CT scan CT scan is the de \201 nitive imaging procedure for pathology within the thorax. I njection of contrast highlights vascular structures and the pres- ence of pulmonary emboli in sub-segmental arteries (with high resolu- tion scanning). It enables improved discrimina tion of pulmonary shadows, e.g. \202 uid, \201 brosis, consolidation, atelectasis, absc ess, empyema, and can be used to guide drainage procedures of air and \202 uid collection s. RESPIRATORY IMAGING 159 See also: Airway obstruction, p348; Atelectasis and pulmonary collapse, p352; Acute chest i nfection (1), p356; Acute respiratory distress syndrome (1), p360; Asthma \200\224general m anagement, p364; Pneumothorax, p368; Haemothorax, p370; Pulmonary embolus, p376; Heart fail ure \200\224assessment, p392; Intrahospital transportation, p644. This page intentionally left blank 161 Chapter 9 Cardiovascular monitoring ECG monitoring 162 Blood pressure monitoring 164 Arterial cannulation 166 Central venous ca theter \200\224insertion 168 Central venous catheter \200\224use 170 Central venous cathete r \200\224complications 172 Pulmonary artery catheter \200\224insertion 174 Pulmonary arter y catheter \200\224use 176 Cardiac output \200\224central thermodilution 178 Cardiac output \200\224peripheral thermodilution 180 Cardiac output \200\224indicator dilution 182 Cardia c output \200\224Doppler ultrasound 184 Cardiac output \200\224pulse contour analysis 186 C ardiac output \200\224other techniques 188 Pressure and stroke volume variation 190 Echocar diography 192 Tissue perfusion monitoring 194 Gut tonometry 196 162 CHAPTER 9 Cardiovascular monitoring ECG monitoring Continuous ECG monitoring is routinely used in every Critical Care Unit. The standard technique is to display a three-lead ECG (commonly lead II). Other limb leads may be used although the electrodes are placed at the shoulders and left side of the abdomen. Other lead con \201 gurations can be used for speci \201 c purposes: \200 \200 \200 Chest \200\223shoulder \200\223V5: Early detection of left ventricular strain. Chest \200 \223manubrium \200\223V5: Early detection of left ventricular strain. Chest \200\223back \200\223V5: p wave monitoring. Modern monitors include alarm functions for bradycardia and tachycardia monitoring, and sof tware routines for arrhythmia detection or ST segment analysis. Causes of changes in heart rate or rhythm Changes in heart rate or rhythm may be an indicat ion of: \200 Increased sympathetic/parasympathetic activity: \200 \200 \200 \200 \200 \200 A dverse drug effects: \200 Antiarrhythmics. \200 Sedatives. Electrolyte imbalance. Fever . Circulatory insuf \201 ciency. Pain. Anxiety. Hypoxaemia. Hypercapnia. Sepsis. \200 ECG MONITORING 163 See also: Tachyarrhythmias, p384; Bradyarrhythmias, p386; Acute coronary syndrome (1), p388 ; Acute coronary syndrome (2), p390. 164 CHAPTER 9 Cardiovascular monitoring Blood pressure monitoring Non-invasive techniques Non-invasive techniques are intermittent but automated. They include oscillotonometry (detection of cuff pulsation as the systolic p ressure), detection of arterial turbulence under the cuff, ultrasonic detection of arterial wall motion under the cuff, and detection of blood \202 ow distal to the cuff. Any cuff system should use a cuff large enough to cover two thirds of the surface of the upper arm. Invasive (direct) arterial monitoring Blood pressure is most usefully monitored from larger limb arteries, e.g. femoral or brachial. However, because of the potential risk of damage to these arteries compromising the distal limb, it is generally safer to use radial, ulnar, or dorsalis pedis arteries. Pressure measured in a distal artery is higher than from a cen tral vessel. The arterial cannula is connected to an appropriate transducer system via a sh ort length of non-compliant manometer tubing. The transducer should be matched to the monit or, i.e. as recommended by the manufacturer of the monitor. The transducer must be zeroed t o atmospheric pressure and be positioned at the level of the 4th intercostal space in the m id-axillary line. The transducer, mano- meter tubing, and cannula should be continuously \202 ushed with 3mL/h saline or heparinised saline (1000IU/L). Damping errors A correctly damped system will return immediately to the pressure wave- form after \202 ushing. Return is slow in an over-damped system, and there is often resonance around the baseline before return to the pressure wave- form in an under-damped system. An under-damped system will over-esti- mate systolic and under-estimate diastolic blood press ure. The converse is true for an over-damped system. Waveform shape cannot be correctly int erpreted unless damping is correct. Interpretation of waveform The shape of the arterial pressure waveform gives useful qualita tive infor- mation about the state