15 critical care handbook of the massachusetts general hospital, 6e

Chitilian, MD Assistant Professor of Anesthesiology Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital Harvard Medical School Boston, Massachusetts

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Critical Care Handbook of the

Massachusetts General Hospital

of, among other things, age, weight, gender, current or prior medical conditions, medication history, laboratory data and other factors unique to the patient The publisher does not provide medical advice or guidance and this work is merely a reference tool Healthcare professionals, and not the publisher, are solely responsible for the use of this work including all medical judgments and for any resulting diagnosis and treatments Given continuous, rapid advances in medical science and health information, independent professional verification of medical diagnoses, indications, appropriate pharmaceutical selections and dosages, and treatment options should be made and healthcare professionals should consult a variety of sources When prescribing medication, healthcare professionals are advised to consult the product information sheet (the manufacturer’s package insert) accompanying each drug to verify, among other things, conditions of use, warnings and side effects and identify any changes in dosage schedule or contraindications, particularly if the medication to be administered is new, infrequently used or has a narrow therapeutic range To the maximum extent permitted under applicable law, no responsibility is assumed by the publisher for any injury and/or damage to persons or property, as a matter of products liability, negligence law or otherwise, or from any reference to or use by any person of this work.

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I would just thank the authors, the residents, the faculty, and above all, our patients who have given us the privilege of allowing us to care for them We will engage in continuous

learning to ensure that our care for our patients is optimal.

Jeanine P Wiener-Kronish, MD

Hovig V Chitilian, MD

Assistant Professor of Anesthesiology

Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital

Harvard Medical School

Boston, Massachusetts

Brian M Cummings, MD

Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital

Yvonne Lai, MD

Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital

Marcos Vidal Melo, MD, PhD

Associate Professor of Anesthesia

Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital

Harvard Medical School

Milad Sharifpour, MD, MS

Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital

Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital

Instructor in Anesthesia

Harvard Medical School

Boston, Massachusetts

Stephen D Wilkins, MD

Clinical Fellow in Cardiac Anesthesia

Department of Anesthesia, Critical Care and Pain Medicine Massachusetts General Hospital

Boston, Massachusetts

Alison S Witkin, MD

Division of Pulmonary and Critical Care Medicine

The Critical Care Handbook of the Massachusetts General Hospital is meant to provide all health care

providers with an overview of this enlarging and exciting field Critical care now encompasses the care

of all patients requiring intense physiologic monitoring These patients include patients who now survivepreviously untreatable cancers, trauma victims of all ages, postoperative patients of all ages, patientswhose age or comorbidities would have previously precluded anesthesia and surgery, and patientsrequiring mechanical support for prolonged periods

The separation of ICUs is somewhat artificial since patients often have multiple problems requiringmultiple medical specialties Likewise, the skills of intensivists are expanding and include transthoracicechocardiography, transesophageal echocardiography, experience with ECMO and destination hearts aswell as experience with liver, heart, face, and kidney transplants In addition, intensivists must now learn

to protect themselves from EBOLA and MERS as well as from influenza and antibiotic-resistant bacteria.Some treatments have changed only slightly over the years, but the future of critical care will involveestimation of patient genetics and immunologic status as well as their microbiomes Molecular techniquesutilized only for research until recently will now come to the clinical arena Critical care practitionerswill need knowledge of these techniques, as well as an appreciation of epidemiology, disease states, andthe economics of care Communication skills need to be taught to improve patient care; patients needmultiple providers when they have multiple organ dysfunction Protocols and checklists improve aspects

of care, as does optimal communication between practitioners and patients and their families

The handbook has been heavily revised to reflect a multidisciplinary approach to care, the need toinclude all care providers to optimize treatment, and the need for ongoing education and learning Thechallenges will continue, but so does the satisfaction in caring for our sickest patients and our goal ofcontinually improving our patients’ lives

