Hemodynamic monitoring is directed at assessing the results of resuscitation and maintaining organ and tissue perfusion. Currently used measures are only estimates of tissue perfusion because the measurement of oxygen and nutrient transfer to cells cannot be made directly at the bedside. Instead, global physiologic measures of central pressures still serve as the principal guides.

Measurement of arterial blood pressure is the mainstay for the assessment of tissue perfusion. In critical illness, this measurement can be made using cuff sphygmomanometers; however, in practice this technique is not useful because the measurement is episodic, and placement of these cuffs on burned extremities is problematic. Diastolic pressures can also be artificially elevated in the elderly and obese. Instead, continuous monitoring for hemodynamic instability through the use of intraarterial catheters is generally preferable when the patient is in the BICU for a prolonged period. Lines are typically placed in either the radial or the femoral artery. The radial artery is the preferred site for critically ill patients because of safety, with the dual arterial supply to the hand as backup should a complication arise. However, it has been shown that radial artery catheters are inaccurate in the measurement of central blood pressure when vasopressors are used and are notoriously inaccurate in children because of greater vascular reactivity. Furthermore, femoral cannulation sites are often unburned due to the insulation provided by undergarments, and they do not preclude mobilization with physical therapy or rehabilitation goals. For these reasons, we recommend femoral arterial blood pressure measurement in most burn patients.

For arterial catheters, systolic, diastolic, and mean arterial pressures (MAPs) should be displayed continuously on the monitor screen. Either systolic or MAP can be used to determine adequacy of pressure, although a MAP greater than 70 mmHg is considered a more accurate descriptor of normal tissue perfusion on the whole. Reasons for this include the finding that, as the arterial pressure wave traverses proximally to distal, the systolic pressure gradually increases while the diastolic pressure decreases; the MAP determined by integrating areas under the curve, however, remains constant. The adequacy of the waveform must also be determined, with a diminished waveform indicative of catheter damping, which requires catheter replacement. Care must be taken to ensure that the diminished waveform is not true hypotension, which can be determined using a manual or cycling sphygmomanometer. Exaggerated waveforms with elevated systolic pressure and additional peaks in the waveform (generally only two are found) may be a phenomenon known as catheter whip, which is the result of excessive movement of the catheter within the artery. Typically, this problem is self-limited, but care must be taken not to interpret normal systolic blood pressure values with evidence of catheter whipping as unexceptional because the effect generally overestimates pressures. Again, use of MAP as the principal guideline for the assessment of blood pressure is optimal, as effects of catheter whip or other problems with intraarterial monitoring are then diminished.

Complications associated with arterial catheters include distal ischemia associated with vasospasm and thromboembolism, catheter infection, and arterial damage/pseudoaneurysm during insertion and removal. Although these complications are uncommon, the results can be devastating. Physical evidence of ischemia in the distal hand or foot should prompt immediate removal of the catheter and elevation of the extremity. If improvement in ischemic symptoms is not seen promptly (within an hour), angiography and intervention must be considered. Should thromboembolism be found, the clot can be removed with operative embolectomy or clot lysis at the discretion of the treating physician. If, during angiography, extensive arterial damage is found with ischemia, operative repair may be indicated. Consideration for anticoagulation must be made while balancing the risk of hemorrhage from open wounds vs. the benefit of tissue salvage.

Evidence of catheter infection hallmarked by purulence and surrounding erythema should instigate removal of the catheter, which often will suffice. With continued evidence of infection, antibiotics and incision and drainage of the site should be entertained. Great caution must be exercised to avoid arterial bleeding if an incision is made over the catheter site. If a pseudoaneurysm is encountered after arterial catheterization and removal without signs of distal ischemia, injection of thrombin or compression with a vascular ultrasound device until no further flow is seen in the pseudoaneurysm will often alleviate the problem without operative intervention.