of the heart and circulation (see \201 gure 9.1): Shor t systolic time Hypovolaemia High peripheral resistance Hypovolaemia Pericardial effusion Airways obstructi on High intrathoracic pressure Poor myocardial contractility High peripheral resistance Marked respiratory swing Slow systolic upstroke Limitations of blood pressure monitoring It is important not to rely on arterial blood pres sure monitoring alone in the critically ill. A normal blood pressure does not guarantee ade quate organ blood \202 ow. Conversely, a low blood pressure may be acceptable if perfusio n pressure is adequate for all organs and blood \202 ow is high. Measurement of cardiac o utput and markers of organ perfusion, in addition to blood pressure, is necessary where the re is doubt about the adequacy of the circulation. BLOOD PRESSURE MONITORING 165 Normal arterial waveform BP Systolic time Short systolic time BP Systolic time Slow systolic upstroke BP Fig. 9.1 Examples of arterial waveform shape. See also: Arterial cannulation, p166; Hypotension, p380; Hypertension, p382. 166 CHAPTER 9 Cardiovascular monitoring Arterial cannulation Indications Performed correctly, arterial cannulation is a safe techni que allowing con- tinuous monitoring of blood pressure and frequent sampling of blood. It i s indicated in any patient with unstable or potentially unstable haemodynamic or respirator y status. Radial artery cannulation The radial artery is most frequently chosen because it is accessi ble and has good collateral blood \202 ow. Allen \200\231s test, used to con \201 rm the ulnar blood supply, is not reliable. Technique of cannulation Use aseptic technique. Hyperextend the wrist and abduct the thumb. After skin cleansing, local anesthetic (1% plain lidocaine) is injected into the skin and sub- cutaneous tissue over the most prominent pulsation. The course of the artery is noted and a 20G Te \202 on cannula inserted along the line of the vessel. The vessel is usually entered in similar fashion to an intravenous cannula. There is usually some resistance to s kin puncture. To avoid accidentally puncturing the poste- rior wall of the artery, the skin and artery should be punctured as two distinct maneuvers. Alternatively, a small skin nick may be made to facilitate skin entry. In the case of elderly patients with mobile, atherom atous vessels, a tech- nique involving deliberate trans \201 xation of the artery may be u sed. The cannula is passed through the anterior and posterior walls of the vessel, thus imm obilizing it. The needle is removed and the cannula withdrawn slowly into the vessel lumen before being advanced. Seldinger-type kits are available for arterial cannulation. A guidewire is \201 rst inser ted through a rigid steel needle. The indwelling plastic cannula is then placed over the gu idewire. The cannula should be connected to a continuous \202 ushing device after successful punct ure. Flushing with a syringe should be avoided since the high pressures generated may lead to a retrograde cerebral embolus. Alternative sites for cannulation Brachial artery End artery supplying a large volume of ti ssue. Thus thrombosis has poten- tially severe consequences. Ulnar artery Should be avoided if the radial artery is occluded. Femoral artery May be dif \201 cult to keep clean. Also supplies a large volume of tissue. A longer catheter should be used to avoid displacement. Dorsalis pedis artery Blood pressure will be at \211 10 \200\22320mmHg higher than in cent ral vessels. Complications \200 \200 \200 \200 Digital ischaemia due to arterial spasm, thrombosis, or embolus. Bleeding (particularly in cases with altered coagulation status). Infection (with prolonged cannulation). False aneur ysm. ARTERIAL CANNULATION 167 See also: Blood gas analysis, p154; Blood pressure monitoring, p164; Routine changes of dis posables, p550. 168 CHAPTER 9 Cardiovascular monitoring Central venous catheter \200\224insertion Ultrasound-guided placement should be considered, especially for dif \201 cult placements. The technique with ultrasound guidance is differ ent to the landmark technique; operators should be able to identify a patent vein and manip ulate both probe and cannula simultaneously (see \201 gure 9.2). There will be situations where an ultrasound device may be unavailable, so place- ment using anatomical landmarks a lone should still be learned. Landmarks Various landmarks have been described. For example: \200 Internal jugular: Halfway between mastoid process and sternal notch, lateral to carotid pul sation, and inside medial border of sternocleidomastoid. Aim toward ipsilateral nipple, adv ancing under body of sternocleidomastoid until vein entered. Subclavian: 3cm below junction of lateral third and medial two thirds of clavicle. Turn head to contralateral side. Aim f or point between jaw and contralateral shoulder