Index

I HEMODYNAMIC MONITORING is one of the cornerstones of patient evaluation in the

intensive care unit and provides diagnostic and prognostic value The choice of monitoringdepends on the diagnostic needs of the patient and the risk–benefit balance of monitor

placement and maintenance This chapter outlines an approach to assessment of hemodynamicsand perfusion in critically ill patients and the technical principles of commonly used

monitoring methods

A Perfusion: The goal of hemodynamic monitoring is to ensure adequate tissue perfusion

for gas, nutrient, and waste exchange to ultimately decrease morbidity and mortality Toget from optimizing a single hemodynamic parameter to improving morbidity and

mortality requires many assumptions (as shown in Fig 1.1 for mean arterial pressure,MAP) For this reason, the intensivist should not rely solely on any one physical monitorand should look for other signs of adequate perfusion such as mental status, urine output,

or laboratory findings (e.g., central venous oxygen saturation, base deficit, lactate)

B Optimizing Perfusion: Hemodynamic monitors by themselves are not therapeutic.

Hemodynamic data should be used to guide therapy Optimizing perfusion may requirefluid administration, diuresis, pharmacologic agents (e.g., vasoconstrictors, inotropicagents), or interventions (e.g., thrombectomy, intra-aortic balloon pump, ventricularassist devices, extracorporeal membrane oxygenation) With this in mind, any monitormust be used dynamically to ensure that employed therapies are optimizing perfusionover time

1 Fluid challenge: The fluid challenge is a time-honored test that bears specific

mention Rapid administration of crystalloid (typically 500 cc–1 L) while monitoringhemodynamics is used to determine if a patient may benefit from fluid, as suggested

by an increase in cardiac output or blood pressure, for example A “passive legraise” test provides similar information To perform this test, a clinician passivelyelevates a supine patient’s legs Blood moves to the central veins from the elevatedlimbs, providing an “autotransfusion” of approximately 150 to 300 cc An

improvement in hemodynamics suggests fluid responsiveness, whereas deterioration

in hemodynamics can be quickly reversed by lowering the legs

II ARTERIAL BLOOD PRESSURE MONITORING

1 Blood pressure describes the pressure exerted by circulating blood within the blood

vessels Since this pressure drives flow, it is used as a surrogate measure of bloodflow and, in turn, organ perfusion (Fig 1.1) This simplified view has limitations andnotably poor correlation with cardiac output in some situations, such as emergencyresuscitation of the hypovolemic critically ill patient Nonetheless, arterial bloodpressure monitoring as a target for perfusion is used in almost all critical caresettings and has been linked to morbidity and mortality outcomes

FIGURE 1.1 The assumptions when extrapolating MAP to a morbidity and mortality benefit.

2 Under normal circumstances, tissue perfusion is maintained across a range of

pressures by autoregulation, which describes the intrinsic capacity of vascular beds

to maintain flow by adjusting local vascular resistance However, pathologicalconditions common in the intensive care unit such as chronic hypertension, trauma,

3 The “gold standard” for blood pressure measurement is aortic root pressure, which is

representative of the stresses faced by the major organs (e.g., heart, brain, kidneys)

As the pressure wave travels distally from the aorta, the measured mean pressuredecreases while the measured pulse pressure (systolic pressure minus diastolicpressure) is increased owing to pulse wave reflection from the high-resistant distalarterioles In addition to being amplified, as one progresses distally, the arterialwaveform is slightly delayed (Fig 1.2) The difference in mean pressure is typicallyminimal given the low arterial resistance, but can be significant in some situations(e.g., high-dose vasoconstrictor administration)

FIGURE 1.2 Arterial waveforms as one travels distally along the arterial tree.

