Hemodynamic Monitoring

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Chapter 6 Hemodynamic Monitoring

Daniel Saddawi-Konefka, MD, MBA , Jonathan E. Charnin, MD

1 What is the purpose of hemodynamic monitoring?

Oxygen and fuels are brought to the tissues and waste products are removed by the flow of blood. The goal of hemodynamic monitoring is to assess whether the circulatory system has adequate performance in this regard to sustain organ function and life. Notably, hemodynamic monitoring provides data to guide therapy but is not by itself therapeutic.

2 How do manual blood pressure cuffs differ from automatic blood pressure cuffs?

Manual auscultation of the blood pressure assigns systolic value to the pressure measured when the first Korotkoff sound is heard and diastolic value to the pressure measured when the fourth Korotkoff sound disappears. Automatic blood pressure cuffs measure oscillations in pressure, caused by blood flow across a range of blood pressures; they determine only the mean blood pressure, which is determined by the point of greatest oscillation. With use of proprietary algorithms, the systolic and diastolic blood pressures are calculated. Both methods are susceptible to error with poor cuff size (which should cover two thirds of the limb segment), motion artifact, arrhythmia, and extremes of blood pressure.

3 How are arterial lines calibrated, and what factors affect readings?

Arterial lines generate blood pressure readings using pressure transducers. To yield useful information, the transducers must first be zeroed and leveled (positioned appropriately). Second, the system should be monitored for damping and resonance.

Zeroing and leveling eliminate the effects of atmospheric pressure and hydrostatic pressure, respectively, on blood pressure readings. Atmospheric pressures are set to zero so that reported values are relative pressures. If the system is not zeroed appropriately, measurements will continuously offset by a fixed amount. Errors can also occur if the transducer is physically lowered so that it will read a higher pressure and vice versa (potential energy is replaced by pressure to maintain energy in the fluid; read about Bernoulli’s equation for more). Therefore it is crucial to position the transducer at the height of interest (e.g., external acoustic meatus to approximate pressure at the circle of Willis).

Damping is the tendency of an oscillating system to decrease oscillation amplitude. In the case of an arterial line, the systolic and diastolic readings tend to converge around the mean pressure. Damping results from medium or large air bubbles in the circuit, compliant tubing between the transducer and cannulation site, loose connections, or kinks. Resonance or whip causes falsely increased systolic readings and falsely decreased diastolic readings. It occurs when the system’s frequency of oscillation (i.e., heart rate) matches the system’s natural frequency of vibration causing whip in the signal. The classic example of this (though not easy to accomplish) is breaking a wine glass by singing a note of the same frequency as the wine glass’s resonant frequency.

4 In what situations should arterial line placement be considered?

image Inability to obtain noninvasive blood pressures.

image Hemodynamic instability. Patients who need monitoring because of extremely high blood pressure, extremely low blood pressure, or extremely volatile blood pressure.

image Need for rigorous blood pressure control. Patients who need blood pressure kept within a tight range (e.g., status post aortic aneurysm repair).

image Need for frequent arterial blood sampling. Patients with severe ventilation compromise, oxygenation compromise, or other condition where it is useful to follow serial laboratory values with an arterial line.

Choice of cannulation site is important because each site has unique benefits and risks. The radial artery is often chosen given the convenient location and good collateral supply to the hand. In patients with severe vasoconstriction, however, a femoral line may be preferable because the radial pressure may underestimate central arterial pressure.

5 How do arterial tracings differ between proximal and distal cannulation sites?

Arterial tracings become more peaked with higher systolic pressures but similar mean pressures as one transduces sites progressively distal from the aorta. The dicrotic notch representing aortic valve closure is seen in the aorta and its largest branches but becomes lost in peripheral arteries. A smaller second wave of pressure during diastole represents pressure waves reflecting off the peripheral resistance arterioles. This phenomenon can sometimes cause a pulse-oximeter to double count the pulse.

6 List indications for central line placement

Central lines are indicated for monitoring the central venous pressure, infusing concentrated vasopressors, delivering total parenteral nutrition, sampling central venous blood for analysis, and obtaining venous access when peripheral access cannot be obtained.