Pulmonary artery catheters placed percutaneously through a central vein (internal jugular, subclavian, or femoral) and floated into the pulmonary artery through the right heart have been used extensively in hemodynamic monitoring in BICUs. By measuring the back pressure through the distal catheter tip wedged into an end-pulmonary branch, an estimate of left atrial pressure can be measured. In addition, dyes or isotonic solutions injected into a proximal port can be used to determine cardiac output from the right heart. These data are used to estimate preload delivery to the heart, cardiac contractility, and afterload against which the heart must pump, which then directs therapy at restoration of hemodynamics. These catheters are used in BICUs under conditions of unexplained shock, hypoxemia, renal failure, and monitoring of high-risk patients.

The use of pulmonary artery catheters, however, has come under scrutiny from reports indicating no benefit from their use. A study of 5735 critically ill adults in medical and surgical ICUs showed an increase in mortality and use of resources when pulmonary artery catheters were used. Most of these patients had medical conditions. The authors of this report suggested that their results should prompt a critical evaluation of the use of pulmonary artery catheters under all conditions. This was followed by a clinical trial in the United Kingdom demonstrating no benefit from the use of pulmonary artery catheters in a general ICU setting. A more recent evaluation of the usefulness of these devices has demonstrated that, with proper training and in the appropriate setting, they can provide data not available through other modalities. ^, Over the past years, use of pulmonary artery catheters has significantly diminished except in special circumstances, such as unexpected response to treatment, as in volume replacement for oliguria. Even in this condition, new technology based on arterial waveform analysis gives an estimate of cardiac output and end-diastolic volume, which generally gives enough information to guide appropriate therapy. However, in the appropriate patients, pulmonary artery catheters may still play a valuable role.

Multiple devices have been developed over the past decade using arterial waveform analysis to continuously measure cardiac output as well as to estimate preload. Stroke volume (SV) variation provides a good estimate of the fluid responsiveness of shock with only arterial access. The transpulmonary thermodilution technique provides an even more complete hemodynamic dataset without the use of a pulmonary artery catheter. Using only a central line and central arterial line, thermodilution allows monitoring of preload with global end-diastolic volume index, intrathoracic blood volume, continuous cardiac output, and extravascular lung water index. Numerous studies have shown that these volumetric indices represent preload more precisely than urine output or cardiac filling pressures. In a study involving 54 burned children, Herndon et al. determined pulse index continuous cardiac output to be the superior measurement for cardiac parameters to transthoracic echocardiography and an objective cardiovascular monitor to guide goal-directed fluid resuscitation.

Transesophageal echocardiography has been used for a number of years as an intraoperative monitor in high-risk cardiovascular patients. It has not been used extensively in other critically ill patients because of the lack of available expertise and paucity of equipment. Since this device can be used as a diagnostic tool for the evaluation of hemodynamic function, it stands to reason that it could be used to monitor critically ill, severely burned patients. A report documented the use of transesophageal Doppler measurements of cardiac output in a series of severely burned patients and showed that intravascular volume and cardiac contractility are significantly diminished the first day after burn in spite of high-volume resuscitation.

Echocardiography has also been studied as a means to supplement urine output monitoring. Investigators in China examined whether esophageal Doppler monitoring of heart function might be an improvement by studying 21 patients with massive burns (79 ± 8% TBSA burned) who were resuscitated with a goal of 1 mL/kg/hour. They found that cardiac output was predictably low after injury and increased linearly with time by increases in preload and contractility and decreased afterload. However, changes in cardiac output were most closely associated with increased cardiac contractility and decreased afterload rather than increases in preload. Additionally, urine output was not closely associated with cardiac output. Held et al. evaluated 11 adult burn patients with a mean TBSA of 37% and found that changes in volume status on echocardiography preceded changes in urine output and vital signs, and they were able to titrate inotropes and vasopressors in elderly patients.

These results call into question the validity of urine output as the primary measure of the adequacy of resuscitation. A similar study by investigators in Sweden probing the role of cardiac function, as measured by echocardiography, and myocyte damage, as measured by troponin abundance in the serum, showed that half of their patients had myocardial damage during resuscitation universally associated with some temporary cardiac wall motion abnormality. However, systolic function was not adversely affected. Bedside echocardiographic equipment and skills are increasingly common in BICUs and are an increasingly common means of hemodynamic monitoring in critical care. However, the intermittent nature of this procedural assessment allows it to only serve as a useful adjunct to add clarity to a difficult clinical scenario and prevents echocardiography from supplanting continuous monitoring modalities, such as thermodilution or waveform analysis. We look forward to further work regarding the optimal method of assessment of resuscitation; for the present, however, urine output remains the standard, and other measures are useful adjuncts.