tip. Advance needle to hit clavicle. Scrape needle around clavicle and advance further until vein entered. Femoral: Locate femoral art ery in groin. Insert needle 3cm medially and angled rostrally. Advance until vein entered. Insertion technique The Seldinger technique (described below) is safer than the \200\230ca theter- over-needle \200\231 technique and should generally be used in ICU patients. Use as eptic technique throughout. Clean area with antiseptic (2% chlorhexidine) and surround with sterile drapes. Anesthetize locally with 1% lidocaine. Flush lumen(s) of catheter with sal ine. If available, use ultrasound to distinguish the vein from an adjacent artery by its co mpressibility, thinner wall and ovoid shape. The internal jugular and femoral veins are the easiest to locate by ultrasound. Use needle to locate central vein. Con \201 rm by aspira tion of non- pulsatile blood into attached syringe or direct ultrasound visualization of th e needle entering the vein (with longitudinal probe position). Pass wire (with \200\230J \200\231 or \202 oppy end leading) through needle into vein. Only minimal resistance shou ld be felt. If not, remove wire and con \201 rm needle tip is still within vein lumen. If arrhythmias occur, the wire may be at the tricuspid valve and should be withdrawn a few cm. 5. Remove needle leaving wire extruding from skin puncture site. Depending on size/type of catheter, a rigid dilator (9 a prior scalpel 6. nick to enlarge puncture site) may be pass ed over the wire to form a track through the subcutaneous tissues to the vein. Remove dilat or. Thread catheter over wire. Ensure end of wire extrudes from catheter to prevent acciden tal loss of wire in vein. Insert catheter into vein to depth of 15 \200\22320cm. Remove wir e. Aspirate and check respiratory swing, then \202 ush with saline. Attach catheter to sk in using appropriate \201 xation device. Suturing is not generally recommended in view of increased risk of infection. Clean and dry area. A chlorhexidine-impregnated cuff may help avoid contamination. Use a sterile transparent semi-permeable dressing. A chest X-ray veri \201 es correct tip position (junction of superior vena cava and right atrium) and exclud es pneumothorax. A satisfactory position should generally be con \201 rmed before use of t he catheter. 1. 2. 3. 4. 7. 8. 9. 10. CENTRAL VENOUS CATHETER \200\224INSERTION 169 Fig. 9.2 Identi \201 cation of internal jugular vein (v) and carotid artery (a) on trans- verse placement of probe. See also: Central venous catheter \200\224use, p168; Central venous catheter \200\224compli cations, p172. 170 CHAPTER 9 Cardiovascular monitoring Central venous catheter \200\224use Types of catheter \200 \200 \200 \200 Single, double, triple, or quadruple lumen. Sheaths for insertion of pulmonary artery cathe ter or pacing wire. Tunneled catheter for long-term use. Multi-lumen catheters allow multip le infusions to be given separately 9 continuous pressure monitoring. Minimizes accidental bolus risk. Large-bore, double-lumen catheters for veno-venous dialysis/ \201 ltration. Co mmon routes are internal jugular, subclavian, and femoral. \200\230Long \200\231 catheters can be inserted via brachial or axillary veins. Can be used for giving TPN and administeri ng drugs that can cause phlebitis. \200 \200 Uses \200 \200 Invasive haemodynamic monitoring. Infusion of drugs that can cause peripheral phlebitis or tissue necrosis if tissue extravasation occurs (e.g. TPN, epinephrine, amiodarone). Rapid v olume infusion (NB. Rate of \202 ow is inversely proportional to the length of the cannul a). Access, e.g. for pacing wire insertion. Emergency access when peripheral circulation is \200\230shut down \200\231. Renal replacement therapy, plasmapheresis, exchange transfusi on. \200 \200 Contraindications/cautions \200 \200 \200 Coagulopathy. Undrained pneumothorax on contralateral side. Agitated, restless patient. Central venous pressure measurement An electronic pressure transducer is preferable to \202 uid manometry. The pressure transducer should be placed and \200\230zeroed \200\231 a t the level of the left atrium (approximately mid-axillary line) rather than the sternum wh ich is more affected by patient position (supine/semi-erect/prone). Venous pul- sation and some respiratory swing should be seen in the trace but not a RV pressure waveform (i.e. cat heter inserted too far). CENTRAL VENOUS CATHETER \200\224USE 171 See also: Temporary pacing (1), p96; Parenteral nutrition, p130; Central venous catheter \200\224insertion, p168; Central venous catheter \200\224complications, p172; Pulmonary art ery catheter \200\224insertion, p174; Routine changes of disposables, p550. 