7 Describe the central venous waveform components. Which part of the waveform cycle should be reported as the central venous pressure?

Central venous pressures have predictable waveforms. These waveforms have upward deflections representing atrial contraction (“a” wave), ventricular contraction that causes the tricuspid valve to bulge into the atrium (“c” wave), and passive venous return of blood during diastole (“v” wave). (Note the somewhat counterintuitive fact that ventricular contraction coincides with the “c” wave, not the “v” wave.) The downslope after the “c” wave is called the “x” descent, and the downslope after the “v” wave is called the “y” descent. See Figure 6-1.

image

Figure 6-1 Central venous waveform components. IVC, Inferior vena cava; PA, pulmonary artery; RA, right atrium; RV, right ventricle; SVC, superior vena cava.

Central venous pressure should reflect the end-diastolic distention of the ventricle. Therefore the pressure measured should be immediately before systole. This corresponds to the valley immediately before the “c” wave and immediately after the “a” wave. In addition, because intrathoracic pressure (which is transmitted to the central veins) varies with respiration, pressures should be measured at end-expiration to minimize this effect on measurements.

8 List the indications for pulmonary artery catheter (PAC) placement

Indications to place a PAC include monitoring pulmonary artery pressures, measuring cardiac output by using thermodilution, assessing left ventricular filling pressures, and allowing sampling of true mixed-venous blood.

9 Describe complications of central line or PAC placement

Central line complications include pneumothorax, arterial puncture, line infection, arrhythmia, hematoma, and deep venous thrombosis. Rarer complications include thoracic duct injury and cardiac tamponade.

All the aforementioned central line complications can also arise during PAC placement. In addition, PAC placement can result in transient right bundle branch block through direct mechanical irritation of the right ventricle; therefore, PACs are relatively contraindicated in patients with left bundle branch blocks because of the potential for complete heart block. Lastly, pulmonary arterial rupture, which is usually fatal, can occur if care with the balloon tip is not taken.

10 Describe normal pressures and waveforms encountered as a PAC is advanced

The inflated balloon on the distal tip will advance with the flow of blood through the superior vena cava, right atrium, right ventricle, and ultimately the pulmonary artery. Different waveforms are obtained at each position as illustrated below.

The right atrial pressures are similar to central venous pressures described in question 7. Right ventricular pressures will have systolic components that are in phase with (i.e., occur synchronously with) systemic arterial systolic pressures and have low diastolic pressures that increase during diastole. Systolic pulmonary arterial pressures will also be in phase with systemic arterial systolic pressures and have similar waveforms that gradually decrease during diastole. The pulmonary artery occlusion pressure (PAOP)—or wedge pressure if no balloon is used—reflects the left atrial pressure and thereby left ventricular filling. Because the wedge pressure reflects the left atrial pressures, it may have “a,” “c,” and “v” waves, although in practice these may difficult to identify. Large “v” waves due to mitral regurgitation can occur on the PAOP trace, but these large “v” waves will occur during late systole and early diastole. See Figure 6-2.

image

Figure 6-2 Normal pressures and waveforms when a PAC is advanced.

11 Normally, central venous pressures gauge right ventricular end-diastolic volume and pulmonary arterial occlusion pressures gauge left ventricular end-diastolic volume. What factors alter these relationships?

Physiologic or pathologic conditions that affect the relationship between measured pressures and ventricular volumes can be understood by appreciating the following concepts:

image Transducers measure atrial pressures as surrogates for ventricular end-diastolic volumes. In patients with severely stenotic valves, pressures will be elevated and therefore overestimate the volumes.

image

image

From this it becomes clear that:

image A less compliant ventricle will have less volume for the same pressure (e.g., left ventricular hypertrophy).

image Volume depends on the relationship between pressure inside and pressure outside the ventricle. In cases where the pressures externally compressing the ventricle are high (e.g., high positive end-expiratory pressure [PEEP], increased intraabdominal pressures), the pressures inside the ventricles will overestimate the volumes. This is also the case with intraventricular dependence where a severely dilated right ventricle could compress the left ventricle.

