233CHAPTER 26 Principles of Invasive Cardiovascular Monitoring humans In doing so, they brought this methodology to the bed side, where it is still used today In the 50 years since Swan and Ganz added[.]
CHAPTER 26 Principles of Invasive Cardiovascular Monitoring humans In doing so, they brought this methodology to the bedside, where it is still used today In the 50 years since Swan and Ganz added balloon flotation and thermodilution to the PAC, its use has been controversial The literature is rife with studies reporting salutary effects of placing them, studies showing no effect, and studies showing harm.48–53 Due to this controversy and the significant risks attendant to its use, the PAC has fallen out of favor; its routine use today is rare outside of specific clinical scenarios discussed later With regard to pediatric patients, the Pulmonary Artery Catheter Consensus Conference, based on a consensus of expert opinions, concluded that the PAC was useful for clarifying cardiopulmonary physiology in critically ill infants and children with pulmonary hypertension; shock refractory to fluid resuscitation and/or low-to-moderate doses of vasoactive medications; severe respiratory failure requiring high mean airway pressures; and, on rare occasions, multiple organ failure They found no data indicating that PAC use increases mortality in children; however, they also failed to find any controlled trials that demonstrated a benefit of PAC use The panel recommended PAC use for selected patients and called for randomized controlled trials, a registry of PAC use, and studies to assess the impact of PAC use on cost and duration of ICU/hospital stay.54 A further review of current studies55 demonstrated level B and level C evidence for most indications Indications Although controversial, current indications for PAC use in children include septic shock unresponsive to fluid resuscitation and vasopressor support,56–58 refractory shock following severe burn injuries,59 congenital heart disease (CHD),58 pulmonary hypertension,60,61 multiple organ failure,62 liver transplantation,63 and respiratory failure requiring high mean airway pressures.58,64 Capabilities of PACs include determination of CVP, pulmonary artery pressure, and pulmonary artery occlusion pressure (PAOP), also referred to as pulmonary capillary wedge pressure PAOP is a measurement of left atrial pressure and left ventricular end-diastolic pressure (when the mitral valve is open) PACs are also used to assess CO, Svo2, oxygen delivery (Do2) and consumption (Vo2), and pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) The PAC is the only bedside tool that can examine the function of the right and left ventricles separately aside from echocardiography, which provides significantly less precise assessments PACs are used to establish diagnoses, guide response to therapy, and assess the determinants of oxygen delivery PACs are especially helpful in cases of discordant ventricular function One of the most common uses of the PAC in infants and children is monitoring pulmonary pressures during and after repair of CHD In addition to flow-directed, balloon-tipped PACs, transthoracic left atrial catheters are often used in these patients.65 Use of PACs has altered the management of children with CHD by identifying residual anatomic defects and diagnosing pulmonary hypertensive crisis.66,67 The ability to monitor PAP provides the means to titrate response to inhaled nitric oxide and other pulmonary vasodilators.68,69 The lack of response to inhaled nitric oxide may suggest a residual structural anomaly in postoperative patients and indicate the need for interventional cardiac catheterization and/or surgical repair.69 In addition to monitoring for pulmonary hypertensive crisis, PACs can be used to assess the effects of changes in concentration of inspired CO2 on mean pulmonary artery pressure (MPAP), pulmonary vascular resistance index (PVRI), and CI.70 233 Monitoring Techniques with the Pulmonary Artery Catheter The functional features of the PAC are several Its use as a method of indicator thermodilution, measurement of cardiac output, monitoring, and blood sampling is well documented Fig 26.8 demonstrates the expected waveforms as the catheter passes through the cardiovascular system Technical concepts are important for understanding the calculations needed for optimum use See Table 26.1 for a summary of the hemodynamic parameters that can be derived from a PAC Catheter Placement PACs typically contain the following ports (see Fig 26.8) The proximal port is located 15 cm from the tip in Fr catheters