172 CHAPTER 9 Cardiovascular monitoring Central venous catheter \200\224complications Infection The incidence of local infection (u sually coagulase negative Staphylococcus or S. aureus) rises after \201 ve days. The cath eter site should be kept clean with an occlusive dressing which should be changed within se ven days using 2% chlorhexidine in alcohol to clean the site. Strict asepsis should be used for dressing changes. The need for maintaining the catheter in situ should be questioned daily. Unless the risk o f new catheter insertion is very high (e.g. marked coagulopathy, anatomical issues), changi ng the catheter over a wire is not recommended. Catheter-related sepsis should be considered if the patient develops an unexplained pyrexia or neutrophilia. Antibiotics may not be needed if mild, localized infection is present. Removal of the catheter is necessary if the site is cellulitic or blood cul- tures taken th rough the catheter are positive. If the catheter is removed, the tip should be sent to the laboratory for culture. Other complications \200 Arterial puncture: Apply \201 rm pressure directly over puncture site for 10 \200\22315mi ns (or longer). If continues, consider correcting any coagulopathy. Rarely, vascular surgic al assistance may be needed. Hemorrhage: This may occur from around puncture site or from a previously failed attempt at insertion. Direct pressure for 5 \200\22310mins is usually su ccessful though correction of coagulopathy may be needed if bleeding persists. If post-thro mbolysis, consider tranexamic acid. Arrhythmias: Usually related to catheter tip \200\230t ickling \200\231 tricuspid valve and normally resolves by withdrawing catheter by a few cm. Pneumothorax: If signi \201 cant (e.g. >one third of hemithorax) or compromising cardiore spiratory status, drain air through catheter or chest drain. Otherwise, observe closely and drain subsequently, if necessary. Air embolism: Avoid by ensuring all Luer Lock connection s are not loosely attached. If air embolism does occur, roll patient to left-hand side and place head down (Trendelenberg position) to prevent air entering pulmonary artery. Give FIO 2 1.0 to speed resorption of air. Try aspirating air through central line or consider angio graphy-guided aspiration if very large. Venous thrombosis: Suggested by unilateral swelling of distal limb; this can be con \201 rmed by ultrasound. Remove catheter and anticoagulat e (unless contraindicated). Haemothorax: Drain if necessary and monitor blood loss. Usually self-resolving. If bleeding persists, ensure coagulopathy is corrected. Seek a cardiothora cic surgical opinion if blood loss is substantial (>1L). Chylothorax (rare): Losses can exc eed 1L/d. Often resolves spontaeously within 5 \200\2237d. If it persists, surgical ligatio n of the lymphatic duct may be necessary. The volume of loss may be reduced with TPN or a l ow-fat enteral diet. CENTRAL VENOUS CATHETER \200\224COMPLICATIONS 173 See also: Central venous catheter \200\224insertion, p168; Central venous catheter \200\224 use, p170; Pneumothorax, p368; Haemothorax, p370; Routine changes of disposables, p550. 174 CHAPTER 9 Cardiovascular monitoring Pulmonary artery catheter \200\224insertion An 8Fr central venous introducer sheath must be inserted under strict aseptic technique. Pulmonary artery catheterisation is easier via in ternal jugular or subclavian veins. 1. Prepare catheter pre-insertion: 3-way taps on all lumens, \202 ush lumens with crystalloi d, in \202 ate balloon with 1.6mL air and check for concentric in \202 ation and leaks, p lace transparent sleeve over catheter to maintain future sterility, pressure transduce dist al lumen and zero to reference point (usually mid-axillary line). Other pre-insertion calib ration steps may be required, e.g. oxygen saturation. Insert catheter beyond the length of the introducer sheath before in \202 ating balloon. Advance catheter smoothly through the right heart chambers, pause to record pressures, and note waveform shape in RA, RV and PA ( see \201 gure 9.3). When a characteristic PAWP waveform is obtained, stop advancing cathe ter, de \202 ate balloon, and ensure that PA waveform reappears. If not, withdraw catheter by a few cm. Slowly re-in \202 ate balloon, observing waveform trace. The wedge recording should not be obtained until at least 1.3mL of air has been injected into the balloon. If not, withdraw catheter 1 \200\2232cm and repeat. If \200\230overwedged \200\231 (pressure continues to climb on in \202 ation), the catheter is inserted too far and the balloon has in \202 ated forward over distal lumen. Immediately de \202 ate, withdraw catheter 1 \200\2232cm and repeat. After insertion, a chest X-ray is usually performed to verify cathe ter position and to exclude pneumothorax. 2. 3. 