Hemodynamic Monitoring Case File

Hemodynamic Monitoring Case File

Eugene C. Toy, MD, Manuel Suarez, MD, FACCP, Terrence H. Liu, MD, MPH

Case 4

A 55-year-old man with a long-standing history of coronary artery disease is admit­ted to the ICU with hypotension following a 24-hour episode of intermittent chest pain. While on an  IV nitroglycerin  drip  ,  he is free  of chest pain. On the second hospital day,  he suddenly develops chest pain, shortness of breath, and a change in mental  status. A catheter is placed in his pulmonary artery and provided the following hemodynamic readings: central  venous pressure (CVP) 12 mm Hg (0-5 mm Hg), pulmonary artery pressure  (PAP)  40/15  mm Hg  (20-25/5-10 mm Hg), pulmonary capillary occlusion "wedge" (PCW) pressure 18 mm Hg (6-12 mm Hg), and cardiac output (CO) 3.0 L/min (4-8 L/min). On physical examination his tem­perature is 38.2°(, heart  rate 140  beats/minute, blood pressure  75/45 mm Hg, and respiration rate 35 breaths/minute. The jugular venous pressure is difficult to assess. Auscultation of the lungs reveals bilateral  rales. The cardiac examination reveals a regular rhythm, a normal S1 and accentuated S2, and a new S3 gallop. The legs are noted to have bilateral pitting edema to the level of the knees, and palpa­tion reveals cool extremities and weak pulses.

⯈What is the most likely diagnosis?

⯈What are the best next steps in treatment of this patient?

ANSWER TO CASE 4:

Hemodynamic Monitoring

Summary: This 55-year-old man with coronary artery disease has an acute decom­pensation in his status. The data from the hemodynamic monitoring (HM) reveal evidence for cardiogenic shock (CS), with hypotension, hypoperfusion, decreased mental status, and cool extremities. There is a new S3 gallop. The PAC data indi­cates a volume overload with an elevated central venous and PCW pressure. 

• Most likely diagnosis: Cardiogenic shock based on elevated pulmonary capil­lary wedge pressure is elevated with low cardiac output. 
• Best treatment steps: Begin cardiac inotropes; optimize IV fluids according to CO and PCWP. Begin therapy with afterload reducing agents and then adjust therapy guided by the PAC and CO readings. An  aortic balloon pump may also be indicated. 

ANALYSIS

Objectives

1. To understand the goals of hemodynamic monitoring. 
2. To appreciate the various forms available for acute hemodynamic monitoring. 
3. To be able to interpret data from hemodynamic monitoring.

Considerations

This is a 55-year-old man with unstable angina requiring a nitroglycerin drip. On the second hospital day, he suddenly decompensates and is noted to be in cardia­genic shock. His BP is low, cardiac output low, and he has developed pulmonary edema. The most likely cause is cardiac pump failure. He is   in need for quick reversal of organ hypoperfusion with fluids and vasopressors, including the possible use of an intra-aortic balloon pump to bridge him to a definitive intervention such as open heart surgery with coronary artery bypass. At this juncture, the patient is critically ill, and timely and accurate diagnosis and intervention are critical to his survival. Invasive hemodynamic monitoring helps optimize fluid and vasopressor/inotropic intervention.

Approach To:

Hemodynamic Monitoring

HEMODYNAMIC MONITORING GOALS 

The goal of HM is to evaluate the vital signs needed to maintain adequate tissue perfusion. HM can be accomplished by noninvasive (preferred) or invasive means. Continuous monitoring allows an early recognition of poor tissue perfusion based 

on low blood flow. Invasive PAC provides data on systemic, pulmonary arterial and venous pressures, and measurements of CO (cardiac output). Since the flow of blood into organs cannot be measured directly, normal BP and, more specifically, mean arterial pressure (MAP >60 mm Hg and urine output) are used as indirect indica­tors of adequate tissue perfusion. In the ICU, hypotension has been identified as the most common cause of hemodynamic instability. Assuming that the CVP and PAOP are adequate estimates of the volume of the systemic and pulmonary circula­tion, respectively, one can create a working relationship between CVP and PAOP, and CO or stroke volume (SV). This can be plotted in a Starling curve, pointing to the optimum range of end diastolic volume and CO for each patient. Misleading readings  can occur with abnormal pressure/volume relationship (compliance) of the RV or LV,  increased intrathoracic pressure (PEEP, auto PEEP, intra-abdominal pres­sure), and valvular heart disease (mitral stenosis).