12 Describe how thermodilution with a PAC can be used to determine cardiac output

The principle of thermodilution cardiac output is that a bolus of cold injectate will lower the temperature of the blood as it flows through the right side of the heart. Cold saline solution is bolused proximal to the right side of the heart, and the temperature is measured by the distal tip of the catheter in the pulmonary artery. Higher blood flow (i.e., cardiac output) means the cold is diluted in a larger volume of blood, and smaller temperature changes will be measured. Conversely, low cardiac output shows high temperature change with the bolus, because the cold bolus comprises a much higher portion of the flow that passes the temperature probe.

The modified Stewart-Hamilton equation describes this:

image

where CO is cardiac output, Tbody is the temperature of the body, Tinjectate is the temperature of the injectate, V1 is the volume of the injectate, K1 represents properties specific to the catheter and measuring system, and AUC is the area under the curve of the temperature change. In simple terms, the equation relates that flow (a volume per time) is equal to quantity (of cold) per time divided by concentration (in this case a temperature concentration). Again, even more simply:

image

13 How is a fluid challenge useful in the intensive care unit (ICU) setting?

Determining volume status (i.e., which side of the Frank-Starling curve a patient is on) in the ICU can be very difficult but remains important to thoughtfully optimize hemodynamics. A time-honored test is to give a fluid bolus and look for a change in hemodynamics. If a bolus has a salutary effect (increase in blood pressure or cardiac output), then the patient is likely on the ascending portion of the Frank-Starling curve, and another fluid bolus may be indicated to improve hemodynamics. If a fluid bolus produces minimal effect, then it is likely that the patient is near the top of the Frank-Starling curve. If a fluid bolus causes deterioration in the blood pressure or cardiac output, then the patient may be volume overloaded on the descending portion of the curve.

A similar test of volume status is the passive leg raise. To perform this test, blood pressure or cardiac output is measured with the patient supine. The patient’s legs are then passively elevated, delivering a reversible autotransfusion of approximately 500 mL of blood (i.e., blood rushes to the central veins from the legs because of hydrostatic pressure). If the passive leg raise improves hemodynamics, additional volume may be indicated. If the effect is negative, it can be quickly reversed by lowering the legs, which is an advantage over the fluid bolus technique where the bolus cannot be quickly removed.

14 Can arterial lines tell us anything more than pressure?

Arterial lines can be used to assess both fluid responsiveness and cardiac output. With the ubiquity of arterial lines in ICUs, these techniques can be very useful.

image Fluid responsiveness: If central veins are not adequately filled (i.e., with a patient with hypovolemia), changes in intrathoracic pressure during positive-pressure variation can result in highly significant effects on cardiac output and blood pressure. Variation in pulse pressure or systolic pressure of greater than 10% to 12% is predictive of positive fluid responsiveness. Note that this technique holds only for sedated patients (with no spontaneous respirations) receiving positive-pressure ventilation with regular cardiac rhythm.

image Cardiac output: As the arterial pulse pressure waveform reflects the stroke volume, algorithms have been developed to continuously monitor cardiac output through waveform analysis. These techniques are challenged by nonlinearity of pressure to volume translation, damping and resonance issues, and flow-independent systemic vascular resistance changes.

15 What are transpulmonary thermodilution and transpulmonary lithium dilution?

Similar to thermodilution with PACs (see question 12), these techniques use the Stewart-Hamilton equation to approximate cardiac output using temperature (in the case of thermodilution) or lithium concentration (in the case of lithium dilution). Boluses (of either cold injectate or lithium) are injected in central veins, and temperatures or lithium concentrations are measured in arterial lines. Commercial applications typically use these techniques to calibrate some form of arterial waveform analysis to provide a continuous determination of cardiac output (e.g., the PiCCO and LiDCO devices).

16 What is impedance cardiac output?

With impedance cardiac output, current is applied across the chest and the impedance (resistance) is calculated. With each heartbeat, the impedance decreases (as the low-resistance blood-filled aorta expands). Combining the impedance change and heart rate allows estimation of the cardiac output. Limitations of this technique in the critically ill have resulted in lack of widespread adoption.