4. Contraindications/cautions \200 \200 Coagulopathy. Tricuspid valve prosthesis or disease. Complications \200 \200 \200 \200 \200 \200 \200 Problems of central venous catheterisation. Arrhythmias (especially when traversing tricusp id valve). Infection (including endocarditis). Pulmonary artery rupture. Pulmonary infarcti on. Knotting of catheter. Valve damage (do not withdraw catheter unless balloon de \202 at ed). Troubleshooting Excessive catheter length in a heart chamber causes coiling and a risk of k notting. No more than 15 \200\22320cm should be passed before the waveform changes. If not, de \202 ate balloon, withdraw catheter, repeat. A knot can be managed by: (i) \200\230un knotting \200\231 with an intraluminal wire, (ii) pulling taut and removing catheter + intr oducer sheath together, or (iii) surgical or angiographic intervention. If catheter fails to advance to next chamber, consider \200\230stiffening \200\231 cath- e ter by injecting iced crystalloid through distal lumen, rolling patient to left lateral, or advancing catheter slowly with balloon de \202 ated. The catheter should never be withdra wn with the balloon in \202 ated. Arrhythmias on insertion usually occur when the catheter tip is at the tricuspid valve. The se usually resolve on withdrawing the catheter or, occasionally, after a slow bolus of 1.5m g/kg lidocaine. PULMONARY ARTERY CATHETER \200\224INSERTION 175 mmHg 30 0 RV RA PA PAWP Fig. 9.3 Waveforms. See also: Pulmonary artery catheter \200\224use, p176. 176 CHAPTER 9 Cardiovascular monitoring Pulmonary artery catheter \200\224use Several prospective multicentre studies in critically ill and decompensated heart failure patients found no impact on mortality with catheter us e. As studies also reported an inadequate knowledge base regarding insertion and data inter pretation, proper training in its use is mandated. Uses \200 \200 \200 Pressure monitoring \200\224RA, RV, PA, PAWP. Flow monitoring \200\224(right ventricular) c ardiac output. Oxygen saturation \200\224 \200\230mixed venous \200\231 (i.e. in RV out \202 ow tract/PA), determination of left to right shunts (ASD, VSD). Derived variables \200 \224SVR, PVR, LVSW, RVSW, DO2, VO2, O2ER. Temporary pacing. Right ventricular ejection frac tion and end-diastolic volume. \200 \200 Specialized catheters \200 \200 \200 \200 Continuous mixed venous oxygen saturation measurement. Continuous cardiac output measuremen t. RV end-diastolic volume, RV ejection fraction calculation. Ventricular (9 atrial) pacing . Management Monitor PA pressure continuously to identify catheter migration and a wedged tra ce. If present, pull back catheter immediately to prevent pul- monary infarction due to art erial occlusion. The risk of local infection rises after \201 ve days. Consider daily the need for the cat heter. Removal 9 change of site is needed if the site is cellulitic or positive cultures ar e grown from either line tip or blood. Withdraw samples of pulmonary artery blood slowly from the distal lumen to prevent \200\230arterialisation \200\231, i.e. pulmonary venous sampling. Wedge pressure measurements \200 In \202 ate balloon slowly, monitoring the waveform to avoid potential vessel rupture, esp ecially in elderly and/or pulmonary hypertension. The trace should only \200\230wedge \200 \231 after \211 1.3mL air \201 lls the balloon. Measure at end-expiration when intrathor acic pressure is closest to atmospheric pressure. For ventilated patients, end-expiration \211 lowest wedge reading; during spontaneous breathing, end-expiration \211 highest rea ding. Measurement is dif \201 cult in the dyspnoeic patient; a \200\230mean \200\231 wedg e reading may be used in this instance. The PAWP cannot be higher than the PA diastolic pre ssure. CVP, PAWP, and cardiac output should not be measured during rapid volume infusion bu t after a period of equilibration (5 \200\22310min). The PAWP does not equal the LVEDP in m itral stenosis. In mitral regurgitation, measure PAWP at the end of the \200\230a \200\231 wave. \200 West \200\231s zones The catheter tip should lie in a zone III region (where PA pressure > PV pressure > alveolar pressure) below left atrial level on a lateral CXR. Suspect a non-zone III position if: (i) following a rise in PEEP, the PAWP rises by >50% of the increment, (ii) the wedge trace shows no detectable cardiac pulsation and/or excess re spiratory variation. A non-zone III position is more likely with PEEP and/or hypovolaemia. PULMONARY ARTERY CATHETER \200\224USE 177 Normal values Stroke volume Cardiac output Right atrial pressure Right ventricular pressure Pulmonary artery pressure Pulmonary artery wedge pressure Mixed venous oxygen saturation 70 \200\223100mL 4 \200\2236L/min 0 \200\2235mmHg 20 \200\22325/0 \200\2235mmHg 20 \200\223 25/10 \200\22315mmHg 6 \200\22312mmHg 70 \200\22375% Derived variables Variable Calculation Normal range Cardiacoutput 2.5 \200\2233.5L/min/m2 Cardiac index (CI) Body surface area SV 40 \200\22360mL/m2 Stroke index (SI) Body surface area 960 \200\2231400dyn.s/cm5 MAP - R AP Systemic vascular resistance \227 79.9 Cardiac output 25 \200\223125dyn.s/cm5 P AP - P A WP Pulmonary vascular resistance \227 79.9 Cardiac output 44 \200\22368g-m.m2 4 \200\2238g-m.m2 LV stroke work index RV stroke work index (MAP-PAWP) x SI x 0.0136 (MPAP-RAP) x SI x 0.0136 Cardiac output x 0.134 x Hba x SaO2 Cardi ac output x 0.134 x [(Hba x SaO2) \200\223 (Hbv x SvO2)] O2 delivery 950 \200\2231300mL/min 180 \200\223320mL/min O2 consumption \210\222 Sa O SvO \210\222 2 Sa O 2 0.25 \200\2230.30 O2 extraction ratio 1 2 Key papers Richard C, Warszawski J, Anguel N, et al. for the French Pulmonary Artery Cathet er Study Group. (2003) Early use of the pulmonary artery catheter and outcomes in patients with shock and acute respiratory distress syndrome: a randomized controlled trial. JAMA 290 : 2713 \200\22320. Harvey S, Harrison DA, Singer M, et al. for the PAC-Man study collaboration. (2005) Assessm ent of the clinical effectiveness of pulmonary artery catheters in management of patients i n intensive care (PAC-Man): a randomized controlled trial. Lancet 366: 472 \200\2237. Binanay C, Califf RM, Hasselblad V, et al. for the ESCAPE Investigators. (2005) Evaluation study of congestive heart failure and pulmonary artery catheterisation effectiveness: the E SCAPE trial. JAMA 294: 1625 \200\22333. See also: Pulmonary artery catheter \200\224insertion, p172; Cardiac output \200\224central thermodilution, p178. 178 CHAPTER 9 Cardiovascular monitoring Cardiac output \200\224central thermodilution Thermodilution is utilized by the pulmonary a rtery catheter to measure right ventricular cardiac output. The principle is a modi \201 c ation of the Fick indicator dilution principle whereby a bolus of cooled 5% glucose is inje cted through the proximal lumen into the central circulation (right atrium), and the temper ature change is detected by a thermistor at the catheter tip, some 30cm distal. A modi \201 cation of the Hamilton \200\223Stewart equation, utilizing the volume, temperature, an d speci \201 c heat of the injec- tate, enables cardiac output to be calculated by an onli ne computer from a curve measuring temperature change in the pulmonary artery. Continuous thermodilution measurement uses a modi \201 ed catheter emitting heat pulses fr om a thermal \201 lament lying 14 \200\22325cm from the tip. 7.5W of heat are added to th e blood intermittently every 30 \200\22360s and these temperature changes are measured by a thermistor 4cm from the tip. Though updated frequently, the cardiac output displayed is us ually an average of the previous 3 \200\2236min. Thermodilution is also used by the PiCCO device, injecting cooled 5% glucose through a cent ral vein, and measuring temperature change in the femoral artery. Thermodilution injection technique The computer constant must be set for the volume and tem perature of the 5% glucose used. Ice-cold glucose (10mL) provides the most accu- rate measu re. Room temperature injectate (5mL) is suf \201 ciently precise for normal and high outpu t states. However, its accuracy does worsen at low output values. 1. 2. 3. 4. Press \200\230Start \200\231 button on computer. Inject \202 uid smoothly over 2 \200 \2233s. Repeat at least twice more at random points in the respiratory cycle. Average three measurements falling within 10% of each other. Reject outputs gained from curves that are irregular/non-smooth. Erroneous readings \200 Valve lesions: Tricuspid regurgitation will allow some of the injectate to re \202 ux back into the right atrium. Aortic incompetence produces a higher left ventricular output; a pr oportion will regurgitate back into the left ventricle, thus the thermodilution-obtained va lue overestimates the effective forward \202 ow from the left ventricle. Septal defects. Loss of injectate: Check that connections are tight and do not leak. \200 Disadvantages \200 \200 \200 \200 Non-continuous (by injection technique). 5 \200\22310% inter- and intra-observer variabilit y. Erroneous readings with tricuspid regurgitation, intracardiac shunts. Frequently repeate d measurements may result in considerable volumes of 5% glucose being injected. CARDIAC OUTPUT \200\224CENTRAL THERMODILUTION 179 See also: Pulmonary artery catheter \200\224insertion, p174; Pulmonary artery catheter \200 \224use, p176; Basic resuscitation, p338; Fluid challenge, p342; Hypotension, p380; Heart f ailure \200\224assessment, p392. 