and 30 cm from the tip in larger catheters It opens into or near the right atrium The proximal port provides access for infusion of fluid or drugs, injection of cold saline solution as indicator (thermodilution method), CVP monitoring, and blood sampling In infants or small children, PAC placement may result in improper location of the proximal port before the right atrium such that the port lies inside the sheath or outside the body Therefore, it is essential to verify not only the placement of the distal tip in the pulmonary artery but also the location of the proximal port The distal port opens at the tip of the catheter It is used for monitoring PAP and PAOP, blood sampling of mixed venous blood gases, and infusion of fluids By monitoring pressure continuously through this port during catheter placement, the location of the tip can be determined from the characteristic pressure tracings shown in Fig 26.8 After placement, PAP should be monitored continuously in order to identify inadvertent migration into the pulmonary capillary bed or “wedged” position It is important to allow the catheter tip to “float” into the wedged position only when actively measuring PAOP in order to minimize risk of pulmonary artery infarct or rupture The balloon inflation port inflates the balloon, which is located cm proximal to the catheter tip The balloon is inflated for flowdirected catheter placement and PAOP monitoring The thermistor is located just proximal to the balloon and connects to a bedside computer to measure changes in the temperature of pulmonary artery blood The oximeter uses a fiberoptic-based sensor to continuously measure the Svo2 Larger catheters also may have cardiac pacing ports An adultsized catheter is available for continuous CO determination when coupled with an appropriate bedside computer Indirectly Measured Variables Measurements from PACs include directly and indirectly measured or derived variables Directly measured variables include CVP, MAP, MPAP, PAOP, CO, arterial oxygen saturation (Sao2), and Svo2 Derived parameters include CI, PVR, SVR, PVRI, and systemic vascular resistance index (SVRI), as well as SV (in mL/ beat) and stroke volume index (SVI; in mL/beat per m2) Stroke index (SI), or SVI, normally is 30 to 60 mL/m2.71,72 SV CO/HR Eq 26.2 SVI SV/BSA Eq 26.3 234 S E C T I O N I V Pediatric Critical Care: Cardiovascular SvO2 % Proximal port 100 80 60 Distal port 40 20 Balloon inflation port Thermistor connector 15 30 45 60 Time Oximeter connector ge ed W e en tric l tv gh Ri rtery 30 mm Hg ion sit po ig R ry a 40 ona Pulm um tri a ht 20 10 • Fig 26.8 Components and functional features of a thermodilution flow-directed pulmonary artery cath- eter The flexible multilumen catheter with the balloon at the distal tip inflated is in the wedge position The proximal ends of the five lumens are labeled The distal port is connected to a pressure measurement system for catheter insertion and subsequent monitoring When the distal tip is within the central venous circulation, the balloon is inflated to enhance flow direction of the tip through the right atrium into the right ventricle and then to the pulmonary artery Recorded pressures (bottom) correspond to these locations, confirming the course of the catheter The last tracing on the right corresponds to the “wedge” position, commonly reflecting pressure transmitted from the left atrium via the pulmonary veins and capillaries Upper right panel shows an example of a continuous Svo2 (venous oxygen saturation) tracing from the fiberoptic monitor available on adult-size catheters (Modified from Daily EK, Tilkian AG Hemodynamic monitoring In: Tilkian AG, Daily EK, eds Cardiovascular Procedures, Diagnostic Techniques and Therapeutic Procedures St Louis: Mosby; 1986.) Left ventricular stroke work index (LVSWI) and right ventricular stroke work index (RVSWI) normally are 56 6 and 0.5 0.06 gm-m/m2, respectively.71,72 Note that all values are for pediatric patients unless otherwise indicated LVSWI SI MAP 0.0136 Eq 26.4 RVSWI SI MAP 0.0136 Eq 26.5 Measurement of Cardiac Output CO is the volume of blood pumped by the heart each minute, or SV multiplied by the number of ejections per minute or HR (CO HR SV) and often is expressed as CI, which is CO divided by the body surface area (BSA) in square meters The normal range for infants and children is approximately 3.3 to L/min/m2.71,72 Two methods for calculating CO are discussed here: the Fick method and thermodilution Fick Method In 1870, Adolph Fick was the first to study the relationship between blood flow and gas exchange in the lungs using a