CVP is often used as the sole guide to monitor hemodynamic function. Tech­niques such as echocardiography, transesophageal echocardiography, Doppler, and volume-based monitoring can be used. No single monitoring technique has been demonstrated to improve patient outcome. Assuring the veracity of the  data is key to the correct interpretation. Time is crucial for an early diagnosis of a hemo­dynamic catastrophe and the early detection and application of effective therapy. Trends of data information are more reliable than single data points.

Monitors 

Critically ill patients require continuous monitoring to diagnose and manage their complex medical conditions. This is most commonly achieved by using direct pres­sure monitoring systems. PaP measurements are much less commonly used today than previously. Monitoring central venous pressure (CVP) and intra-arterial blood pressure (BP) are common approaches to evaluate hemodynamic functioning. Most facilities use   noninvasive monitoring of hemodynamic functions of BP and other basic vital signs.

Continuous Vital Signs 

Modem electronic devices continually monitor up to 5 vital signs (heart rate, res­piration rate, skin temperature, oxygen saturation, and blood pressure). The data is entered into a 5-point index that is constantly on display. Nursing staff can review these vital signs and the patient status index regularly to identify patients expe­riencing distress. This improves the prospect to stabilize the patient and initiate goal-directed therapy to recover from these abnormalities. This should lead to fewer unplanned admissions to ICUs and minimize cardiac arrests occurring out-of-intensive care units, thereby significantly improving cost and life savings. 

Monitoring of Cardiac Function 

The assessment of ventricular function is based on the measurement of both blood volume and pressure. The ejection fraction (EF) as left ventricular dp/dtmax has been widely accepted as an index of the contractile performance of the left ven-tricle. Measurement of left ventricular dp/dtmax is a satisfactory index of ventricular max contractility. 

ECG Monitoring 

All ICU patients have continuous ECG monitoring. The diagnosis of arrhythmias and the commencement of rapid treatment is a  goal of hemodynamic monitoring. The waveform display is arranged to monitor Leads I, II,  or V,  whichever provide the tallest QRS complex. Alarms must be set and placed in the "on position" at all times. High and low alarm settings are assessed and documented in the graphic record. Upper and lower alarm limits are selected for each individual patient. The rhythm strip is for rhythm interpretation; for the evaluation of morphology a 12-lead ECG is needed.

Intra-arterial Blood Pressure Monitoring

Intra-arterial BP monitoring is used to obtain direct and continuous BP measure­ments in ICU patients who have hypertension or   hypotension. Arterial blood gas measurements and blood sampling can be obtained repeatedly. If no collateral cir­culation exists and the cannulated artery becomes occluded, ischemia and infarc­tion of the area distal to that artery could occur. To  check collateral circulation to the hand, use the Allen test to evaluate the radial and ulnar arteries or use an ultrasonic Doppler to evaluate any of the arteries. In the Allen test, the radial and ulnar arteries are compressed simultaneously. The Allen test confirms the presence of collateral circulation. If this collateral circulation is not confirmed, arterial blood gases (ABGs) using that site should be avoided. The preparation of sites for arterial lines and their subsequent care are the same as for CVP catheters. Complications in the use of arterial lines include local obstruction with distal ischemia, external hemorrhage, massive ecchymosis with compartmental syn­drome, dissection, air embolism, blood loss, pain, arteriospasm, and infection. Blood pressure readings are   more commonly obtained by automatic self-inflating cuff devices. Under most circumstances these produce comparable blood pressure results when compared to arterial lines. BP cuffs should be avoided on arms with shunts to avoid occlusion. 