180 CHAPTER 9 Cardiovascular monitoring Cardiac output \200\224peripheral thermodilution The PiCCO device utilizes thermodilution m easured peripherally. The most frequent artery used is the femoral into which a catheter (u sually 5Fr in adults) is inserted. Specialized catheters are also available for axil- lary and radial artery insertion. A bolus of cold saline injected into a central vein is detecte d by a thermistor in the arterial catheter. Advantages \200 Reasonably accurate and less invasive than pulmonary artery placement. Other information de rived. Disadvantages \200 \200 Can underestimate low output values. Inaccurate with moderate/severe valvular regurgitation , intracardiac shunts, aortic aneurysm, aortic stenosis, pneumonectomy, pulmonary embolism, rapidly changing temperature, extracorporeal circulations. Large size of cannula can compr omise distal leg perfusion. Derived indices As the injectate is administered centrally and the temperature difference i s measured in a proximal artery, most of the temperature change occurs in the intrathoracic compartment. Hence, other volumetric variables such as intrathoracic blood volume and extr avascular lung water can be estimated. Global end-diastolic volume (GEDV) = volume of blood within the four heart chambers. Intrat horacic blood volume (ITBV) = GEDV + blood volume within the pulmonary vessels. ITBV and GEDV are measures of cardiac preload that studies suggest are comparable, if not s uperior, to standard cardiac \201 lling pressures (CVP, PAWP), particularly as they are l ess in \202 uenced by mechanical ventilation. EVLW provides a bedside measure of the degree of pulmonary oedema. It correlates with the s everity of ARDS and outcomes. There are little data at present showing its utility in guidi ng treatment and improving outcomes. Extravascular lung water (EVLW) = water content in the lungs. CARDIAC OUTPUT \200\224PERIPHERAL THERMODILUTION 181 See also: Extravascular lung water measurement, p156; Cardiac output - central thermodiluti on, p178; Basic resuscitation, p338; Fluid challenge, p342; Hypotension, p380; Heart failur e - assessment, p392. 182 CHAPTER 9 Cardiovascular monitoring Cardiac output \200\224indicator dilution Dye dilution Mixing of a given volume of indicato r to an unknown volume of \202 uid allows calculation of this volume from the degree of i ndicator dilution. The time elapsed for the indicator to pass some distance in the cardiova scular system yields a cardiac output value, calculated as: \227 60 l \227 t C m \200 where I = amount of indicator injected, Cm = mean concentration of the indicator, an d t = curve duration. Traditionally, indocyanine green was injected into a central vein wit h arterial blood repeatedly sampled to construct a time-concentration curve with a rapid up stroke and an exponential decay. Plotting the dye decay curve semi-logarithmically and extr apolating values to the origin produces cardiac output. The LiDCO device is based on a similar principle, though using lithium as the \200\230dye \200\231. This can be injected into a peripheral vein and sampled from the radial artery. Advantages \200 Reasonably accurate and less invasive than pulmonary artery placement. Other information de rived. Disadvantages \200 \200 Accumulation of lithium with multiple repeated measurements. Measurement/calibration takes >10min. Ideally, 2 \200\2233 lithium time- concentration curves should be performed per cal ibration to improve the coef \201 cient of variation. Can underestimate low output values. Inaccurate with moderate/severe valvular regurgitation, major pulmonary embolism, intracar diac shunt. Paralyzing agents and severe hyponatraemia interfere with lithium measurement. Lithium should not be used in pregnant patients. \200 CARDIAC OUTPUT \200\224INDICATOR DILUTION 183 See also: Basic resuscitation, p338; Fluid challenge, p342; Hypotension, p380; Heart failur e \200\224 assessment, p398. 184 CHAPTER 9 Cardiovascular monitoring Cardiac output \200\224Doppler ultrasound Doppler ultrasound An ultrasound beam of known fr equency is re \202 ected by moving blood corpuscles with a shift in frequency proportional to blood \202 ow velocity. The actual velocity is calculated from the Doppler equation; this requires the cosine of the vector between the direction of the ultrasound beam and tha t of blood \202 ow. This has been applied to blood \202 ow in the ascending aorta and a ortic arch (via a suprasternal approach), descending thoracic aorta (oesophageal approach), and intracardiac \202 ow (e.g. transmitral from an apical approach). Spectral analysis o f the Doppler frequency shifts produces velocity-time waveforms, the area of which represen ts \200\230stroke distance \200\231, i.e. the distance traveled by a column of blood with each left ventricular systole. (See \201 gure 9.4). The product of stroke distance and ao rtic (or mitral valve) cross-sectional area is stroke volume. Cross- sectional area can be measured echocardiographically. However, as both operator expertise and equipment is requir ed, this additional measure- ment can be either ignored or assumed from nomograms to provid e a reasonable estimate of stroke volume. Advantages \200 \200 \200 Quick and minimally invasive. Reasonably