induction of general anesthesia. It is now a requirement for monitoring of ventilated patients on the ICU and transportation of ventilated patients both intrahospi tal and interhospital. Capnometry A pH-sensitive chemical indicator in a plastic housing placed in the system betw een the ETT and the method of ventilation. Exposure to CO2 causes a change in the paper \200\231s color: purple to yellow. The degree of color change permits approximation of the CO2 concentration; however, it cannot be used for accurate measurement. Location of CO2 mon itors Capnometers are classi \201 ed as side-stream or main-stream, depending on the posit ion of the sensor relative to the gas delivery system. Both have advantages and disadvantag es. Con \201 rmation of endotracheal intubation The presence of a capnogram aids in the identi \201 cation of an appropriately placed ETT, and is mandatory for oral or nasal tubes and tracheostomy. Following intubation, six expired CO2 waves need to be observed to con \201 rm cor- rect placement, as oesophageal and gastric CO2 may be present from bag and mask ven tilation, the presence of car- bonated drinks, etc. The use of capnometry for such pro- ced ures is acceptable, but caution must be observed as tracheal secretions or gastric contents may contaminate the capnometer, interfering with its chemical reaction and causing it to r ead positive for CO2 in error. Presence and adequacy of ventilation The presence of a regul ar capnogram of correct morphology allows continuous monitoring of ventilation. This is in prefer- ence to the display of the PetCO2 value alone. Accidental disconnection or extubati on, endotracheal obstruction, ETT cuff leak and ventilator malfunction can all be detected due to a change in the morphology of the capnogram. Capnography can be used to monitor vent ilation in the spontaneously ventilating subject. Sampling can be under- taken at the nose, by adaptation of the nasal cannulae, Side-stream The senor is located distal to the patient. The gas is sampled through capillar y tubing attached to the distal end of the ventilation apparatus, as close to the subject \200\231s airway as possible. This may be a T-piece inserted into a breathing system, or a luer lock on an HME. The tubing can also be inserted into the nostril of a spontaneously br eathing patient. Optimal gas \202 ow is considered to be 50 \200\223200ml/min, which need s to be considered if using expired gas \202 ow to measure tidal volume. It also causes a delay in reading of 150ms. Main-stream Mounted on the breathing system, the capnometer con- sists of an infrared gener ator and sensor within a cuvette through which the respiratory gasses pass. The sensors are heated to 39 C to prevent water condensation that may 93 CHAPTER 6.2 End-tidal CO2 monitoring correlation between this method and other standards. This may have a role as a non-invasive alternative in the mechanically ventilated patient. Capnography also has a role at cardiac arrest. It re \202 ects well the adequacy of resuscitation. PetCO2 values of <10mm Hg at 20min post-resuscitation are associated with signi \201 cant mortality. Return of a sponta neous circulation can be signi \201 ed by a sudden increase in PetCO2. Alveolar recruitmen t and PEEP The application of PEEP as a method to recruit and main- tain the patency of col lapsed alveoli is a strategy that can be used in respiratory failure, particularly ALI. Dea d space is initially large in ALI, as re \202 ected by a high (a-et)PCO2. The application of PEEP has been shown to reduce this gradient. Other studies have demonstrated that titrat ion of PEEP does not improve (a-et)PCO2 in all cases of ALI/ ARDS. However, the gradient re duces in those that have a demonstrable in \202 ection point on the pressure \200\223volum e curve, and those patients that improve oxygenation with the application of PEEP. or by placing the sampling line within the face mask. When oxygen is concurrently at low \202 ow rates, the PetCO2 is a good predictor of PaCO2. At high oxygen \202 ow rates, the mixing with the exhaled gasses leads to under-reading of the PetCO2. In patients at high r isk of apnoea, e.g. type II COPD, neuromuscular disorders, CO2 monitoring can contribute to a prompt response in the event of hypopnoea or respiratory arrest. If neuromuscular