To  Catheterize or Not to Catheterize? 

In the past, the pulmonary artery balloon tip thermodilution catheter (PAC) has been the gold standard for evaluating the circulatory function of patients in the I CU. A higher mortality rate has been reported in patients in  whom a PAC was inserted. A PAC-based hemodynamic management is successful in improving patient out­come in instances of planned major surgery. This benefit has not been translated to the ICU patient. PAC-based monitoring has its limitations, the most significant being the misinterpretation of the   data being produced even by well-trained inten­sivists and cardiologists.  PAC monitoring provides measurements of PaP, pulmonary capillary wedge pressure (PCWP or PAOP), and right atrial pressure.  Flow variables such as CO and mixed venous O2 (Svo2) are also measured. Calculations of sys­temic and pulmonary vascular resistance (SVR, PVR), right ventricular stroke work, and left ventricular stroke work (LVSW) are obtained. There was no significant dif­ference regarding outcome or postoperative complications whether managed with a CVP or a PAC (see Table 4-1). 

^*Different subsets of diseases have different values

⁺Pulm BF = pulmonary blood flow; System BF = systemic blood flow.

Central Venous Catheter

PAC has not improved survival or organ function and has been associated with more complications than CVP-guided therapy. Newer CVP catheters provide a continuous recording of O2 saturation, which helps in maintaining venous O2sat goals at > 70% to 7 5%. CVP catheters should be placed with guidance from ultrasound. PIC lines are also an option as a central line.

PAOP does not always reflect left ventricular end diastolic volume (LVEDV) . The CVP and PAOP parallel each other closely in patients with EF >50%. I n EF <40%, the correlation between the CVP and PAOP decreases due to changes in myocardial compliance caused by myocardial hypertrophy or a stiff LV. Patients with PAC placement receive more fluid in the first 24 hours and have an increased incidence of renal failure and thrombocytopenia than with CVP use. A line infection is one of the most important risks. Complications arose in approximately 20% of the instances in which a catheter was left in place for more than 6 days.

Mixed Venous Oxygen Saturation

Continuous monitoring of venous oxygen saturation (Svo2) by reflectometry immediately detects trends and abrupt changes in the oxygen supply-to-demand ratio. Svo2 has been promoted as an indicator of changes in CO. Normal values for Svo2 range from 70% to 75%. A linear correlation has been demonstrated between the CO and Svo2. Svo2 reflects the overall oxygen reserve of the whole body. A normal Svo2 value does not rule out an impaired oxygen supply to individual organs. The pulmonary artery carries blood from all vascular beds of the body; thus, Svo2 represents the amount of oxygen in the systemic circulation that is left after passage of the blood through the tissues. Svo2 thus serves as a measure of global oxygenation. The determinants of Svo2 are SA02, systemic VO2, CO, and Hb.

Svo2 = ( SAO2 - VO2)/(1.39 x Hb x CO)

An increase in VO2 and a decrease in Hb, CO, and arterial oxygenation will result in a decrease of Svo2. Interpretation of Svo2 values might be difficult in conditions where DO2/VO2 relationships are altered. Arterial-venous microcirculatory shunting in sepsis may increase Svo2 tissue oxygenation while regional tissue dysoxia is present.

Monitoring of the Right Ventricle

The right ventricle is responsible for accepting venous blood and pumping it through the pulmonary circulation. Circulatory homeostasis depends on an adequate function and synchronization of both ventricles. Changes in the dimensions of one ventricle influence the geometry of the other. Monitoring of CVP has demonstrated its value in judging right ventricular function. Moreover, the use of RVEDV and RVEF is unaffected by arbitrary and poorly reproducible zero points for pressure transducers. RVEF by thermodilution measurements is easy to perform.