B Noninvasive Blood Pressure Monitoring: Various techniques can be used to measure

blood pressure noninvasively including manual palpation, determination of Korotkoffsounds with a sphygmomanometer and stethoscope or Doppler ultrasound, and automatedoscillometric methods, which are most common in the ICU

1 Function: The oscillometric method uses a pneumatic cuff with an electric pressure

sensor, most commonly over the brachial artery The cuff is inflated to a highpressure and then slowly deflated Arterial pulsations are recorded as oscillations.The pressure that produces greatest oscillation recording is closely associated with

mean arterial pressure Systolic and diastolic pressures are then calculated, often

using proprietary algorithms

cuff sizing and placement Most blood pressure cuffs display reference lines for anacceptable arm length, and the cuffs should be sized as recommended in relationship

to arm circumference Cuffs that are too small may overestimate blood pressure,whereas cuffs that are too large may underestimate blood pressure

3 Limitations and risks:

a The pressure measured by a noninvasive cuff is the pressure at the cuff site.

When an extremity pressure is measured to estimate coronary perfusion, eitherthe extremity should be elevated to the level of the heart, or the fluid columnshould be accounted for (e.g., a pressure measured at a site 10 cm below theheart will be 10 cmH2O or approximately 7.4 mmHg greater than the pressure atthe heart)

b Tissues, including vessels and nerves, can be damaged by cyclical compression

of pneumatic cuffs with frequent cycling Automatic methods may not be reliable

in rapidly changing situations, such as extremes of blood pressure, rapidlychanging blood pressure, or patients with dysrhythmias

3 Function: Necessary equipment to monitor invasive arterial blood pressure includes

an intra-arterial catheter, fluid-filled noncompliant tubing, transducer, continuousflush device, and electronic monitoring equipment The flush device typicallyprovides an infusion of plain or heparinized saline at a rate of 2 to 4 milliliters perhour through the tubing and catheter to prevent thrombus formation Commonlyemployed arterial line pressure bag/transducer systems provide this slow flush bydesign The transducing sensor is “connected” to arterial blood by a continuous line

of fluid and measures a pressure deflection in response to the transmitted pressurewave of each heartbeat The accuracy of intra-arterial blood pressure measurementdepends on the proper positioning and calibration of the catheter–transducer

monitoring system

a Positioning: The arterial pressure transduced is at the level of the transducer, not

at the level of the cannulation site This is because of the fluid-filled tubingbetween the patient and transducer, which maintains energy by exchangingpotential energy for pressure (Bernoulli’s equation) For example, if thetransducer is lowered, the fluid in the tubing exerts an additional pressure on thetransducer and the measured pressure will be higher Therefore, the arterialtransducer should be placed at the level of interest For example, positioning atthe fourth intercostal space on the midaxillary line (“the phlebostatic axis”)corresponds to the level of the aortic root, and positioning at the externalacoustic meatus corresponds to the Circle of Willis

opened to air and the recorded pressure (the atmospheric pressure) is set to zero

c Dynamic calibration: The two components of dynamic calibration of an

oscillating system are resonance (which increases pulse pressure amplitude) anddamping (which decreases pulse pressure amplitude)

1 Resonance: When an arterial pulsation “hits” the elastic arterial wall, this

vibrates and, just like a musical fork, generates an infinite series of sinewaves of increasing frequency and decreasing amplitude Typically, thenatural frequencies of arterial pulse waveforms are in the 16-to-24-Hzrange The transducer system has its own natural frequency, commonly over

200 Hz As the natural frequency of the arterial pulse approaches that of thetransducer system, the system will resonate—pressure waveforms will beamplified versions of the intra-arterial waveform (“whipped” waveformswith erroneously wide pulse pressures) The resonant frequency of a systemcan be tested with a fast flush test Displayed on a strip chart recorder, theresonant frequency of the system can be calculated by measuring the distancebetween two subsequent peaks of the trace Tachycardia or a steep systolicupstroke will increase the natural frequency of the arterial pulse and maycontribute to resonance

2 Damping: The damping coefficient is a measure of how quickly an

oscillating system comes to rest A high damping coefficient indicates thatthe system absorbs mechanical energy well and will cause attenuation of thewaveform Factors that increase damping include loose connections, kinks,and large air bubbles The ideal damping coefficient depends on the naturalfrequency of the system, though it is 0.6 to 0.7 for commonly used systems