Mixed venous saturation is the gold standard for the measurement of total tissue perfusion but has fallen out of favor because it requires a pulmonary artery catheter. As such, peripheral surrogates, such as base deficit and serum lactate, have become the standard values followed to monitor shock. These can be measured in minutes using point-of-care techniques and rapid guide interventions.

The base deficit is a value calculated using the Henderson-Hasselbalch equation based on the relationship between pH, pCO ₂ , and serum bicarbonate:

It is the stoichiometric equivalent of base required to return the pH to 7.4. Base deficit is routinely calculated on blood gas analysis and provides a reasonable estimate of the degree of tissue anoxia and shock at the whole-body level, particularly in hemorrhagic shock. A rising base deficit indicates increasing metabolic acidosis and may stratify risk of mortality in patients after major trauma. The same can be said for the use of base deficit in resuscitation of burn patients. ^, These studies showed a correlation between higher base deficit and increased mortality, and some have suggested that this value is a better monitor of resuscitation than the time-honored monitors of urine output and arterial blood pressure. Recent studies of burn patients showed the base deficit was higher in nonsurvivors during resuscitation, although the authors could not identify a specific boundary for the effect. ^, Despite its utility as an indicator of shock, base deficit remains a nonspecific indicator of metabolic acidosis and may be elevated with many confounding conditions other than shock, including hyperchloremia, uremia, and alcohol, cocaine, and methamphetamine use. Interpretation can be difficult under these circumstances.

Lactate is another common measure used to determine the adequacy of tissue perfusion. Under acute low-flow conditions, cells transition from primarily aerobic metabolism to anaerobic metabolism for energy production (i.e., adenosine triphosphate). A byproduct of anaerobic metabolism is lactic acid. Under ischemic conditions, plasma lactate concentration will increase, leading to a decrease in pH. Measurement of lactate is commonly performed to determine the adequacy of generalized perfusion; increases suggest ischemia. Investigators showed that lactate does increase, along with base deficit, in burn patients during resuscitation, and higher levels are associated with poorer outcomes. Later in the course, however, lactate concentrations must be used with some caution because elevated levels do not necessarily indicate ischemia. Under hypermetabolic conditions common in the severely burned, pyruvate dehydrogenase activity is sufficiently inefficient that lactate levels might be elevated without ischemia. Isolated elevations of lactate should then be interpreted with caution, and confirmation of ischemia or shock by physical or other laboratory findings sought. There are, however, recent indications that an early, even minor increase in lactate during the resuscitation period may affect outcome.

Preload is the force that stretches the cardiac muscle prior to contraction. This force is composed of the volume that fills the heart from venous return. Due to the molecular arrangement of actin and myosin in muscle, the more the incoming venous volume stretches the muscle, the further it will contract. This is best demonstrated on a Frank-Starling curve ( Fig. 26.1 ), first described by Otto Frank in a frog heart preparation in 1884; Ernest Starling extended this observation to the mammalian heart in 1914. The relationship demonstrated in the Frank-Starling curve justifies the use of preload augmentation by volume resuscitation to increase cardiac performance. However, when the end-diastolic volume becomes excessive, cardiac function can decrease; probably the muscle fibers overstretch and pull the contractile fibers past each other, thereby reducing the contact required for contractile force. The preload necessary to reduce cardiac function in experimental settings is in excess of 60 mmHg, which is rarely encountered in patients.

Frank-Starling curve. The solid line depicts the pressure–volume relationship of the heart, showing that as pressure to the heart (preload) increases, the volume pumped by the heart increases. Immediately after burn, contractility diminishes, shifting the curve downward (dashed line) . It must be noted that, with this change, the volume pumped by the heart still increases with increased pressure (preload), validating the use of increased atrial pressure as a means of increasing cardiac output after severe burn.