blocka de is being used, then CO2 moni- toring can aid assessment of its adequacy. Breathing by th e patient during mandatory ventilation leads to a charac- teristic dip within phase III; th e so-called \200\230curare cleft \200\231. Titration of paralyzing therapy to prevent this phenomenon can be achieved as a goal and can lead to lower dose requirements. Estimation o f PaCO2 In normal individuals, the difference between PaCO2 and PetCO2, the (a-et)PCO2 is b etween 2 and 5mm Hg. It increases with age and disorders that may increase anatomical or al veolar dead space, e.g. COPD, pulmonary embolism, low cardiac output and mechanical ventila tion itself. It decreases with large tidal volume low frequency ventilation, as well as in pregnancy and in young children. Because of this potential variability, it is best to have a con- current assessment of the PaCO2. Ongoing monitoring can then use the PetCO2 as a sur rogate. Dynamic changes in the patient \200\231s cardiovascular or pulmonary condition may result in a change in the (a-et)PCO2, so the PetCO2 cannot be ubiquitously relied on as a g uide to PaCO2. Though a raised PetCO2 does usually signify a raised PaCO2, the contrary is not always true. The shape of the volumetric capnogram Alterations in the shape of the capnogram can give in for- mation regarding pulmonary function. If all the alveoli had the same PCO2, the phase I I time would be short and the plateau of phase III would be horizontal. The lung is not hom ogenous due to wide ranges of V/Q ratios. Those areas of the lung that have a low V/Q ratio (underventi- lated, high PCO2) empty after those with a high V/Q ratio (well ventilated, l ow PCO2). This causes an upward slope of phase III. The respiratory units with the lower V/ Q tend to be located distally to those with a higher V/Q. Consequently they empty later in the respiratory cycle. The slope of phase III is thus dependent on the emptying patterns of alveoli with differing time constants and PCO2 concentrations. A lengthening of phase II a nd an increase in the gradient of phase III due to an increase in the heteroge- neity of al veolar ventilation and perfusion is seen in many causes of respiratory failure; notably acu te asthma, COPD, ALI/ARDS, pulmonary \201 brosis and pneumonia. In pulmo- nary embolism, the reduction in perfusion leads to a marked increase in the affected V/Q. The absence of C O2 in the affected units does not alter the morphology of the SBT-CO2 trace; it just reduce s its height. Cardiovascular status The delivery of CO2 to the lungs and the regional per- fusion of the alveoli are re \202 ected in the partial pressure of CO2 in the alveoli. If ventilation re mains constant (V), then changes in perfusion (Q) will result in a change in PECO2. This ha s been demonstrated clinically in both dynamic changes in cardiac output and the detection of pulmonary embolism. Changes that occur in hypermetabolic states such as thryotoxicosis o r severe sepsis can increase PECO2 as a re \202 ection of the increase in tissue CO2 produ ction. A novel method of cardiac output monitoring, the partial rebreathing technique (PRCO ), uses a variation of the indi- rect Fick method. Studies suggest that there is good 94 S STIRLING & R LANGFORD Pulse oximetry Introduction Pulse oximetry is an essential monitoring technique widely used to determine t he oxygen saturation of arterial blood. It is a standard monitor in intensive care as well as anaes- thesia and other critical care areas. Variable absorption due to change in volume with each pulse Absorption by arterial blood Absorption by venous blood n o i t p r o s b A Principles The use of the pulse oximeter is based on two main physi- cal principles, i.e. t he Beer \200\223Lambert law and the different absorption spectra of oxy- and deoxy-Hb. Beer \200\231s law describes how the intensity of transmitted light decreases exponentially as the concentration of a sub- stance increases. Lambert \200\231s law states that the intensi ty of transmitted light decreases exponentially as the distance traveled through a substanc e increases. These laws are applied using two light-emitting diodes in the pulse oximeter p robe which is usually placed on a \201 nger, toe, ear or nose. Red light (660nm) and infr ared light (940nm) are transmitted in sequence several hundred