4 Complications: Arterial cannulation is relatively safe Risks depend on site of

cannulation For radial cannulation, reported serious risks include permanentischemic damage (0.09%), local infection and sepsis (0.72% and 0.13%,respectively), and pseudoaneurysm (0.09%) Fastidious attention to the adequacy ofdistal perfusion is of great importance Thrombotic sequelae are associated withlarger catheters, smaller arterial size, administration of vasopressors, duration ofcannulation, and multiple arterial cannulation attempts With regard to infectious risk,aseptic technique was not standardized in the studies that yielded the aforementionedpercentage of risk, and longer duration of cannulation increased risk Less seriousrisks include temporary occlusion (19.7%) and hematoma (14.4%) Axillary andfemoral sites are associated with higher risks of infection Brachial cannulation hasbeen associated with median nerve injury (0.2%–1.4%)

5 Respiratory variation: Increased intrathoracic pressure decreases preload, arterial

pressure, and pulse pressure This effect is marked in hypovolemic patients who aremore susceptible to increased intrathoracic pressures Variation of more than 10% to12% in systolic pressure or pulse pressure is suggestive of fluid responsiveness.Importantly, this is validated for patients with regular cardiac rhythm and breathingpattern, and it is dependent on the ventilatory pressure delivered

6 Arterial waveform analysis has been used to gauge stroke volume and is discussed

later in this chapter

Measurements exist, with noninvasive measurements tending to yield higher

measurements during hypotension and lower measurements during hypertension Thesediscrepancies persist even with appropriate cuff sizing in critically ill patients

Retrospective data suggests that the higher noninvasive systolic blood pressures duringhypotension are overestimates of perfusion, since the incidence of acute kidney injury andmortality is higher with noninvasive versus invasive systolic blood pressures There was

no difference in acute kidney injury or mortality when mean pressures were compared.This suggests that mean pressures should be targeted when the hypotensive critically illpatient is treated with a noninvasive cuff

III CENTRAL VENOUS PRESSURE MONITORING

A Indications for placement of a central venous catheter (CVC) include administration of

certain drugs, concentrated vasopressors, or TPN; need for dialysis; need for long-termmedication administration such as chemotherapy or intravenous antibiotics; need for IVaccess in patients with difficult peripheral access; or need for sampling central venousblood

B Site: Common central venous cannulations sites are the internal jugular, subclavian, and

femoral veins The ideal site of cannulation varies with the characteristics of the patientand the indications for insertion For example, the subclavian site is relatively

contraindicated in coagulopathic patients as it is not directly compressible, and thefemoral vein may be ideal in emergency situations because of ease of cannulation Table1.1 summarizes the advantages and disadvantages of the most commonly used sites forvenous access

(Fig 1.3) The a-wave corresponds with atrial contraction and correlates with the p

tricuspid valve into the right atrium) and correlates with the end of the QRS complex on

EKG The v-wave corresponds to atrial filling against a closed tricuspid valve and occurs with the end of the T wave on EKG The x-descent after the c-wave is thought to

FIGURE 1.3 The central venous pressure (CVP) waveform.

2 Utility and controversy: In spite of recent evidence suggesting limited utility, CVP

has been used clinically to assess fluid status for decades The physiologicdeterminants of CVP include patient position, the circulating volume status,interactions between systemic and pulmonary circulations, and the dynamic changes

responsiveness

3 Clinical confounders: When used clinically, CVP measurements are used to estimate

end diastolic volume, given in the following relationship:

VRV = CRV · (CVP – Pextracardiac)

where VRV is end diastolic volume, CRV is compliance of the right ventricle, CVP

approximates the pressure inside the ventricle, and Pextracardiac is extracardiacpressure Given this relationship, the general categories of physiologic perturbationthat alter the direct relation between CVP and volume are:

Additionally, some PA catheters have pacing ports and can be used for temporarytransvenous pacing

B Technique: Pulmonary artery catheters are positioned by floating a distally inflated

balloon through the right atrium and right ventricle into the pulmonary artery Figure 1.4shows the characteristic pressure waveforms seen as the pulmonary catheter is advanced.During placement, attention to the pressure tracing, electrocardiogram, systemic bloodpressure, and oxygen saturation is essential to ensure proper placement of the catheterand to minimize known complications

central venous pressure; IJ, internal jugular; PA, pulmonary artery; PCW, pulmonary capillary wedge; RA,right atrium; RV, right ventricle

1 Fluoroscopic guidance may be useful in certain situations, such as the presence of

recently placed permanent pacemaker (generally within 6 weeks), the need forselective PA placement (e.g., following pneumonectomy) and the presence ofsignificant structural or physiologic abnormality (e.g., severe RV dilation, largeintracardiac shunts or severe pulmonary hypertension)

C Waveforms during Placement: The right atrial pressure waveform is the same as the

CVP waveform previously described The pressure waveform in the right ventricle (RV)has a systolic upstroke (in phase with the systemic arterial upstroke) with low diastolicpressures that increase during diastole, owing to ventricular filling The pulmonary arterypressure waveform will also be in phase with systemic pressures during systole, but willdiffer from the RV tracing as the pressure decreases during diastole Often, the diastolicpressure will increase when the balloon enters the pulmonary artery, but the better marker

of this advancement is the transition to a downward slope during diastole The pulmonaryartery occlusion pressure (PAOP) or wedge pressure waveform will resemble the CVP

trace with a-, c-, and v-waves, though these are often difficult to distinguish clinically.

D Physiologic Data

a Method: A rapid bolus of cold saline is injected proximal to the right heart, and

temperature is monitored at the distal tip of the PA catheter With higher cardiacoutput, more blood is mixed with the cold fluid bolus and the temperature

Hamilton Equation:

recorded over time will be attenuated, as described by the Modified Stewart- b where CO is cardiac output, Tbody is the temperature of the body, Tinjectate is the

temperature of the saline bolus, V is the volume of the bolus, K reflectsproperties of the catheter system, and AUC is the area under the curve oftemperature change

c Reliability: Averaging of serial measurements is recommended for each CO

determination as the calculated CO may vary by as much as 10% without achange in clinical condition It is important to minimize variations in the rate andvolume of injection, which also introduce error Colder solutions (i.e., increased

Tbody – Tinjectate) decrease error, though attention should be paid to potentialtachy- or bradyarrhythmias Tricuspid regurgitation may affect calculations as aresult of recirculated blood between the right atrium and ventricle Intracardiacshunt can likewise introduce error

2 PA occlusion pressure: Occluding the PA recreates a static fluid column between the

distal tip of the catheter and the left atrium, allowing equilibration of pressuresbetween the two sites In this manner, PAOP approximates left atrial pressure, asurrogate for left ventricular end-diastolic volume For an accurate measure of PAOP,

the proper atrial trace, similar to the “a-c-v” trace of the CVP waveform, should be

visualized As highlighted in the discussion of CVP measurements, volume is onlyone parameter that influences the PAOP measurements of other variables (e.g.,cardiac compliance, intrathoracic pressures, valvular lesions, ventricularinterdependence)

3 Mixed venous oxygen saturation (S o 2 ): As cardiac output increases, tissue oxygen

demand is met with less per-unit oxygen extraction and S o2 increases This is aloose correlation as S o2 also depends on hemoglobin concentration and oxygenconsumption, as outlined by the Fick Equation:

a surrogate for S o2 It is typically higher than the S o2 by about 5% as it does notinclude the oxygen-depleted blood from the heart itself (returned to the right atriumfrom the coronary sinus).