mathematic model.73 Fick hypothesized that the amount of oxygen extracted by the body from the blood must equal the amount of oxygen taken up by the lungs during breathing Fick also reasoned that the flow of blood through the lungs must equal the CO to the remainder of the body in the absence of a shunt If the amount of oxygen consumed by the body and the amount of oxygen extracted by the body from the blood can be determined, then the CO can be determined In Fick’s time, oxygen consumption was measured using a basal metabolism spirometer, and the oxygen content in arterial and venous blood was measured using a rudimentary method.73 Although Fick’s method remains the gold standard, it is rarely used in the ICU because it is less practical than the more commonly used thermodilution method described in the next section However, Fick’s method is commonly used in the cardiac catheterization laboratory because the required data are easily measured in this setting, although oxygen consumption is often estimated CHAPTER 26 Principles of Invasive Cardiovascular Monitoring 235 TABLE Hemodynamic Parameters 26.1 Parameter Formula Normal Range Units CI CO/BSA 3.5–5.5 L/min/m2 SI CI/heart rate 1000 30–60 mL/m2 Arterial-mixed venous O2 content difference avDo2 Cao2 – Cvo2 30–55 mL/L O2 delivery Do2 CI O2 O2 consumption Cardiac index Stroke index 620 50 mL/min/m2 Vo2 CI avDo2 120–200 mL/min/m2 O2 extraction ratio ERO2 avDo2/Cao2 0.26 0.02 Arterial oxygen content (1.34 Hb Sao2) (Pao2 0.003) mL/L Venous oxygen content (1.34 Hb Svo2) (Pvo2 0.003) mL/L Fick principle VO2 CO (Cao2 – Cvo2) Systemic vascular resistance index SVRI 80 (MAP – CVP)/CI Pulmonary vascular resistance index PVRI 80 (MPAP – PAOP)/CI LV stroke work index LVSWI SI MAP 0.0136 56 6 gm-m/m2 RV stroke work index RVSWI SI MPAP 0.0136 0.5 0.06 gm-m/m2 800–1600 dyne • s/cm5/m2 80–200 dyne • s/cm5/m2 avDo2, Arterial-mixed venous content difference; BSA, body surface area in m2; Cao2, O2 content of systemic arterial blood in mL/L; CI, cardiac index; CO, cardiac output; Cvo2, O2 content of mixed venous blood in mL/L; CVP, central venous pressure in mm Hg; DO2, oxygen delivery; ERO2, O2 extraction ratio; Hb, hemoglobin; LVSWI, left ventricular stroke index; MAP, mean systemic arterial pressure in mm Hg; 80 is the conversion factor used for the units in the table; MPAP, mean pulmonary arterial pressure in mm Hg; PAWP, pulmonary artery wedge pressure in mm Hg, which is approximately equal to the left atrial pressure under many circumstances; Pvo,2, partial oxygen pressure in mixed venous blood; PVRI, pulmonary vascular resistance index; RVSWI, right ventricular stroke work index; SI, stroke index; Svo2, venous oxygen saturation; SVRI, systemic vascular resistance index; Vo2, oxygen consumption Modified from Katz RW, Pollack MM, Weibley RE Pulmonary artery catheterization in pediatric intensive care In: L.A Barness, ed Advances in Pediatrics Chicago: Year–Book; 1984 As noted previously, Fick’s equation is based on the assumption that the amount of oxygen extracted by the body from the blood equals the amount of oxygen taken up from the lungs during breathing The oxygen content of blood is generally expressed in milliliters of O2 per deciliter of blood The difference in oxygen content of arterial blood (Cao2) and venous blood (Cvo2) is termed the arterial-mixed venous oxygen content difference (avDo2) By multiplying the avDo2 by the amount of blood pumped through the lungs or body (CO) we can calculate the oxygen consumption Note that CO is generally expressed in L/min Therefore, we must multiply the avDo2 by 10 to convert it to milliliters of O2 per liter of blood if we are to perform the calculation using CO in L/min The oxygen content of the blood is a function of the hemoglobin (Hb) concentration of blood in g/dL, the Sao2 or Svo2 expressed in decimal form, and the arterial or venous partial pressure of arterial oxygen (Pao2 or Pvo2) expressed in mm Hg The oxygen-carrying capacity of adult Hb is 1.34 mL O2/g Hb, and the Bunsen solubility coefficient of O2 in plasma at 37°C equals 0.003 mL/mm Hg per dL A true Svo2 is measured in the pulmonary artery; however, in the presence of an intracardiac leftto-right Svo2 shunt, Svo2 should be measured in the SVC (1.34 Hb Sao2) (Pao2 0.003) Cao Eq 26.6 (1.34 Hb Svo2) (Pvo2 0.003) Cvo Eq 26.7 avDo2 Cao2 – Cvo2 Eq 26.8 The avDo2 is the difference between Cao2 and Cvo2 and normally ranges from 2.8 to 7.8 mL/dL in children.72 As noted earlier, the amount of oxygen extracted (consumed) by the body from the blood equals avDo2 multiplied by the amount of blood that flows