Measurement of Extravascular Lung Water and Intrathoracic Blood Volume

Depressed left ventricular performance increases hydrostatic pressure in the pulmonary circulation, influencing fluid flux across a damaged pulmonary microvascular membrane. Extravascular lung water ( EVLW) can be measured at the bedside using a double-dye technique with indocyanine green. Intrathoracic blood volume appears to be a more reliable indicator of preload than cardiac filling pressure.

Echocardiography

Assessing global and regional LVF is the domain of echocardiography, via either transthoracic echocardiography (TTE) or transesophageal echocardiography (TEE) . Two-dimensional echocardiography provides significant information including left ventricular cavity size, fractional shortening, and abnormalities in regional wall motion. Two-dimensional colored echocardiography enables a quantification of shunts, CO, and provides a noninvasive assessment of concomitant valvular disease. The presence and extent of ischemic heart disease is determined by monitoring segmental wall motion. These abnormalities are indirect markers of myocardial perfusion that can persist for prolonged periods in the absence of infarction. TEE provides more accurate information on ventricular size than standard TTE. End diastolic volume (LVEDV) is a better predictor of myocardial performance than PAOP. Echocardiography is the first diagnostic method which should be used when suspicion of aortic dissection, endocarditis, or pulmonary embolism with hemodynamic instability are believed to exist. Hypovolemia, left ventricular failure, global systolic function, and the size of both ventricles can be rapidly identified using TTE/TEE. Valvular abnormalities and functionally important heart disease can be readily determined.

Monitoring of Organ Perfusion and Microcirculation

Monitoring of tissue oxygenation and organ function in the clinical setting is based on measuring variables of global hemodynamics, pulse oximetry, capillary refill, urine output, or by the use of indirect biochemical markers. These parameters remain insensitive indicators of dysoxia and are considered poor surrogates for measuring O2 at the tissue levels. The net balance between cellular O2 supply and O2 demand determines the status of tissue oxygenation. Regional tissue dysoxia can persist despite the presence of adequate systemic blood flow, pressure, and arterial oxygen content.

Oxygen Delivery and Oxygen Consumption

Total body perfusion and oxygenation relies on an adequate arterial oxygen saturation (SaO2) , appropriate hemoglobin (Hb ) concentration, and cardiac output. The total amount of oxygen delivered to the peripheral tissue per minute (DO2) can be calculated as DO2 = CO x CaO2, with CaO2 = (Hb x 1.39 x SaO2) . In steady state conditions, the uptake of oxygen from the arterial blood (YO) represents the sum of all oxidative metabolic reactions in the body. VO2 can be measured by analysis of the expired gas or calculated from CO and arterial and mixed venous blood samples. The VO2/DO2 is the oxygen extraction ratio. VO2/DO2 dependency occurs when the increase in oxygen extraction can no longer fully compensate for the fall in DO2. The relationship between DO2 and VO2 can therefore be used to assess the adequacy of tissue oxygenation. The determination of DO2 and VO2 requires right heart catheterization to measure CO.

Blood Lactate Level

Arterial blood lactate levels in critically ill patients have proven very useful Lactate is formed from pyruvate by the cytosolic enzyme lactate dehydrogenase. A lactate concentration > 2 mmol/L is generally considered a biochemical indicator of inadequate oxygenation. Circulatory failure with impaired tissue perfusion is the most common cause of lactic acidosis. Mechanisms other than impaired tissue oxygenation may cause an increase in blood lactate, including an activation of glycolysis, a reduction in pyruvate dehydrogenase activity, or liver failure. The complex process of tissue lactate production and its utilization mandates an understanding of the usefulness and limitations of blood lactate levels. Elevated lactate levels should prompt the clinician to initiate procedures for assessment of the circulatory status.