E Complications: In addition to complications associated with central venous access, PA

catheter placement is associated with increased risk of arrhythmia including right heartblock (especially in patients with recent MI or pericarditis) and pulmonary arteryrupture Pulmonary artery rupture risk is increased with pulmonary hypertension,advanced age, mitral valve disease, hypothermia, and anticoagulant therapy and requiresemergent thoracotomy Catheter-related complications, including knotting or balloonrupture with subsequent air or balloon fragment emboli, have also been reported

F Relative contraindications to PA placement include left heart block (as a superimposed

right heart block would lead to complete block), presence of a transvenous pacer orrecently placed pacemaker or ICD leads, tricuspid or pulmonary stenosis or prosthetictricuspid or pulmonary valves (given the associated difficulty in passing the catheter andballoon), patient predisposition for arrhythmia, coagulopathy, or severe pulmonaryhypertension Need for MRI is also a contraindication as most PA lines containferromagnetic material

G Controversy on PA Catheters: Although hemodynamic data derived from PA catheters

enhances understanding of cardiopulmonary physiology, the risk-to-benefit profile hasbeen questioned Since the mid-1990s, several large outcome studies assessing thebenefit of PA catheters have been conducted, and none shows clear evidence of benefit.Moreover, PA catheter-guided therapy has been associated with more complications thandoes central venous catheter-guided therapy Although these results are not sufficientlyconvincing to completely discourage use of PA catheters, they underscore the importance

of only using PA catheters when the benefit of management guidance derived from PAdata are strongly believed to outweigh the associated risks

V ALTERNATIVES TO BLOOD PRESSURE MONITORING: Numerous alternative

hemodynamic monitors have been developed to assess either cardiovascular function or tissueperfusion

is multiplied by heart rate to obtain cardiac output Notably, as this is only the flow in adescending aorta, a certain percentage (typically around 30%) is added to determine totalcardiac output Modern probes are roughly the size of nasogastric tubes, much smallerthan ordinary trans-esophageal echocardiography probes, and can provide “correctedflow time” and stroke volume variation in addition to cardiac output, which can be used

to gauge fluid responsiveness

1 Advantages: Esophageal Doppler monitoring allows continuous measurement with

2 Disadvantages: Esophageal Doppler monitoring can only be performed in intubated

patients, requires frequent repositioning if the patient is moved, is operatordependent, and is not widely available

Using an intermittent partial rebreathing circuit, the change in CO2 production and end

tidal CO2 concentration in response to a brief, sudden change in minute ventilation ismeasured The changes in end tidal CO2 are used to calculate cardiac output

1 Advantages: This method is low risk, noninvasive, and can be performed every few

minutes The partial rebreathing CO2 cardiac output method has also shownreasonably good agreement with gold standard thermodilution in clinical trials insome settings

2 Disadvantages: As currently designed, this method requires tracheal intubation for

measurement of exhaled gases Measurements can be affected by changing patterns ofventilation and intrapulmonary shunting Furthermore, this technique has a relativelylong response time

D Transpulmonary Thermodilution and Transpulmonary Indicator Dilution: With these

techniques, the same principles for PAC thermodilution are employed, but only a CVCand arterial line are required A bolus of either cold saline (with the PiCCO device) orlithium chloride (with the LiDCO device) is injected into the central line and the dilutionover time in a peripheral artery is used to derive the cardiac output These are commonlyused in conjunction with pulse contour analysis (see next section) to provide continuousassessment of cardiac output

1 Advantages: Both of these methods have been shown to correlate reasonably well

with PA catheter thermodilution Transpulmonary thermodilution has the addedbenefit of providing assessment of extravascular lung water and intrathoracic bloodvolume

2 Disadvantages: Both methods require repetitive blood draws, and calibration may

be affected by neuromuscular blocking agents Notably, the PiCCO system typicallyrequires placement of an axillary or femoral arterial catheter

principle that stroke volume and cardiac output can be gauged from characteristics of thearterial waveform, using calculations that are based on estimates of compliance of thearterial tree Commercially available devices require calibration, typically againstthermodilution or indicator dilution methods

