Cardiopulmonary Monitoring

In the event of a cardiac arrest, the first procedure is to take your own pulse. Samuel Shem

The goal of cardiopulmonary monitoring is to assess the adequacy of the cardiac and pulmonary systems in meeting the metabolic needs of the patient. Although these goals may appear simple, this monitoring can become amazingly complicated in practice. The most important idea to grasp is that no single measurement can exist in isolation; all data must be considered with other data and the presentation of the patient. The ideal monitor is noninvasive, reliable, and conveys physiologic information in “real time.” Often, in critically ill patients, noninvasive monitoring is either unreliable or inaccurate and invasive monitoring is required.

I

Cardiac Monitoring

A

Cardiac Rhythms

1.
Continuous electrocardiogram (EKG)—allows for rapid recognition and analysis of heart rate, rhythm and various amplitudes and intervals
2.
EKG abnormalities ( Figs. 13.1–13.7 )
- a.
  ST segment changes
  - (1)
    ST segment depression is seen in subendocardial myocardial infarction (MI) and myocardial ischemia.
  - (2)
    ST segment elevation is seen in transmural MI.
- b.
  T wave changes
  - (1)
    Peaked T waves are seen with hyperkalemia.
  - (2)
    Inverted T waves can be seen in many cardiac and noncardiac conditions, such as MI, myocarditis, and myocardial contusion.
  - (3)
    Biphasic T waves can be seen in hypokalemia or ischemia.
- c.
  QTc interval
  - (1)
    Prolonged QTc interval (>440 ms) is associated with increased risk of ventricular arrhythmias, such as torsades de pointes.
  - (2)
    Prolonged QTc can be caused by various electrolyte disturbances and by drugs, such as antipsychotics (haloperidol, quetiapine), antidepressants (amitriptyline, nortriptyline, bupropion), antihistamines (loratadine, diphenhydramine), and antiarrhythmics (amiodarone).
3.
Common cardiac arrhythmias
- a.
  Atrial fibrillation: irregularly irregular ventricular rate with the absence of discernible P waves
  - (1)
    Hemodynamically unstable patients require immediate synchronized direct current cardioversion.
  - (2)
    In stable patients, rhythm control can often be achieved pharmacologically with beta-blockers, calcium channel blockers, digoxin, or antiarrhythmics, such as amiodarone.
  - (3)
    If rhythm control is unattainable, rate control is the next goal. Although current American Heart Association guidelines suggest similar outcomes for rate and rhythm control for patients with new onset atrial fibrillation, surgical patients frequently have an inciting event (operation, acute volume overload) and potential contraindications for anticoagulation that may make rate control more desirable.
  - (4)
    New-onset atrial fibrillation that persists beyond 48 hours may require anticoagulation to prevent sequelae of embolization.
- b.
  Atrial flutter: narrow complex tachycardia caused by reentry circuit in right atrium, typical rate of 130–170 (approximately 150 most common)
  - (1)
    Treatment is similar to atrial fibrillation.
  - (2)
    Have high suspicion for atrial flutter in patients with narrow complex tachycardia with rate of 150.
- c.
  Supraventricular tachycardia (SVT): narrow complex sinus tachycardia, typical rate of 170–280 (higher than in atrial flutter)
  - (1)
    Adenosine can help to differentiate SVT from atrial flutter and sinus tachycardia as the slowing of the ventricular rate makes the underlying rhythm more easily visible.
  - (2)
    Mainstay of treatment is identification and treatment of the underlying cause, but rate-control agents such as beta-blockers, calcium channel blockers, or amiodarone can also be used if the SVT is symptomatic.
- d.
  Mobitz I second-degree heart block—progressive PR interval prolongation before a nonconducted P wave
  - (1)
    It may be asymptomatic or symptomatic, but hemodynamically stable patients should be closely monitored with transcutaneous pacing pads in place.
  - (2)
    Hemodynamically unstable patients should be promptly treated with atropine and temporary transcutaneous cardiac pacing started.
  - (3)
    Patients may eventually require permanent pacemaker placement if heart block does not resolve.
- e.
  Mobitz II second-degree heart block—PR interval unchanged before a nonconducted P wave
  - (1)
    Treatment is similar to Mobitz I heart block.
  - (2)
    However, Mobitz II has a higher likelihood of progressing into third-degree heart block, and therefore more aggressive treatment is indicated.
- f.
  Ventricular tachycardia: wide QRS complex (>120 ms) tachycardia with atrioventricular dissociation
  - (1)
    Stable patients can be chemically cardioverted with antiarrhythmics, such as lidocaine or amiodarone.
  - (2)
    Hemodynamically unstable patients require immediate synchronized direct current cardioversion.
  - (3)
    All wide complex tachycardias should be treated as ventricular tachycardia until proven otherwise.
- g.
  Ventricular fibrillation: irregular ventricular rhythm with no distinction between QRS complex, ST segment, and T waves
  - (1)
    When recognized, immediate treatment is asynchronous defibrillation, then follow advanced cardiac life support (ACLS) protocol if rhythm is sustained after defibrillation.