Preload is estimated clinically by central venous pressure (CVP), pulmonary artery wedge pressure, echocardiography, or transpulmonary thermodilution. These measures can be used to optimize preload, balancing vascular volume loading and cardiac performance against interstitial and pulmonary edema.

The force with which the heart contracts is referred to as cardiac contractility. It is directly related to the number of fibers contracting, their preload, and afterload. Contractility is diminished in patients with low preload or high afterload, in coronary artery disease with loss of myocardium from infarction and ischemia, in burn patients during acute resuscitation due to myocardial depressant factor, in septic shock with Takotsobu cardiomyopathy, or in patients who are severely malnourished. Calculating the left ventricular stroke work (LVSW) from pulmonary artery catheter-derived values provides the best estimate of cardiac contractility and can be determined with the following formula:

where SV is cardiac index divided by heart rate, and PCWP is pulmonary capillary wedge pressure. In current practice, contractility is usually tracked with arterial waveform continuous cardiac output monitoring or echocardiography.

Afterload is the force impeding or opposing ventricular contraction and, in conjunction with cardiac output, creates blood pressure. This force is equivalent to the tension developed across the wall of the ventricle during systole. Clinically, afterload is measured by arterial resistance as an estimate of arterial compliance. Arterial resistance is measured as the difference between inflow pressure (mean arterial) and outflow pressure (venous) divided by the flow rate (cardiac output [CO]):

where SVR is systemic vascular resistance. Pulmonary artery catheters, arterial waveform analysis, or echocardiography calculate this value.

For the heart to function properly, the electrical conduction system must be intact to provide rhythmic efficient contractions to develop sufficient force to propel blood through the circulatory system. For example, if the heart rate approaches 200 beats/minute, the heart will have insufficient time to fill completely, thereby reducing myocardial fiber stretch and heart function. Also, if frequent premature ventricular contractions are present, the heart will not perform optimally, for similar reasons. Heart rate and rhythm are monitored continuously as routine in every critically ill patient using electrocardiography. A combination of sympathetic and adrenergic tone, combined with high atrial stretch, can predispose burn patients to atrial arrhythmias, such as atrial fibrillation. Judicious fluid and electrolyte management, β-blockade, rate control, and the appropriate use of antiarrhythmics are cornerstones of treatment. An important note is that mortality is higher in BICU patients after developing atrial fibrillation.

Severe burns affect cardiac performance in a plethora of ways. The first is to reduce preload to the heart through volume loss into burned and nonburned tissues. It is for this reason that volumes predicted by resuscitation formulas must be used to maintain blood pressure and hemodynamics. In addition, severe burn induces myocardial depression characterized by a decrease in tension development and velocities of contraction and relaxation. Cardiac output is then reduced. These effects are most evident early in the course of injury during resuscitation; however, they are followed shortly thereafter by a hyperdynamic phase of increased cardiac output primarily caused by a decrease in afterload through vasodilation and an increase in heart rate.

One response to severe burn is a dramatic increase in catecholamine production; this has been linked to a number of metabolic abnormalities, including increased resting energy expenditure, muscle catabolism, and altered thermoregulation. A nonspecific β-blocker, propranolol, has been used to reduce heart rate and myocardial work in severe burns. Propranolol administration also reduces peripheral lipolysis and muscle catabolism, which are additional benefits. Wurzer et al. demonstrated that propranolol reduced cardiogenic stress by reducing cardiac index and MAP in severely burned, injured children without reducing peripheral oxygen delivery or increasing lactic acidosis events or organ dysfunction. Further trials are in progress to define the benefits of β-blockade as well as its interaction with other anabolic medications, such as oxandrolone. Regardless, β-blockade is becoming an increasingly standard component of burn care.

Intubation entails passing an endotracheal tube (ETT) from either the nose or the mouth through the pharynx and into the trachea. This tube is subsequently connected to a mechanical ventilator to induce inspiration and passive exhalation. In burn patients, indications for intubation are, in general, to improve oxygenation and ventilation or to maintain a compromised airway, such as in a severe inhalation injury or obtunded patient ( Table 26.1 ).