Respiratory Monitoring

Breathing waves are generated by the standard 3 ECG leads used in the ICU for ECG rhythm monitoring. A change in breathing rate disturbs the electrical triangle formed by the leads measuring respiratory rate and apneas. This method is not accurate and is predisposed to deliver incorrect data. Noninvasive monitoring of pulmonary function is most important in the mechanically ventilated patient. The respiratory system requires the generation of pressure for the inflation needed to overcome resistive and elastic properties of the lung. Resistance is located mainly in the airways. Several techniques are available to measure respiratory mechanics, but the most practical method is the rapid airway occlusion technique. This technique estimates the elastic recoil pressure of the alveoli by measuring the inspiratory plateau airway pressure ( Pplat ) . Functional lung monitoring has questionable prognostic value and is of limited use in daily clinical practice. Bedside monitoring of static compliance and Pplat should be used routinely to detect the presence of alveolar overdistention and at least qualitatively assess the risk for volume-induced lung injury (VILI ) . An important factor in respiratory mechanics is intrinsic positive end-expiratory pressure ( PEEPi) . This is commonly measured using end-expiratory airway occlusion. PEEPi causes decreased cardiac output, alveolar overdistention, increased work of breathing, and patient-ventilator asynchrony. If neglected, PEEPi leads to an underestimation of compliance.

O2 Saturation

The determination of O2 saturation (O2sat) via pulse oximeters is a valuable adjunct to clinical oxygen monitoring. When properly applied, it reliably indicates the patient's HR and arterial oxygen saturation. ECG synchronization reduces motion artifacts when the ECG R wave is detected. The diagnosis of hypoxemia requires an arterial blood gas analysis and is commonly defined as a PAO2 of <60 mm Hg or O2Sat <90%. Pulse oximetry is commonly used for assessing hypoxemia. However, this modality measures the saturation of hemoglobin and not P AO2, reflecting oxygen dissolved in the blood, which includes both bound and unbound O2. Thus, a patient with severe anemia may have a normal PAO2 but a low O2 content. Low pulse oximetry values <90% coincide with significant hypoxemia, but normal oxygen saturation does not exclude hypoxemia, especially in patients receiving a high FIO2.

Normal PAO2 levels are 80 to 100 mm Hg in a healthy patient. Pulse oximetry values may remain normal until PAO2 decreases to <60 mm Hg. For this reason, the alveolar-arterial oxygen gradient should be evaluated in patients receiving a high FIO2. A widening alveolar-arterial oxygen gradient is a sign of an increasing hypoxemia. Pulse oximetry may be unreliable in cases of severe anemia, carbon monoxide poisoning, methemoglobinemia, or peripheral vasoconstriction.

End- Tidal CO2 Monitoring

End-tidal CO2 monitoring is now standard in intraoperative care. This lack of bedside monitoring is particularly significant because the most common form of respiratory monitoring is normal pulse oximetry. Capnometry or end-tidal volume CO2 (EtCO2 ) monitoring is used to evaluate the PACO2 level during surgery and in intubated patients in the ICU. In children, manual ventilation with EtCO2 monitoring resulted in increased PACO2 readings falling within the intended range.

Urinary Bladder Pressure

Measurement of intra-abdominal pressure (IAP) is accomplished via the use of Foley bladder balloons in critically ill patients. Monitoring IAP to avoid and detect abdominal compartment syndrome is increasingly recommended and is advocated in monitoring patients after abdominal surgery. IAP is usually estimated indirectly by measuring intrabladder pressure (IBP).

Electroencephalographic Monitoring

Continuous EEG monitoring ( CEEG ) is a powerful tool for evaluating cerebral function in obtunded and comatose patients. Ongoing analysis of CEEG data is a major task because of the amount of data generated and the near real-time interpretation of a patient's EEG. Methods such as the computerized detection of seizures have increasingly allowed focused reviews of EEG epochs of interest. These allow personnel and inexperienced staff to recognize significant EEG changes in a timely fashion.

Esophageal Pressure Measurement

The chest wall includes the abdomen. Abdominal pathology affects respiratory mechanics. Eesophageal pressure and airway pressure define the contribution of each of these compartments to respiratory mechanics and particularly to compliance. Supine measurements are less reliable.