B

Blood Pressure Monitoring

1.
Sphygmomanometer—Indirect measurement of arterial blood pressure based on the assumption that pressures in the cuff are equal to pressures in the encompassed artery. The accuracy depends on the correct choice of cuff size and may be unreliable in patients with atherosclerotic disease.
2.
Arterial catheter
- a.
  Advantages—This allows for continuous direct measurement of arterial blood pressure and arterial access for blood sampling.
- b.
  Limitations/complications—Air bubbles in the catheter-transducer system or clotting in the catheter can lead to decreased resonant frequency and erroneous measurements. Measurement also can be positional. Possible complications include thrombosis and distal ischemia, hematoma, stricture, arteriovenous fistula, or infection.
- c.
  Arterial pressure measurements can vary based on the location of the artery (i.e., aorta vs. radial), but mean arterial pressure (MAP) should be the same regardless of the distance of the artery from the heart and may be more clinically relevant than systolic or diastolic blood pressure.

C

Hemodynamic Monitoring Basic Principles

1.
Most often, hemodynamic monitoring is used to evaluate patients with potential inadequate oxygen delivery, whether that be caused by volume status, cardiac function, or high demands. Oxygen delivery is calculated by the following formula:

${DO}_{2} = {CaO}_{2} \times CO \times 10$ ${DO}_{2} = {CaO}_{2} \times CO \times 10$

where CaO ₂ = arterial content of oxygen and CO = cardiac output.

Although most values contributing to arterial oxygen content are known or easily determined, the factors that affect cardiac output are more complex, and this is where additional monitoring devices have the most utility. The ideal method of hemodynamic monitoring would be noninvasive, accurate, and continuous and give information about multiple components of oxygen delivery. Although many methods of evaluating the hemodynamic status of a patient have been developed, we have yet to find an ideal mode that meets all of these requirements. Overall, the information we seek from hemodynamic monitoring devices usually can fall into three basic categories:
- a.
  Intravascular volume status
- b.
  Cardiac function
- c.
  Adequacy of oxygen delivery
2.
Basic clinical examination of the patient: Assessment of pulse, blood pressure, and urine output can offer a rapid assessment of the adequacy of perfusion.
- a.
  Intravascular volume status
  - (1)
    Neck vein distension
  - (2)
    Passive leg raise—predictor of fluid responsiveness but requires measuring either cardiac output, stroke volume, or pulse pressure
    - (a)
      Patient is placed in a semirecumbent position.
    - (b)
      The legs are passively raised to 45 degrees, and stroke volume, cardiac output, or pulse pressure are monitored for change.
    - (c)
      Maximal effect occurs at 30–90 seconds, and a 10% increase in stroke volume, cardiac output, or pulse pressure suggests that the patient would be responsive to fluids.
    - (d)
      Utility is limited in severely hypovolemic patients and those with positional restrictions.
- b.
  Cardiac function—difficult to ascertain on simple physical examination. Tachycardia may represent a compensatory response to hypovolemia but may also be a result of cardiac failure.
- c.
  Adequacy of perfusion—adequate urine output can indicate sufficient renal perfusion, which is often sacrificed under situations in which brain and cardiac perfusion must be preserved. This can be unreliable in patients with intrinsic renal disease or those receiving diuretics.
3.
In critically ill patients, clinical examination provides limited information, and additional monitoring devices are frequently required. We will discuss representative examples from the current methods of hemodynamic monitoring later, but new devices are constantly being developed, and existing devices are continually being updated. However, in general, these devices still provide data answering the three aforementioned questions. As a guide, these parameters are as follows:
- a.
  Parameters that assess preload (volume status): central venous pressure (CVP, estimation of right atrial filling pressure), pulmonary artery wedge pressure (PAWP, estimation of left atrial filling pressure), stroke volume (SV), right and left ventricular end-diastolic volume (RVEDV and LVEDV, respectively), stroke volume variation (SVV), pulse pressure variation (PPV), global end-diastolic volume (GEDV), intrathoracic blood volume (ITBV)
- b.
  Parameters that assess afterload: systemic vascular resistance (SVR), pulmonary vascular resistance (PVR)
- c.
  Parameters that assess contractility: cardiac output (CO), cardiac index (CI), right and left ventricular ejection fractions
- d.
  Parameters that assess oxygenation: hemoglobin, partial pressure of arterial oxygen and CO ₂ (Pa o ₂ , Pac o ₂ ), mixed venous oxygen saturation (Sv o ₂ ), central venous oxygen saturation (Scv o ₂ )