It is important to intubate appropriate patients prior to respiratory arrest. It should be considered, however, that in a large study series more than 33% of burn patients were extubated within 1 day of intubation for transfer to a burn center without reintubation. These patients were subjected to risk without reward, and loss of an ETT in a heavily sedated or paralyzed patient is potentially fatal. ^,

Securing an ETT to a burned and/or edematous face can be challenging. In a large survey of burn centers, ETTs were reported secured with linen nonadhesive tape in 59% of cases, manufactured devices in 48%, and orthodontically in 24%. Our center has used nasal intubation with a septal tie for 20 years without accidental extubation or septic sinusitis.

Tracheotomy can provide a long-term durable airway with less patient discomfort. In a survey of US burn centers, the average tracheostomy was performed at 2 weeks; however, there is a consensus that there are indications for earlier tracheostomy. Interestingly, however, in a study of 600 adult medical ICU patients, Terragni et al. found that early tracheotomy did not result in significant improvement in ventilator-associated pneumonia, although the duration of ventilator-assisted respiration was reduced, as was ICU time.

There has been a proliferation of management strategies for pulmonary failure with an alphabet soup of ventilator modes; however, the physiology of human lungs remains constant regardless of the proliferation of devices. Maintaining a broad physiologic view allows optimal matching of the ventilator’s actions to the patient’s needs. Lungs have three main functions:

• ■

  Ventilation

• ■

  Oxygenation

• ■

  Expectoration

Mechanical ventilation is an essential component of burn pulmonary critical care. Mechanical ventilatory support allows life to be sustained with lungs that would otherwise inexorably lead to death. It is important that the use of mechanical ventilation provides minimal interruption to burn care, including both wound care and physical therapy. The safety of mobilization of mechanically ventilated patients is well documented, including out-of-bed mobilization of intubated patients ( Fig. 26.2 ).

Respiratory considerations for in-bed and out-of-bed exercises. Green circle indicates low risk for adverse event. Yellow triangle indicates potential risk and consequences of an adverse event but may be outweighed by the potential benefits of mobilization. Red octagon indicates a significant potential risk of an adverse event, and mobilization should only be carried out with specific consideration by senior staff nurse or physical therapist. H FOV , High-frequency oscillation ventilation; PEEP , positive end-expiratory pressure.

(From Hodgson CL, Stiller K, Needham DM, et al. Expert consensus and recommendations on safety criteria for active mobilization of mechanically ventilated critically ill adults. Crit Care. 2014;18[6]:658.)

The principal clinical difference between mechanical ventilation and spontaneous ventilation is the effect of positive pressure, as opposed to normal physiologic negative pressure. The use of positive pressure improves ventilation by recruiting alveoli and increasing functional residual capacity (i.e., the number and volume of open alveoli at the end of expiration), thus improving V/Q mismatch and reducing shunting of blood past nonventilated lung areas ( Fig. 26.3 ). Positive-pressure ventilation (PPV) also allows maintenance of higher MAP-O ₂ to overcome high A-a gradients. Adverse effects of PPV lie in its propensity to produce trauma to the airways (barotrauma) and its effects on intrathoracic pressure, which can impede venous return to the heart, thus reducing cardiac output.

A schematic of intrapulmonary shunting. Blood comes into the lung and then divides into capillaries and passes close to alveoli, where gas exchange occurs. In the case of alveolar collapse, the passing blood does not undergo gas exchange and thus returns to the pulmonary vein unchanged (low oxygen concentration, blue color ). This then mixes with oxygenated blood from the aerated alveoli (high oxygen concentration, red color ) to return to the heart and the periphery. In effect, more collapsed lung equates to less oxygenation through this process.

In a survey of US burn centers, pressure-support ventilation (PSV) and volume assist control were the most common starting ventilator modes for all burn patients; however, 53% of centers report using an open lung technique, such as high-frequency percussive ventilation (HFPV), high-frequency oscillation ventilation (HFOV), or airway pressure release ventilation (APRV) in the setting of inhalation injury.

