Arterial lines

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, as 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 intra-arterial catheters is generally preferable while in the ICU for a prolonged period. Lines are typically placed in either the radial or the femoral artery. The radial artery is the preferred site for most critically ill patients because of safety with the dual arterial supply to the hand should a complication occur. However, it has been shown that radial artery catheters are inaccurate in the measurement of central blood pressure when vasopressors are used⁹ and are notoriously inaccurate in children because of greater vascular reactivity.¹⁰ For these reasons, we recommend femoral arterial blood pressure measurement in most burned patients.

For arterial catheters, systolic, diastolic, and mean arterial pressures should be displayed continuously on the monitor screen. Either systolic or mean arterial pressure can be used to determine adequacy of pressure, although a mean arterial pressure >70 mmHg is considered a more accurate description 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 and the diastolic pressure decreases; the mean pressure 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, requiring 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 placed on the arm or leg. Exaggerated waveforms with elevated systolic pressure and additional peaks in the waveform (generally only two are found) may be a phenomenon referred to 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 typically normal systolic blood pressure values with evidence of catheter whipping as normal, as the effect generally overestimates pressures. Again, use of mean arterial pressures as the principal guideline for the assessment of blood pressure is optimal, as the effects of catheter whip or other problems with intra-arterial 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 removal of the catheter and elevation of the extremity. If improvement in ischemic symptoms is not seen promptly (within an hour), angiography and intervention should 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.

Evidence of catheter infection hallmarked by purulence and surrounding erythema should instigate removal of the catheter, which will often 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, compression with a vascular ultrasound device until no further flow is seen in the pseudoaneurysm will often alleviate the problem without operative intervention.¹¹

Cardiac output measurement

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 ICUs. 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 ICUs 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.¹² This was followed by a clinical trial in the UK which showed no benefit to the use of pulmonary artery catheters in a general ICU setting.¹³ These data then suggest that this modality has little utility in today’s practice. In fact, over the past few years, the use of pulmonary artery catheters has significantly diminished except in special circumstances such as unexpected response to treatment such as 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.¹⁴

The base deficit is a value calculated using the Henderson BICU Hasselbach equation based on the relationship between pH, pCO₂, and serum bicarbonate:

The base deficit is the stoichiometric equivalent of base required to return the pH to 7.40. 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.¹⁵ The same can be said for the use of base deficit in burn resuscitation in burned patients.^16,17 These studies showed a correlation of higher base deficit and increased mortality, and some have suggested this value is a better monitor of resuscitation than the time-honored monitors of urine output and arterial blood pressure.¹⁸ Recent studies in burned patients showed that base deficit was higher in non-survivors during resuscitation, although the authors could not identify a specific boundary for the effect.^19,20 Despite its utility as an indicator of shock, base deficit remains a non-specific indicator of metabolic acidosis, and may be elevated with alcohol, cocaine, and methamphetamine use. Interpretation may be difficult under these circumstances.

Lactate is another common measure used to determine adequacy of tissue perfusion. Under acute low-flow conditions, cells transition from primarily aerobic metabolism to anaerobic metabolism for energy production (ATP). A byproduct of anaerobic metabolism is lactic acid. Under ischemic conditions, then, 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 burned patients during resuscitation, and higher levels are associated with poorer outcomes.²⁰ Later in the course, however, lactate concentrations must be used with some caution, as elevated levels do not necessarily indicate ischemia. Under hypermetabolic conditions, which are common in the severely burned, pyruvate dehydrogenase activity is sufficiently inefficient such 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 should be sought.