D

Specific Devices for Cardiac Monitoring

1.
Echocardiogram
- a.
  Transthoracic echocardiogram (TTE)
  - (1)
    Measurements
    - (a)
      Volume status can be assessed by looking at cardiac chamber sizes and the diameter of the inferior vena cava (IVC).
      - (i)
        Collapsed cardiac chambers and/or small (<1.2 cm) or collapsed IVC indicate low volume status.
      - (ii)
        However, IVC diameter <1.2 cm has a sensitivity of only 25% for hypovolemia.
    - (b)
      Cardiac function: Global function as well as regional wall motion abnormalities can be measured. Echocardiogram can also differentiate right heart from left heart function in cases where there is a discrepancy.
  - (2)
    Advantages
    - (a)
      Can be performed quickly at bedside
    - (b)
      Noninvasive
  - (3)
    Limitations/complications
    - (a)
      Patient’s body habitus can limit the views of a bedside echocardiogram.
    - (b)
      Accuracy is highly dependent on operator skill and experience.
- b.
  Transesophageal echocardiogram (TEE)
  - (1)
    Measurements—Similar to TTE, TEE can provide information about intravascular volume status and cardiac function. TEE also provides superior visualization of the heart, especially the posterior heart structures including valves as the transducer is in much closer proximity to the heart.
  - (2)
    Advantages—TEE provides real-time measurement of CO, SV, ventricular systolic function, volume assessment, and anatomic abnormalities, although measurements are valid only at that particular point in time.
  - (3)
    Limitations/complications
    - (a)
      Accuracy is highly dependent on operator skill and experience.
    - (b)
      Complications include dental, hypopharyngeal or esophageal injuries, malpositioning of endotracheal tube, or upper gastrointestinal (GI) bleeding.
    - (c)
      It should not be used in patients with known esophageal stricture, esophageal varices, or recent esophageal surgery.
2.
Venous catheter–based monitoring
- a.
  Central venous catheter
  - (1)
    Measurements
    - (a)
      Volume status: Trends in CVP may be useful in monitoring resuscitation, but single measurements may be inaccurate and of limited utility in the critically ill patient.
    - (b)
      Cardiac function: It does not give any information about cardiac function.
    - (c)
      Oxygen delivery: Although many papers, including the guidelines for sepsis, use central venous saturation as a measurement of adequacy of oxygen delivery, this value only reflects saturation in either the superior or IVC, depending on the location of the catheter (see mixed venous saturation later).
  - (2)
    Advantages
    - (a)
      Allows for continuous measurement of CVP and evaluation of trended data
    - (b)
      Provides central venous access for certain solutions and medications, as well as central venous blood sampling
  - (3)
    Limitations/complications
    - (a)
      Invasive procedure with potential for pneumothorax, hemothorax, and/or arterial puncture
    - (b)
      Risks of infection
- b.
  Pulmonary artery catheter (PAC)
  - (1)
    Description—PAC contains multiple lumens/ports, thermistor and thermal filament for CO monitoring, and balloon for PAWP or pulmonary artery occlusion (PAO) pressure measurement. Proximal infusion port and right atrial lumen can be used for infusing fluids and drugs, as well as measuring right atrial pressures. Pulmonary artery (PA) lumen allows for measurement of PA pressures.
    - (a)
      The catheter is inserted into a large vein (internal jugular, subclavian, or femoral vein) and advanced through the right atrium, the right ventricle, and into the PA.
    - (b)
      It is the gold standard for cardiac output monitoring. It uses thermodilution technique as mentioned later.
  - (2)
    Measurements
    - (a)
      Volume status:
      - (i)
        CVP
      - (ii)
        PAWP or PAO—measured at the very end of the catheter, distal to the balloon. Although measurement of PAO is not directly related to left atrial (LA) pressures, the catheter functions on the assumption that the pulmonary vascular system is a low-pressure system, and therefore LA pressures are effectively transmitted to PAO measurements.
      - (iii)
        RVEDV, SV—calculated values that give information regarding volume status
    - (b)
      Cardiac function: CO and CI are measured by thermodilution in which the change in temperature of blood passing by the thermistor is used to calculate flow and cardiac output. Historically, this was performed by injection of cold saline at a proximal port. Modern-day PACs have a built-in thermal filament that heats surrounding blood in the right ventricle, and temperature change is detected by the thermistor located at the tip of the catheter. These types of catheters can often provide continuous, rather than static, CO monitoring.
    - (c)
      Oxygen delivery:
      - (i)
        Sv o ₂ —The Swan-Ganz catheter is the only hemodynamic monitoring device that can measure a true mixed venous saturation, as this is defined by the saturation in the PA. Unlike venous saturations obtained from central venous catheters, the oxygen content in the PA reflects the return from both the SVC and IVC.
      - (ii)
        Sv o ₂ provides an index of tissue perfusion and oxygenation. Increasing Sv o ₂ correlates positively with cardiac output and tissue perfusion and negatively with systemic shunt states (e.g., sepsis and hepatic failure).
      - (iii)
        Although Sv o ₂ cannot reflect changes in regional perfusion, a decreasing Sv o ₂ is a generally ominous sign. Knowledge of the Sv o ₂ will also allow calculation of arteriovenous oxygen content and physiologic shunt (Qsp/Qt), both of which can assist in managing respiratory failure.
  - (3)
    Advantages
    - (a)
      Able to monitor advanced hemodynamic parameters to determine cardiac and pulmonary function, as well as overall fluid status.
    - (b)
      Specialized catheters allow atrial, ventricular, or atrioventricular sequential cardiac pacing.
    - (c)
      Continuous evaluation of variables
  - (4)
    Limitations/complications
    - (a)
      Other parameters (i.e., peripheral resistance) are calculated from the measured values noted previously. Therefore their best utility is to assess trends.
    - (b)
      PAWP is an estimation of left-heart filling pressure based on pulmonary vasculature being a low-pressure system. Thus cardiac and pulmonary disease states, such as valvular diseases or pulmonary hypertension, may lead to inaccurate measurements.
    - (c)
      Respiratory failure requiring high levels of positive end-expiratory pressure increases intrathoracic measurements (including all measurements and calculations obtained from PA catheter) to an unknown and unpredictable degree.
    - (d)
      For patients with low cardiac output, right-sided cardiomegaly, or unusual anatomy, or who are accessed from the left internal jugular vein, it may be difficult to achieve proper placement of the catheter.
    - (e)
      In addition to the risks of central venous catheter insertion, PA catheters can cause cardiac dysrhythmias, as well as PA rupture.