through the lungs (QP) Assuming QP equals the flow of blood through the systemic circulation (QS), then QP is a measure of CO (Note that pulmonary and systemic blood flows cannot be assumed to be identical in children with CHD with single-ventricle physiology or those with anatomic shunts.) extraction 10 (Cao2 – Cvo2) CO O Eq 26.9 The amount of oxygen taken up by the lungs equals the amount of oxygen consumed by the body According to Fick, the amount of oxygen extracted by the body from the blood (Eq 26.10) equals oxygen consumption (Vo2) (Cao2 – Cvo2) CO VO2 in L/min 10 Eq 26.10 CO VO2/[10 (Cao2 – Cvo2)] Eq 26.11 As noted in Eqs 26.6 and 26.7, the amount of dissolved oxygen in blood (Pao2 or Pvo2) contributes an almost negligible amount to the oxygen content and can be left out (unless very high) for ease of computation By rearranging Eq 26.11, a rough estimate of CO can be calculated rather easily at the bedside without use of a PAC: CO VO2/(1.34 Hb (Sao2 – Svo2) 10) Eq 26.12 Oxygen consumption can be measured using the metabolic cart or taken from standardized tables.17 Hb concentration can be 236 S E C T I O N I V Pediatric Critical Care: Cardiovascular measured directly Sao2 can be taken from the pulse oximeter Svo2 can be measured by the oximeter at the distal end of the PAC or determined from a venous blood gas sample from a catheter in the internal jugular or subclavian vein These data also can be used to calculate the intrapulmonary shunt fraction, which is the fraction of blood that passes through unventilated areas of lung: Qs/Qt (Cpvo2 Cao2)/(Cpvo2 Cvo2) Eq 26.13 where Cao2 is systemic arterial oxygen content and Cvo2 is mixed venous oxygen content Cpvo2 is the theoretical oxygen content in a normal pulmonary vein and can be estimated using the alveolar gas equation: Cpvo 1.34 Hb Spvo2 Ppvo2 0.003 Eq 26.14 where Spvo2 is pulmonary vein O2 saturation and Ppvo2 is pulmonary vein po2 For the normal lung, Ppvo2 can be estimated from the alveolar air equation (Eq 26.15), and Spvo2 is presumed to be 1.0: Ppvo Pao2 (Pio2 Pwp) Paco2/R Eq 26.15 where Pao2 is alveolar partial pressure of oxygen, Pio2 is inspiratory pO2, Pwp is vapor pressure of water (47 mm Hg at 37°C), Paco2 is arterial CO2, and R is respiratory quotient, which is normally assumed to be 0.8 The normal shunt fraction is 3% to 7% Thermodilution Method In 1921, Stewart74 first described an indicator-dilution method for measuring CO Flow was calculated by measuring the change in concentration of an indicator over time The “ideal” indicator is “stable, nontoxic, uniformly distributed, and does not leave the system between sites of injection and detection However, it should be rapidly cleared in a single circulation time to prevent recirculation interfering with measurement.”75 In 1953, Fegler76,77 demonstrated that a change in the heat content of blood could be used as an indicator for CO measurement A bolus of cold liquid of a known temperature is injected into or proximal to the right atrium A thermistor near the PAC tip in the pulmonary artery or a pulmonary artery branch measures a change in the temperature of the blood as the bolus passes by the end of the catheter A computer calculates the flow by integrating the change in temperature at the thermistor The first law of thermodynamics, the conservation of heat, is the fundamental principle underlying thermodilution Thermodilution makes several assumptions: physiologic conditions must remain constant during the period of observation; all heat exchange occurs between the indicator and the blood without heat loss to the surrounding tissues; mixing of the injectate and blood is complete upstream of the temperature measurement; and the temperature sensor is sufficiently sensitive, accurate, and rapidly responsive to depict accurately the change in temperature over time Measurement of CO using the thermodilution method can be understood by examining a modified version of the StewartHamilton equation.75 V1 is injectate volume (in mL); Tb is temperature of the pulmonary artery at baseline (in degrees Celsius); Ti is temperature of the injectate (in degrees Celsius); K1 is the density factor that equals the specific heat of the injectate multiplied by the specific gravity of the injectate, divided by the product of the specific heat and specific gravity of blood; and K2 is a constant that figures in the dead space of the catheter and the loss of heat from the injectate as it moves through the catheter The denominator of the equation is the integral of the change in the temperature of the blood (Tb) over time (t): CO V1(Tb Ti)K1K2/∫DTb(t)dt