Transesophageal echocardiography

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 as a monitor in critically ill severely burned patients. A report documented the use of transesophageal Doppler measurements of cardiac output in a series of severely burned patients showing that intravascular volume and cardiac contractility are significantly diminished the first day after burn in spite of high-volume resuscitation.²¹ For the monitoring of burn resuscitation, it seems that most use urine output; however, the question has been raised whether this is the proper measure. Investigators in China asked whether esophageal Doppler monitoring of heart function might be an improvement. They studied 21 patients with massive burns (79 ± 8% TBSA burned) who were resuscitated with a goal of 1.0 mL/kg/h. 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.²² These results call into question the validity of urine output as the primary measure of adequacy of resuscitation. Perhaps this is not the best method? A similar study by investigators in Sweden studying the role of cardiac function measured by echocardiography and myocyte damage measured by troponin abundance in the serum showed that half their patients had myocardial damage during resuscitation that was universally associated with some temporary cardiac wall motion abnormality. However, systolic function was not adversely affected.²³ We look for further work in the future with regard to the optimal method of assessment of resuscitation; for the present, however, urine output remains the standard.

Ventilatory modes

The complexity of mechanical ventilators has increased dramatically since the first generation of volume cycle ventilators used in the 1960s. The development of positive end-expiratory pressure (PEEP) to maintain functional residual capacity was followed by the development of modes using partial ventilatory support, such as intermittent mandatory ventilation in the 1970s. Efficient microprocessors were then developed that permitted modes of ventilation such as pressure support ventilation, time-cycled pressure control ventilation, and inverse ratio ventilation. Most recently, new processors have been used to combine modes of ventilation, such as pressure support and time-cycled pressure control ventilation, and open lung strategies such as airway pressure-release ventilation, with some success (Fig. 33.1). However, even with these new developments, the function of mechanical ventilators remains identical to those first used. Whether these new ventilators have had any appreciable effect on mortality remains to be fully elucidated, though new evidence suggests that choice of mode may make a difference.

Figure 33.1 Photographs of different ventilators commonly used in burn units in the USA. (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 PEEP. The I : E ratios can also be reversed. (b) The second type is the VDR ventilator, which delivers HFPV or CPAP. (c) The third type is the APRV ventilator, which can be set to all the servo settings as well as APRV.

The principal difference in mechanical ventilation from spontaneous ventilation, which each of us use every minute of the day, is the effect of positive pressure as opposed to normal physiologic negative pressure. The use of positive pressure improves gas exchange by recruiting alveoli and increasing functional residual capacity (i.e., the number and volume of open alveoli at the end of expiration), thus improving ventilation perfusion mismatch and reducing intrapulmonary shunting of blood past non-ventilated lung areas (Fig. 33.2). Adverse effects of positive-pressure ventilation 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 and thus reduce cardiac output.

Figure 33.2 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.

Control and assist control ventilation

The volume control modes are volume-cycled settings that deliver a preset tidal volume at a minimum respiratory rate and inspiratory flow rate regardless of the patient’s own respiratory efforts (Fig. 33.3). Of the two, the control mode will not trigger with patient effort. This mode is typically very uncomfortable for patients owing to ventilator–patient dyssynchrony, with use limited to patients under general anesthesia or heavy sedation. The assist control mode differs in that a breath will be delivered regularly according to a prescribed rate, but will also deliver a breath upon patient negative-pressure effort to open a flow valve, at which time the ventilator will fire, allowing the patient to control his or her own ventilatory rate with a preset minimum rate as a back-up. This mode is typically used in heavily sedated patients who cannot generate enough tidal volume under pressure support modes or intermittent mandatory ventilation mode, or in those patients in whom the clinician wishes to minimize the work of breathing. It must be noted, however, that the work of breathing can be increased dramatically in patients with excessive ventilator triggering; increasing the minimum number of breaths to match the patient’s effort can minimize this effect. When using this mode of ventilation, it is important to monitor lung compliance by measuring serial peak and plateau pressures for diagnostic purposes as well as minimization of barotrauma.

Figure 33.3 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 (sub-baseline 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.