Arterial pulse contour analysis–based monitoring

FloTrac/Vigileo

(1)
Description—analyzes arterial line waveform using a specific proprietary algorithm that converts the pressure signal into a flow measurement based on assumptions about arterial compliance. This then allows for calculations of cardiac output.
- (a)
  It does not require external calibration that refers to methods such as thermodilution or drug-dilution techniques that are performed to calibrate cardiac output monitoring devices for accurate measurements.
(2)
Measurements
- (a)
  Volume status:
  - (i)
    SVV and PPV are naturally occurring phenomena in which the arterial pulse pressure and stroke volume fall during inspiration and rise during expiration due to changes in intrathoracic pressure.
  - (ii)
    SVV and PPV are indicators of relative preload responsiveness. If there is 13%–15% variation during inspiration, the patient is likely hypovolemic and will respond to volume. Conversely, if there is <10% variation, the patient is unlikely to be volume responsive.
  - (iii)
    SV
- (b)
  Cardiac function: CO, CI—Unlike devices that use thermodilution, instruments based on arterial waveform analysis do not measure but instead calculate CO using proprietary algorithms that vary from company to company.

(3)

Advantages

(a)
Much less invasive compared with PA catheter because it requires only an arterial line.

(b)

Provides continuous CO, SV, SVR, SVV, and PPV ( Table 13.1 for normal values)

TABLE 13.1

Reference Values for Various Hemodynamic Parameters

Central venous pressure (CVP)	0–8 mm Hg
Pulmonary artery pressure (PAP)	15–30/6–12 mm Hg
Pulmonary artery wedge pressure (PAWP)	6–12 mm Hg
Cardiac output (CO)	4.0–8.0 L/min
Cardiac index (CI)	2.5–4 L/min/m ²
Mixed venous oxygen saturation (Sv o ₂ )	70–80%
Stroke volume variations (SVV)	≤10%
Pulse pressure variations (PPV)	≤10%
Global end-diastolic volume index (GEDVI)	680–800 mL/m ²
Intrathoracic blood volume index (ITBVI)	850–1000 mL/m ²