times per second, with pause s when both diodes are off to enable compensation for ambient light. Red and infrared light are used at these wavelengths because of their widely differing absorption spectra for oxy - and deoxy-Hb (Fig. 6.3.1). Absorption due to tissue Time Fig. 6.3.2 Absorption of light transmitted through the \201 nger during pulse oximetry. The non-pulsatile or constant components of the meas- urements are discarded, leaving a pul satile trace from which arterial oxygen saturation can be derived. Limitations Calibration Pulse oximeter calculation of oxygen saturation is based on referen ce ranges using laboratory measurements from healthy volunteers, and hence cannot include d angerously hypoxaemic levels. It is well established that below satura- tions of 70%, puls e oximeters become progressively more inaccurate. Initial set-up, however, on powering up i s an automatic process requiring no external calibration proce- dure. (Red) 660nm (Infrared) 910nm 10 Perfusion In hypothermia, hypotension, hypoperfusion, peripheral vasoconstriction (both iat rogenic and primary) and venous congestion, the pulsatile component of the signal can be of insuf \201 cient amplitude and quality, such that the reading is inaccurate or absent. Hbo2 Hb Movement Movement of the probe or cable by the patient or attend- ing staff can cause artif acts, loss of signal and errors. The most recent pulse oximetry algorithms have markedly im proved performance in low perfusion states and movement. 0.1 Wavelength (nm) 600 700 900 1000 Fig. 6.3.1 Absorption spectra of light as used in the pulse oximeter. The transmitted light travels through the tissue to a photo- detector where the measurement s take place. In the shorter wavelength region (red), oxy-Hb absorbs less light than deoxy- Hb, with the reverse being true in the infrared region (longer wavelength). At the photodet ector, the transmitted light is electronically processed and the relative absorption of the blood at the two wavelengths compared. The isobestic point is the wavelength at which two substances absorb a particular wavelength of light to the same extent and was used in some oximeters to correct for Hb concentration. It can also be used as a reference point where l ight absorption is independent of saturation. The pulse oximeter is designed to measure sol ely arterial oxygen saturation, which requires that the signals from other tissues and capi llary and venous blood be \201 ltered out. This is done using the pulsatile nature of art erial blood. Arrhythmias Atrial \201 brillation can make the measurement of maximum and minimum absorp tion more difficult and prone to error. Electrical interference Surgical diathermy is a common cause of loss of saturation reading. Abnormal haemoglobins Carboxy-Hb has a similar absorption coef \201 cient to oxy- Hb, tren ding towards a falsely high reading at 96%. In the case of carbon monoxide (CO) poisoning, e.g. in smoke inhalation, it is essential to remember this point. Where there are suspecte d high levels of carboxy-Hb, an arterial blood co-oximeter measurement should be taken to c on- \201 rm the correct carboxy-Hb and O2 levels as the SpO2 reading may be misleading. M ethaemoglobin (MetHb) has a similar absorption at both emitted wavelengths, giving 95 CHAPTER 6.3 Pulse oximetry This delay is also seen when oxygen saturations improve some seconds after \200\230pinking up \200\231 of a patient following an interven- tion to treat cyanosis and low saturations . This time lag has been shown to be reduced when an ear probe is used instead of a \201 nger probe. Despite the expert opinion perception of the advantage of continuous monitoring , there is no published trial evidence of alteration of outcome with pulse oximetry. a saturation reading of 84%. These readings are the same regardless of the true oxy-Hb sat uration. Recently launched multi-wavelength pulse oximeters can now measure car- boxy-Hb an d MetHb, and consequently have improved accuracy in SpO2 measurement. Other intravascular s ubstances A number of substances can give falsely low saturations when present in tissues a nd circulating blood, e.g. methylene blue and indocyanin green. Clinical applications Critical care unit It is now normal practice to have all patients con tinuously monitored using a minimum of