Near-Infrared Spectroscopy

Near-infrared spectroscopy (NIRS ) is a noninvasive way to measure oxygenated and deoxygenated Hb as well as the redox state of cytochrome 3 as an average value of arterial, venous, and capillary blood. It has been used primarily in studies of cerebral or muscle oxygenation after different types of hypoxic injuries.

CLINICAL CASE CORRELATION

• See also Case 5 (Vasoactive Drugs) , and Case 16 (Acute Cardiac Failure).

COMPREHENSION QUESTIONS

4.1 A 45-year-old man is admitted to the ICU after a motor vehicle accident. The nurse calls to notify you of a continuous venous O2 saturation which has been dropping steadily over the last few hours from 7 5 % to 65%. What is the most likely cause ?

A. CHF Stage 1

B. Noncompressible arterial disease

C. Peripheral venous disease

D. Systemic hypoperfusion

E. COPD

4.2 A 20-year-old pregnant woman develops a urinary tract infection with positive blood cultures. She is admitted to the ICU with a blood pressure of 88/52 mm Hg, which has persisted despite fluid challenge. Her condition deteriorates as she develops increasing respiratory distress. She appears to be developing adult respiratory distress syndrome (ARDS) and is intubated for mechanical ventilation. The resident staff inserts a right heart catheter to measure pulmonary vascular pressure. Which of the following HM findings is likely to be seen in this case ?

A. Low wedge pressure, low cardiac output

B. Low wedge pressure, high cardiac output

C. High wedge pressure, low cardiac output

D. High wedge pressure, high cardiac output

E. Normal cardiac output, normal wedge pressure

ANSWERS TO QUESTIONS

4.1 D. Systemic hypoperfusion with increased oxygen uptake by the tissues and decreased delivery of oxygen, especially to organs with high oxygen demands decreases venous O2 saturation. The increased difference in oxygen consumption and lower venous O2Sat is a direct indicator of some form of decreased oxygen delivery, such as decreased cardiac output, severe anemia, respiratory failure (ARDS) , hypoxemia, and sepsis.

4.2 A. In a patient with ARDS due to sepsis, one would expect the PCWP to range between normal and low and the CO to be low. Considering the gram-negative uroseptic syndrome described in this case, endotoxin from the infecting bacteria has acted as a circulating cardiosupressant. The Svo2 measurements will direct treatment, with a goal of attaining an Svo2Sat >70% to 75 %. Dobutamine is indicated to treat the decreased CO, which was induced by the gram-negative septic shock.

CLINICAL PEARLS

⯈ There  is no specific monitoring technique that  is  known to  improve patient outcome. 

⯈ PAC has not been associated with improvement in patient outcomes. 

⯈ PAC is now rarely used except in selected cases. 

⯈ An MAP of 60 mm Hg is a target associated with signs of adequate urine output and MS. 

⯈ CVP and PAOP are  comparable in patients with EF >50%  and indicate end diastolic volume. 

⯈ Maintaining plateau alveolar pressures at <30 cm H20 reduces alveolar strain, and barotrauma. 

⯈ PEEP is clinically titrated by measuring its effects on gas exchange and on hemodynamics. 

⯈ The mechanical characteristics of the respiratory  system are compliance, resistance, and intrinsic PEEP; all can be measured using standard venti­lators and bedside maneuvers. 

⯈ Monitoring esophageal pressure can  help  assess the extent of alveolar strain from PEEP. 

References

Loscalzo J. Hamson's Pulmonary and Critical Care Medicine. New York, NY: McGraw-Hill; 2010. 

Pulmonary-Artery versus Central Venous Catheter to Guide Treatment of Acute Lung Inj ury. The National Heart, Lung, and Blood Institute Acute Respiratory Distress Syndrome (ARDS ) Clinical Trials Network. N Eng! ] Med . 2006;3 5 4 : 2 2 1 3 -2224. 

Summerhill EM, Baram M. Principles of pulmonary artery catheterization in the critically ill. Lung. 2005 ; 1 83 : 209-2 1 9 . [PMID: 1 607 8042]

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