(4)
Limitations/complications
- (a)
  It requires good arterial waveform for accurate analysis.
- (b)
  Inaccurate in patients with hemodynamic instability or rapid vascular motor tone changes. In hemodynamically unstable patients, calibrated devices may be more useful.
- (c)
  Because there is no external calibration, the data may be less accurate than monitors with calibration.

b.
Pulse contour cardiac output (PiCCO)
- (1)
  Description—hybrid-type device that provides continuous cardiac monitoring through arterial waveform analysis like FloTrac/Vigileo but is externally calibrated through transpulmonary thermodilution technique
  - (a)
    Thermodilution technique—Cold saline is injected through the central line, and change in temperature is measured downstream via an arterial line to calculate cardiac output.
  - (b)
    That measurement is then used to calibrate the cardiac output measurements calculated from arterial contour analysis.
- (2)
  Measurements
  - (a)
    Volume status: SVV, PPV, GEDV, ITBV
    - (i)
      SVV and PPV are similar to parameters measured by other arterial waveform analysis devices.
    - (ii)
      GEDV is an estimate of end-diastolic volume in all four cardiac chambers and ITBV consists of GEDV plus pulmonary blood volume. These two measures are calculated via thermodilution technique noted previously and represent preload volume.
    - (iii)
      Low GEDV or ITBV indicates hypovolemia.
    - (iv)
      SV
  - (b)
    Cardiac function: CO, CI
  - (c)
    Oxygen delivery: Scv o ₂ —not as accurate as Sv o ₂ as it only contains SVC blood sample (see PAC section earlier).
- (3)
  Advantages
  - (a)
    It provides continuous CO, SV, SVV, SVR, PPV, GEDV, and ITBV (see Table 13.1 for normal values).
  - (b)
    External calibration may make calculated CO more accurate.
- (4)
  Limitations/complications
  - (a)
    Unreliable in patients with poor arterial signal, rapid changes in vascular tones, arrhythmia, severe peripheral vascular disease, aortic valve pathology, and mechanical circulatory assist devices
  - (b)
    Requires arterial line and central venous catheter
c.
Lithium dilution cardiac output (LiDCO)
- (1)
  Description—provides continuous cardiac monitoring through arterial waveform analysis, which is externally calibrated via lithium dilution technique
  - (a)
    Lithium is injected peripherally, and blood samples are drawn to plot a lithium concentration time curve to obtain the cardiac output.
  - (b)
    As with PiCCO, this measurement is used to calibrate the cardiac output values calculated from arterial contour analysis.
- (2)
  Measurements
  - (a)
    Volume status: SVV, PPV, ITBV, SV
  - (b)
    Cardiac function: CO, CI
  - (c)
    Oxygen delivery: Scv o ₂
- (3)
  Advantages
  - (a)
    Requires only arterial line and a peripheral venous line
  - (b)
    Provides continuous CO, SV, SVV, SVR, PPV, and ITBV (see Table 13.1 for normal values)
- (4)
  Limitations/complications
  - (a)
    It is inaccurate in patients with poor arterial signal, aortic regurgitation, arrhythmia, and severe peripheral vasoconstriction.
  - (b)
    Calibration can be affected by nondepolarizing muscle relaxant and lithium therapy.
  - (c)
    It is contraindicated in patients less than 40 kg or in first trimester of pregnancy.

4.
Bioreactance
- a.
  Nicom, Cheetah
  - (1)
    Description—measures the phase changes in an alternating electrical current crossing the patient’s torso, which has been shown to correlate with pulsatile changes in aortic fluid volume. Four electrodes are placed on the patient’s chest, and a 75-kHz electrical signal is generated on one side and detected across the chest by the other electrode. The delay in transmission is then measured by the device—the higher the cardiac stroke volume, the more significant the phase shifts become. Because the signal is only affected by pulsatile flow, the delay can be used to calculate how much blood is coming out of the LV into the aorta, or stroke volume.
  - (2)
    Measurements
    - (a)
      Volume status: SV
    - (b)
      Cardiac function: CO, CI
    - (c)
      Oxygen delivery: none
  - (3)
    Advantages: noninvasive continuous monitoring of cardiac output, stroke volume
  - (4)
    Limitations/complications
    - (a)
      It depends highly on positioning of the electrodes.
    - (b)
      Limitations include electrical interference (i.e., electrocautery), fluid in thorax (pleural effusions, pulmonary edema, pericardial tamponade), changes in peripheral vascular resistance, obesity, arrhythmias, and motion artifacts.
    - (c)
      Validation studies suggest poor correlation compared with PA catheter.

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