an ECG and a pulse oximeter. Bilirubin This has a similar absorption to deoxy-Hb, giving a falsely low reading. Ambient light Lights (e.g. in theater or an anesthetic room) can affect saturation readings especially if light is bright, direct or \202 ickering (appearing pulsatile). Anesthesia Pulse oximetry has become a mandated standard both during and after general and regional anesthesia Ward care, imaging units, endoscopy and patient transportation Continuous oxygen saturation monitoring is becoming more commonplace especially in higher risk patients, and during tra nsportation and procedures involving sedation. Nail polish and nicotine These can absorb infrared light and cause inaccuracies. Nail polis h should therefore routinely be removed pre- operatively. If this is not possible or if the patient has false nails, the pulse oximeter probe can be placed side to side across the \201 nger, avoiding the nail-bed. Resuscitation Pulse oximetery can be used during CPR to give a general assessment of adequa cy of perfusion. Physical risks Pressure damage There have been case reports of pressure necrosis in patient s being monitored for long periods, more commonly if poor tissue perfusion is a factor. Investigations Saturation monitoring can be used during sleep studies to investigate sleep apnoea as well as in cardiac investigations such as exercise tolerance tests. This simple t o use, portable, non-invasive device has become one of the most widely used monitors in hos pital medicine. Whilst it has its limitations, it has proved an invaluable addi- tion to cl inical observation and other forms of monitoring. Magnetic resonance imaging (MRI) Burns have been reported due to electrical induction when non-MRI-compatible probes have been used. Clinical problems Complacency SpO2 readings in the \200\230normal range \200\231 can be fa lsely reassur- ing. Because of the sigmoid shape of the Hb dissociation curve, the pulse ox imeter can be relatively poor at detecting a signi \201 cant fall in PaO2 particularly at the steeper portions of the curve (Fig. 6.3.1). Further reading Aitkenhead AR, Smith G, eds. Textbook of anesthesia, 3rd edn. Churchill Livinstone, London 1996. Yentis, SM, Hirsch, NP, Smith, GB. Anesthesia and intensive care A \200\223Z, 2nd edn. Elsevier, 2000. Jubran A. Pulse oximetry. Review article. Critical Care 1999; 3: R11 \200\2237. Time lag There can be a lag of up to 30s between clinical evidence of cyanosis and pulse ox imetry detection of a drop in saturation. This page intentionally left blank 97 Chapter 7 Cardiovascular monitoring Chapter contents ECG monitoring 98 Arterial pressure monitoring 102 Insertion of central venous catheters 10 4 Common problems with central venous access 106 Pulmonary artery catheter: indications and use 108 Pulmonary artery catheter: insertion 110 Echocardiography 112 Clinical application of echocardiography in the ICU 116 Doppler 118 Pulse pressure algorithms 120 Non-invasive methods 122 Measurement of preload status 124 Detection of \202 uid responsiveness 126 98 E ASHLEY ECG monitoring ECG monitoring is carried out in order to measure heart rate, rhythm and conduction disturb ances, and to monitor pacemaker function. It is essential for the detection of myocardial i schaemia and may also give an indication of electrolyte imbalance. aVF I aVL aVR aVL aVR II III \210\22290 \210\22260 \210\222120 aVL II 5 mm \210\22230 III \210\222150 aVR 0.2 sec 180 5 mm 0 0.5 mV R aVF I aVF I +150 +30 +60 +120 0.04 sec 1 mm 1 mm 0.1 mV +90 aVR III II aVL 25 mm/sec 10 mm/mV aVL III II aVR I aVF III II P-R seg- ment T aVF P S-T segment U Fig. 7.1.2 ECG leads. Q P-R interval Augmented unipolar leads The reference electrode is formed by combining leads I, II and III , i.e. the center of Einthoven \200\231s equilateral triangle, where the potential differen ce is zero. right arm aVR left arm aVL left leg aVF S-T interval S QRS interval Q-T interval Fig. 7.1.1 Normal ECG con \201 guration. Unipolar chest leads The reference electrode is formed by the combined aV leads. V1 4th int ercostals space, right sternal edge V2 4th intercostals space, left sternal edge V3 midway between V2 and V4 V4 5th intercostals space, left mid-clavicular line V5 5th intercostals s pace, left anterior axillary line V6 5th intercostals space, left mid-axillary line