Eq 26.16 The computer generates a CO curve with the area under the curve inversely related to the magnitude of the CO In settings of low CO, less warm blood flows with the injectate, and the injectate stays cooler The difference between the injectate temperature and that of the blood remains large, and the CO curve has a high domed shape, with a slow return to baseline temperature In situations of high CO, more pulmonary artery blood flows with the injectate, and the temperature of the injectate approaches or equals that of the blood more rapidly In these situations, because the difference between the final temperature of the injectate and that of the blood is small, the CO curve rapidly returns to baseline following a sharp spike from the cold injectate In extreme lowflow states, the change in temperature of the injectate resulting from handling alone, before the injectate even enters the catheter from the proximal port, may be greater than the change caused by warming of the injectate by the flow of blood A correction factor is added to the equation to account for warming of the injectate because of handling alone However, the correction factor may be inaccurate if the injection is too slow or the syringe is held in the injector’s hands too long Therefore, CO readings should be made as quickly as possible and should be repeated until three successive readings are within 15% of each other Other sources of error include a falsely elevated CO because of inadvertent warming of the thermistor when it is up against the wall of the pulmonary artery The thermodilution method generally should not be used in patients with an intracardiac shunt However, if the shunt fraction is less than 10%, the error likely is negligible.24 Measurements in the cardiac catheterization lab are frequently considered the gold standard for CO determination, but the PAC has clinical advantages for intermittent measurements at the bedside It is important to keep in mind the shortcomings of the PAC, including accuracy of measurements and risks to the patient Stetz et al.78 evaluated PAC thermodilutional determination of CO and found that a difference of 15% or less across three measurements suggested acceptable precision More recent follow-up studies have found that this level of precision is infrequently achieved; for example, Dhingra et al.79 evaluated thermodilutional PAC measurements against direct Fick calculations and found a percentage of error of 62% An animal model study80 found that PACs were only able to consistently detect CO changes of at least 30% Calculation of Oxygen Delivery and Consumption Metabolic derangements, such as fever, sepsis, and shock, interfere with DO2 to and VO2 by the tissues Svo2 is a measure of the oxygenation of blood returning to the heart Svo2 can be measured continuously by a fiberoptic oximeter (see description of PAC ports in the catheter placement section) and normally ranges CHAPTER 26 Principles of Invasive Cardiovascular Monitoring from 65% to 75% The oxygen extraction ratio (ERO2) is avDo2 (see Eq 26.8) divided by Cao (Eq 26.11) and usually is approximately 25%71,72: ERO2 avDo2/Cao2 Eq 26.17 Do2 also can be expressed as the product of CI and Cao2 and Vo2 as the product of CI and avDo2 The normal value for Do2 is 620 50 mL/min per square meter Vo2 typically ranges from 120 to 200 mL/min per square meter.71,72 Do2 CI Cao2 Eq 26.18 Vo2 CI avDo2 Eq 26.19 Interpretation of Waveforms The waveforms corresponding to the right atrium and systemic arterial blood pressure were discussed in previous sections The pressure in the right atrium ranges from approximately to 12 mm Hg As the PAC passes into the right ventricle, the diastolic pressure drops to to 10 mm Hg and the systolic pressure increases to 13 to 42 mm Hg As the catheter enters the pulmonary artery, the diastolic pressure increases to to 21 mm Hg while the systolic pressure remains relatively similar to that of the right ventricle, 11 to 36 mm Hg Once the catheter tip advances into the pulmonary capillary bed and the pulmonary artery is occluded by the inflated balloon, the measured pressure decreases to to 14 mm Hg.24 By recognizing the changes in the various tracings, the movement of the catheter tip can be followed through the chambers of the right heart and into the pulmonary circulation without simultaneous imaging The waveforms are affected by the components of the respiratory cycle As expected, the effects of respiration differ during unsupported breathing (negative pressure) versus mechanical ventilation (positive pressure) During normal unsupported ventilation, PAP