(Reprinted with permission from Shapiro BA, Lacmak RM, Care RD, et al. Clinical application of respiratory care. St Louis, MO: Mosby Year Book; 1991)⁹⁵

Time-cycled pressure control ventilation

Volume-cycled ventilation delivers a set volume of air regardless of pressure required. In the case of poor lung compliance, such as in acute respiratory distress syndrome (ARDS), this mode could lead to excessive ventilatory 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 is the case with volume-controlled ventilation. Therefore, pressure control has the advantage of limiting inspiratory pressure despite changes in compliance. It also 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. This puts the onus on the provider to monitor changes in compliance to make appropriate changes in the preset pressure to compensate. This differs from the volume-cycled mode, which requires monitoring of peak and plateau pressures. Another disadvantage is that this mode is not as well tolerated by awake patients as other modes.

Intermittent mandatory ventilation

Intermittent mandatory ventilation (IMV) was developed to allow spontaneous ventilation interspersed with volume-cycled or time-cycled pressure control mechanical ventilation. It was developed initially as a method to wean patients from the ventilator by depending more and more on patient effort for ventilation. The addition of a synchronized mode (SIMV) to avoid placing a mechanical breath on top of a spontaneous patient breath greatly improved this mode. It has the advantage of maintaining some patient work in breathing to preserve respiratory strength when mechanical ventilation is required, and as a weaning tool to progressively increase patient effort while reducing mechanical support in preparation for discontinuing mechanical ventilation. This method can be problematic in patients with low pulmonary compliance, and the IMV mode may not allow for sufficient spontaneous tidal volumes due to extremely limited inspiratory capacity. The addition of pressure support to augment spontaneous ventilatory efforts can be used (Fig. 33.4).

Figure 33.4 Airway pressure tracings of four volume-cycled modes. The thick solid lines represent ventilator breaths. The thick dotted lines are spontaneous breaths. The thin dotted lines illustrate the spontaneous breathing pattern in the absence of ventilator breaths. IMV, intermittent mandatory ventilation; SIMV, synchronized intermittent mandatory ventilation.

(Reprinted with permission from Shapiro BA, Lacmak RM, Care RD, et al. Clinical application of respiratory care. St Louis, MO: Mosby Year Book; 1991)⁹⁵

Pressure support ventilation

Pressure support ventilation is a patient-triggered pressure-limited flow-cycled ventilatory mode (Fig. 33.5). Each pressure support breath is triggered by patient negative-pressure effort, at which time a valve opens producing a high-flow pressure-limited breath. The breath ends when the patient’s inspiratory demand falls below a preset limit, allowing spontaneous respiration and complete patient control. It also differs from the other modes in that it is flow-cycled as opposed to volume- or pressure-cycled. Because this mode is triggered and completed entirely by patient response, it cannot be used in patients with decreased respiratory drive, such as paralyzed patients and those who are heavily sedated. This mode has a number of advantages because of improved synchrony with patient effort. It can be used to provide full ventilatory support by setting a flow and pressure such that an adequate tidal volume is delivered. It can also be used effectively during weaning by reducing the pressure incrementally to allow for progressively greater patient effort. Caution must be exercised when reducing ventilatory support so that a sufficient tidal volume is still delivered to maintain adequate ventilation.

Figure 33.5 Schematic illustration of pressure support ventilation. IN, initiation by spontaneous breath; LIM limit (pressure); CYC, cycle (derived from decreasing inspiratory flow); EX, expiration.

(Reprinted with permission from Shapiro BA, Lacmak RM, Care RD, et al. Clinical application of respiratory care. St Louis, MO: Mosby Year Book; 1991)⁹⁵

Inverse ratio ventilation

Inverse ratio ventilation is a mode of ventilation designed to improve oxygenation at a given level of inspired oxygen. Conventional ventilation uses the times of inspiration and expiration at a ratio of 1 : 4 or 1 : 2, giving a longer time for expiration, as it is generally a passive process. Inverse ratio ventilation reverses this ratio to give a longer inspiratory time (1 : 1 or 2 : 1) by using rapid inspiratory flow rates and decelerating flow patterns during the inspiratory phase. The effect of inverse ratio ventilation is to increase mean airway pressures and thus recruit alveoli in an effect similar to PEEP. Secondly, in severe lung disease, ventilation in the lung is unequal due to peribronchial narrowing. Thus, some underventilated alveoli that are actually open are not able to exchange gases efficiently, increasing the intrapulmonary shunt and reducing arterial oxygenation. Inverse ratio ventilation can improve this by selective air-trapping or intrinsic PEEP in these compromised air spaces. Inverse ratio ventilation can be done either in a volume-cycled or a time-cycled pressure ventilation mode, but it is most commonly used with pressure controlled ventilation to reduce peak airway pressures.