1 Advantages: Pulse contour analysis devices are continuous and employ catheters

(central venous catheters and arterial lines) that are commonly employed in ICUpatients

2 Disadvantages: This technique requires mechanically ventilated patients and has

shown questionable accuracy in patients who are hemodynamically labile andpatients on vasoactive medications The altered arterial waveform of patients withaortic insufficiency may also decrease the accuracy of this technique

F Impedance Cardiography (Also Known as Electrical Impedance Plethysmography):

With impedance cardiography, a high-frequency, low-magnitude current is applied to thechest, and impedance is measured As the aorta fills with blood with each heartbeat,impedance decreases, and this change is used to determine stroke volume and cardiacoutput Advances in phased-array and signal-processing technologies have improvedimpedance cardiography, largely overcoming artifact due to electrode placement, HR andrhythm disturbances, and differences in body habitus, though its use is still fairly limited

in the ICU Both electrical velocimetry and bioreactance employ similar principles.Electrical velocimetry relates the velocity of blood flow in the aorta to determine cardiacoutput, whereas bioreactance uses changes in electrical current frequency (rather than inimpedance) to measure changes in blood flow during the cardiac cycle

measures gastric CO2, which decreases with low perfusion states Tissue oxygenation(StO2) measures percentage of oxygenated hemoglobin at the microcirculation/tissuelevel While there are many others, most of these are still used primary for research andnot for clinical applications at this time

monitoring is performed to assure patient safety, maintain homeostasis, and titrate treatment.

B The main components of respiratory physiology are gas exchange, respiratory

mechanics, control of breathing, and pulmonary circulation This chapter will address the monitoring of gas exchange and respiratory mechanics, and comment on control

7 Arterial partial pressure of CO2 (PaCO2)

a The PaCO2 reflects the balance between carbon dioxide production ( CO2) and

c Minute ventilation affects PaCO2 only to the extent that it affects the alveolar

in acute respiratory distress syndrome (ARDS)

9 Arterial pH is determined by bicarbonate (HCO3−) concentration and PaCO2, as

predicted by the Henderson–Hasselbalch equation:

a Care must be taken to avoid sample contamination with air, as the PO2 and PCO2

of room air at sea level are approximately 155 and 0 mmHg, respectively Careshould also be taken to avoid contamination of the sample with saline or venousblood

body temperature pH-stat management is an alternative method in which

measurements obtained at 37°C are corrected to the patient’s actual bodytemperature before use to achieve those same numerical targets It may benecessary to enquire with your laboratory to determine whether the laboratory isreporting temperature-compensated values (that may facilitate the use of the pH-stat strategy)

b Most intensive care units (ICUs) utilize the α-stat strategy The choice of

ventilation strategy is becoming increasingly important with the use of inducedhypothermia as a therapeutic tool Because of increased gas solubility during

hypothermia, the α-stat strategy results in relative hyperventilation The pH-stat

approach results in increased cerebral blood flow There are different conditions

in which each of these methods can be more or less advantageous

B Venous Blood Gases reflect PCO2 and PO2 at the tissue level

1 There is a large difference between Pao2 and venous Po 2 (Pvo 2 ) PvO2 is affected by

oxygen delivery and oxygen consumption, whereas PaO2 is affected by lung function.Thus, PvO2 should not be used as a surrogate for PaO2

2 Normally, venous pH is lower than arterial pH, and venous Pco 2 (Pvco 2 ) is higher

than PaCO2 However, the difference between arterial and venous pH and PCO2 isincreased by hemodynamic instability During cardiac arrest, for example, it has beenshown that PvCO2 can be very high even when PaCO2 is low, a consequence of lowcardiac output for a similar CO2 production

3 When venous blood gases are used to assess acid–base balance, mixed venous or

central venous samples are preferable to peripheral venous samples Mixed venous

blood is the blood present in the pulmonary artery, a result of the mix of blood from