decreases during inhalation and increases during exhalation In contrast, during mechanical ventilation, PAP increases during inhalation and decreases during exhalation The cyclical changes induced by the respiratory cycle cause the tracings to take on a sinusoidal pattern once the tip of the catheter enters the thorax The effects of respiration on PAC determinations can be minimized by measuring pressures at the end of expiration, when pleural pressures are closest to zero Because CVP is a measure of preload or filling of the right ventricle, it reflects changes in volume status, right ventricular function, and pulmonary vascular tone Similarly, PAOP measures filling pressures of the left atrium and ventricle When the pulmonary artery is occluded, the pressure from the left atrium is transmitted back to the catheter tip During diastole, when the mitral valve is open and the aortic valve is closed, a continuous fluid-filled column is formed from the catheter tip to the left ventricle and PAOP is equivalent to the left ventricular enddiastolic pressure In patients with cardiogenic shock, an elevated PAOP may reflect decreased function of the left ventricle In this situation, rather than providing further fluid resuscitation or preload, increasing contractility or decreasing afterload may be preferable Afterload is the load that the heart must eject blood against 237 According to Laplace’s law, ventricular wall stress (T) is proportional to ventricular transluminal pressure (P intraluminal pressure – extraluminal pressure) and radius (r) and is inversely related to twice the wall thickness (t): T P r/2t Eq 26.20 For a given pressure, wall stress is increased by an increase in radius (ventricular dilation); therefore, volume administration may increase ventricular diameter and, consequently, wall stress Similarly, during spontaneous breathing, the transluminal pressure and, consequently, the wall stress increase, whereas during mechanical ventilation (positive pressure), the transluminal pressure and wall stress both decrease Ventricular hypertrophy increases wall thickness and therefore decreases wall stress Resistance To understand resistance, returning to Ohm’s law is helpful: voltage (V) varies directly with resistance (R) and current (I): V IR Eq 26.21 Rearranging Eq 26.21 by substituting pressure for voltage and flow for current gives Eq 26.22: R (Pin Pout)/Q Eq 26.22 where R is resistance, Pin is pressure going into a vessel, Pout is pressure exiting the vessel, and Q is flow According to Poiseuille’s law, the resistance of flow through a tube varies directly with the viscosity of the fluid and the length of the tube and is inversely proportional to the radius to the fourth power multiplied by pi (π): R 8hl/pr4 Eq 26.23 where h is viscosity, l is length, and r is radius Unfortunately, Poiseuille’s law assumes uniform viscosity, length, and radius, none of which holds true in the case of pulmonary or systemic circulation; however, the principles behind the law are valuable in understanding the major determinants of resistance By substituting the appropriate values into Eq 26.20, the formulas for SVR and PVR can be derived CO is substituted for Qs and Qp in the absence of a right-to-left or left-to-right shunt or singleventricle physiology In the case of the equation for PVR (Eq 26.25), PAOP is substituted for pulmonary vein pressure in determining Pout: SVR (MAP CVP)/CO Eq 26.24 PVR (MPAP PAOP)/CO Eq 26.25 SVR and PVR are measured in mm Hg minute L21 (or mm Hg/L per min) These units also are referred to as hybrid resistance units or Wood units after the cardiologist Paul Wood.21 By multiplying by 80, hybrid resistance units or Wood units can be converted to the centimeter-gram-seconds (cgs) system, where resistance is measured as dyne • s/cm5, also known as absolute resistance units PVR and SVR often are indexed for BSA (in m2) The SVRI and PVRI are measured as dyne • s/m2 per cm5: SVRI 80 (MAP CVP)/Cl Eq 26.26 PVRI 80 (MPAP PAOP)/Cl Eq 26.27 ... pressure measurement system for catheter insertion and subsequent monitoring When the distal tip is within the central venous circulation, the balloon is inflated to enhance flow direction of the tip... extracted by the body from the blood must equal the amount of oxygen taken up by the lungs during breathing Fick also reasoned that the flow of blood through the lungs must equal the CO to the remainder... used in the cardiac catheterization laboratory because the required data are easily measured in this setting, although oxygen consumption is often estimated CHAPTER 26 Principles of Invasive