The beneficial effects of inverse ratio ventilation have been questioned. It may be that the same effect can be gained simply by increasing peak airway pressures with PEEP or peak inspiratory pressures. In fact, some studies have showed no benefit of inverse ratio ventilation compared to conventional volume ventilation in terms of oxygenation.²⁴ These studies did show some slight improvements in ventilation (Paco₂). For this reason, inverse ratio ventilation cannot be recommended except in the setting of ARDS refractory to other therapies.

High-frequency oscillatory ventilation

High-frequency oscillatory ventilation (HFOV) is a mode of ventilation that delivers subtidal, high-frequency oscillatory breaths at a constant mean airway pressure (mPAW), otherwise described as ‘CPAP with a wiggle.’ It is thought to maintain open alveoli by recruiting collapsed portions of diseased lung and applying the equivalent of continuous positive airway pressure used during conventional ventilation. Optimizing mean airway pressure limits peak pressure, thereby making this a ‘lung-protective’ mode. A recent randomized prospective trial in adult patients with ARDS demonstrated that HFOV resulted in brief improvements in oxygenation although mortality and complication rates were not different.²⁵ In burned patients, one center reported early success in reversing profound hypoxemia in patients with ARDS while facilitating early excision and grafting with intraoperative use.²⁶ However, this same group recently showed that in burned patients with inhalation injury this method often failed secondary to hypercapnia rather than hypoxia.²⁷ It stands to reason, therefore, that this method might be tried in those with oxygenation difficulties, and perhaps continued if hypercapnia is less of an issue.

High-frequency percussive ventilation

High-frequency percussive ventilation (HFPV) is a pressure-limited, time-cycled mode of ventilation that delivers subtidal pressure-limited breaths at a high frequency (400–800 beats/min) superimposed on a conventional inspiratory and expiratory pressure-controlled cycle (10–30 breaths/min). The purported advantages are to mobilize airway secretion and casts for better pulmonary toilet, and to provide adequate gas exchange at lower airway pressures. This method of ventilation has been tested primarily in burned patients with inhalation injury, and was first reported in 1989.²⁸ In this study, HFPV was used as a salvage therapy in patients with inhalation injury and as primary therapy in another group. Improvements in oxygenation and a lower rate of pneumonia were seen. Another study documented an improvement in mortality in burned patients with inhalation injury treated with HFPV compared to historical controls.²⁹ Other outcomes in this study were significant decreases in the work of breathing, and lower inspiratory pressures in addition to improvements in oxygenation and the rate of pneumonia. This method of ventilation seems to be particularly efficacious in the treatment of inhalation injury, particularly in comparison to HFOV.

A recent study was performed in severely burned ventilated patients testing the effects of low tidal volume conventional ventilator strategy against an HFPV ventilator strategy. This study again found differences in improved oxygenation early in the course, but no differences in mortality or other outcome measures. However, the study did show that the need for ventilator ‘rescue,’ defined as the need for changing ventilator modes due to inadequate oxygenation or ventilation despite maximizing the mode, was higher in the ARDSnet group, thus favoring the use of HFPV in this population (i.e. less rescue indicated). At the very least, these data show that HFPV is useful in the population of burned patients, and may in fact be beneficial by reducing the risk of pulmonary failure at least in the short term.³⁰

Airway pressure-release ventilation

Related posts:

Pathophysiology of burn shock and burn edema Enzymatic debridement of burn wounds Functional sequelae and disability assessment Reconstruction of the burned breast Pre-hospital management, transportation and emergency care The burn problem: A pathologist’s perspective

Stay updated, free articles. Join our Telegram channel

Join