17 How is Fick’s principle used to measure cardiac output?

Fick’s principle allows highly accurate calculation of cardiac output. Total oxygen consumption, VO2, in the tissues must equal the delivered oxygen less the returned oxygen. Delivered oxygen is equal to cardiac output multiplied by arterial oxygen content (Ca, determined in a systemic artery). Returned oxygen is equal to cardiac output multiplied by mixed venous oxygen content (Cv, determined in the pulmonary artery). Oxygen consumption can also be measured directly with a closed-loop spirometer. Setting these two equal to each other allows us to generate the following equation:

image

Rearranging, cardiac output is calculated as

image

In this equation, oxygen content of blood, C, is calculated as

image

where Hgb is hemoglobin (measured in grams per deciliter), SpO2 is hemoglobin saturation with oxygen (measured as a percentage), and PO2 is the partial pressure of oxygen in blood (measured in millimeters of mercury).

18 How is cardiac output measured with transesophageal aortic Doppler?

Doppler technology relies on frequency change of sound waves as they reflect off moving objects. With transesophageal aortic Doppler, the Doppler probe lies in the esophagus and reflects sound from the blood pumping through the aorta, and the velocity of blood in the aorta is determined. By assuming an aortic diameter (based on height, weight, age, sex) or measuring the diameter directly, a flow can be calculated (flow = area × velocity time integral × heart rate). Because this calculates flow only in the descending aorta, a certain percentage (typically around 30%) is added to determine total cardiac output.

19 Describe some of the general applications of bedside ultrasound examination to monitor hemodynamics in the ICU

Ultrasound scan provides real-time noninvasive data, and its utility in critical care is tremendous and growing. To cover ICU ultrasound applications in detail is beyond the scope of this chapter, but certain key points bear mentioning. Ultrasound can be used to directly assess cardiac output and function (stroke volume, contractility/ventricular function, valvular function) with transthoracic or transesophageal echocardiography. Fluid responsiveness can be gauged by measuring the change in inferior vena cava diameter during a respiratory cycle, similar to assessing systolic pressure variation, in a patient receiving mechanical ventilation. The presence of pulmonary edema can be seen, but not quantified, by ultrasound examination of the lung. Technical limitations make the usefulness of ultrasound quite variable. Also, it is a very operator-dependent technology, and specific training coupled with frequent practice is required.

20 Is it possible to look more directly at tissue perfusion?

Many methods to assess tissue perfusion (the ultimate gauge of adequate hemodynamics) have been and are being developed. Although many more exist than are covered in this chapter, some include

image Gastric tonometry: Carbon dioxide accumulation in gut mucosa typically results from decreased perfusion. Therefore measurement of gastric carbon dioxide via nasogastric or orogastric tube can provide a gauge for whether tissue perfusion is adequate.

image Tissue oxygenation (StO2): Tissue oxygenation is a measure of the ratio of oxygenated to nonoxygenated hemoglobin in a sample of tissue. It differs from arterial oxygenation (SpO2), which represents systemic arterial oxygenation, in that tissue oxygenation is a local measurement at the microcirculation level. Think of this as a pulse oximeter for muscle or deeper tissues.

image Confocal microscopy: With use of confocal techniques, these microscopes can look under the skin or mucosa of the tongue. Confocal microscopes can actually watch red blood cells moving through capillaries. This technology is not currently being used for bedside monitoring, but it is often cited as a research tool when studying perfusion.

Key Points Hemodynamic Monitoringimage

1. Hemodynamic monitoring is our way of attempting to determine whether tissue perfusion is adequate; it provides data to guide therapy but is not by itself therapeutic.

2. Arterial transducers must first be zeroed and leveled to eliminate the effects of atmospheric pressure and hydrostatic pressure on readings. In addition, system readings should be monitored for damping (where readings erroneously converge to the mean pressure) and resonance (where readings erroneously diverge from the mean pressure).

3. Central lines have predictable waveforms with upward deflections during atrial contraction (the “a” wave), ventricular contraction (the “c” wave), and passive venous return (the “v” wave). To gauge end-diastolic volume, the pressure reported should be at the valley immediately preceding the “c” wave.

4. Fluid responsiveness in critically ill patients can be assessed with fluid challenge, the passive leg raise test, systolic pressure variation, pulse pressure variation, ultrasound examination of the inferior vena cava, and several other methods.

Bibliography

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