accurate if performed properly. Measures left hear t output, unaffected by mitral (unless measured transmitrally) or tricuspid regurgitation, or by intracardiac shunts. Continuous (via oesophageal approach) \200\224but may need repos itioning. Other information on contractility, preload, and afterload from waveform shape (s ee \201 gure 9.5). Multiple studies showing outcome bene \201 t in high-risk surgical pa tients from guided \202 uid optimization intra-operatively or post-operatively. \200 Disadvantages \200 \200 Non-continuous (unless via esophagus). Learning curve to ensure correct signal is insonated (85 \200\22310 oesophageal, 815 \200\22320 with suprasternal and 30+ with intracardiac). N ot reliable with turbulent \202 ow conditions, e.g. intra-aortic balloon counterpulsation (oesophageal), aortic stenosis or regurgitation (suprasternal or intracardiac from aortic out \202 ow tract), mitral regurgitation (transmitral). Oesophageal approach assumes propo rtionality of \202 ow going to upper and lower body is maintained (approximately 30:70) \200\224this will change during aortic cross-clamping (vascular surgery) or use of epidural . Suprasternal measurements can be hampered by short neck or obesity (probe has to get behi nd sternum to insonate ascending aorta or arch correctly), mediastinal air (e.g. post-cardi ac surgery), and co-located vessels, e.g. innominate artery. CARDIAC OUTPUT \200\224DOPPLER ULTRASOUND 185 VELOCITY mean acceleration peak velocity stroke distance TIME flow time cycle time Fig. 9.4 Doppler velocity-time waveform. Corrected \202 ow time (FTc) = \202 ow time corrected to heart rate of 60bpm. Normal ra nge is 330 \200\223360ms. The FTc is inversely related to systemic vascular resistance. Thu s, FTc falls with decreased preload or increased afterload (e.g. hypovolaemia, vasoconstric tion, obstruction such as PE or tamponade) while FTc increases with vasodilatation. Peak ve locity (and acceleration) are markers of left ventricular contractility and are age-depende nt (see table below). Values increase with positive inotropy and fall with negative inotrop ic states (e.g. heart failure, beta-blockade). Age 20 50 70 Normal range of peak velocity (cm/s) 90 \200\223120 60 \200\22380 50 \200\22370 Preload increase Preload decrease Predominant change Afterload reduction Afterload increase Positive inotropy Negative inotropy Fig. 9.5 Effect of haemodynamic changes on Doppler velocity-time waveform shape. See also: Basic resuscitation, p338; Fluid challenge, p342; Hypotension, p380; Heart failur e \200\224assessment, p398. 186 CHAPTER 9 Cardiovascular monitoring Cardiac output \200\224pulse contour analysis Pulse contour analysis The concept of this te chnique is that the contour of the arterial pres- sure waveform is proportional to stroke v olume. This, in theory, allows continuous, minimally invasive monitoring of cardiac output. However, it is greatly in \202 uenced by arterial compliance (the relationship between pr essure and volume) and aortic impedance. Thus, the major challenges for technologies using this approach are to assess arterial compliance con- sistently and accurately in an individ ual patient and to be able to track changes in cardiac output closely when there are concur rent changes in compliance, e.g. during rapid blood loss, vasodilatation, or changes in tem perature. This is complicated further by the changing morphology of the blood pressure waveform as it travels down peripheral arteries, especially during alterations in blood \202 ow redistr ibution, e.g. with severe vasoconstriction. It is also essential to ensure a correctly damp ed pressure waveform and non-kinking of the arm (radial) or groin (femoral). There are several commercial devices which use their own proprietary algorithms to estimate stroke volume. The LiDCOplus and PiCCO systems use an alternative cardiac output measuring technique (lithium dye dilution and peripheral thermodilution, respectively) in tandem for initial calibra- tion and subsequent re-calibration. The Flotrac and LiDCOrapid systems de rive an estimate of cardiac output without a calibrating device using the patient \200\231s blood pressure, gender, age, and body surface area. There are sig- ni \201 cant concerns regarding the overall accuracy of these systems and their ability to follow trends, especia lly those devices not calibrated against a reference technique. Even in calibrated systems, the literature suggest the need for frequent routine recalibration (1 \200\2234 hourly) or earlier if circulatory status changes to any signi \201 cant extent. Advantages \200 \200 Continuous \202 ow monitoring. Uses data from an arterial cannula already in situ for pre ssure monitoring. Assessment of \202 uid responsiveness using pulse pressure, systolic pr essure, or stroke volume variation. Disadvantages \200 