When oxygenation begins to decline, initial maneuvers are to increase the F io ₂ to greater than 40%, possibly to 100%. Concentrations of oxygen greater than 60% are considered toxic to airway epithelium over the course of hours; other means to increase oxygenation should be employed. As discussed earlier, treating hypoxia is first accomplished by increasing the MAP-O ₂ by raising the level of PEEP incrementally until the desired level of oxygenation is reached while keeping the F io ₂ to less than 60%. Once a level of 15 to 20 mmHg of PEEP is reached, other means of increasing MAP-O ₂ will need to be employed. These consist of ventilator modes, including inverse ratio ventilation (IRV), HFOV, HFPV, and APRV.

After MAP-O ₂ is maximized, therapies are directed at improving V/Q mismatching with inhaled nitric oxide or prone positioning. ECMO has been used with success in burn patients when the lungs are too injured to perform life-sustaining gas exchange. This must be done with caution given the need for anticoagulation and the limitation imposed in the ability to mobilize the patient, both of which can compromise other critical aspects of burn care.

Ventilator modes.

Regardless of the mode ( Fig. 26.4 ), the basic pulmonary physiology principles balancing ventilation, oxygenation, and expectoration against VILI are the purview of a skilled burn surgeon and respiratory therapist.

Different ventilators commonly used in burn units in the United States. A, The most commonly used is the servo type ventilator, which can be set to give volume- or pressure-regulated breaths with or without pressure support or positive end-expiratory pressure. The inspiratory to expiratory ratios can also be reversed. B, The volumetric diffusive respiration ventilator delivers high-frequency percussive ventilation or as continuous positive airway pressure. C, The airway pressure release ventilation (APRV) ventilator can be set to all the servo settings as well as APRV.

Volume-control mode delivers a preset tidal volume regardless of patient effort and is typically used only under general anesthesia due to ventilator–patient dyssynchrony; it takes all respiratory control away from the patient ( Fig. 26.5 ). Assist-control mode differs by allowing the patient to demand additional breaths with a ventilator-preset minimum rate. All breaths are delivered at a prescribed volume, irrespective of any patient attempts for a larger or smaller breath. Some patients find this mode uncomfortable due to the inability to control their volume and require greater sedation than they would under other modes.

Airway pressure curves illustrating the three mechanical functions: IN , initiation of the cycle; LIM , the preset limit (i.e., pressure or volume) imposed on the positive pressure cycle; CYC , the function (i.e., time or volume) ending the cycle. Each mechanical function is governed by one of four physical factors: volume, pressure, flow, and time. A, A time-initiated, volume-limited mode. B, A pressure-initiated (subbaseline pressure produced by the patient’s effort to breathe), volume-limited mode. C, A time-initiated, volume-limited, time-cycled mode that extends inspiration beyond the time that the volume is delivered. A plateau is reached after flow has stopped but before the ventilator cycles into exhalation.

(From Shapiro BA, Lacmak RM, Care RD, et al. Clinical Application of Respiratory Care . St. Louis, MO: Mosby Year Book; 1991.)

Volume-cycled ventilation delivers a set volume of air regardless of pressure required. In the case of poor lung compliance, such as ARDS, this mode could lead to excessive ventilator pressures and significant airway injury. For this reason, time-cycled pressure control ventilation was developed; this delivers inspiration at a given flow rate to a preset pressure. The breath is terminated at a set cycle time, not on the basis of volume of flow, as with volume-controlled ventilation. Therefore pressure control has the advantage of limiting inspiratory pressure despite changes in compliance. It has the disadvantage of a variable tidal volume during dynamic changes in lung compliance, which can lead to inadequate or excessive minute ventilation if compliance respectively worsens or improves.

Intermittent mandatory ventilation (IMV) allows spontaneous ventilation interspersed with volume-cycled or time-cycled pressure control mechanical ventilation. The addition of synchronization, in synchronized IMV (SIMV), avoids placing a mechanical breath on top of a spontaneous patient breath, greatly improving this mode. It was hoped that maintaining some patient work in breathing would preserve respiratory strength while mechanical ventilation was required and that this mode would support weaning to progressively increasing patient effort while reducing mechanical support, although outcome data did not support a role in weaning ( Fig. 26.6 ).

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