Ventilated Patients
I. Cinel, S. Jean, and R.P. Dellinger
Introduction
Endotracheal intubation and mechanical ventilation are required for the majority of critically ill patients in tertiary care intensive care units (ICUs) [1]. During mechani- cal ventilation, patients often have imbalances in regional lung ventilation due to heterogeneity of lung mechanics. The current methods generally available for assess- ing lung function in mechanically ventilated patients include arterial blood gas anal- ysis and graphic waveforms from ventilators (flow, pressure and volume over time as well as pressure-volume, pressure-flow and flow-volume loops). At best, these meth- ods reflect only overall lung function, while failing to give information on disparate regional functionality. Unlike data collected from the ventilator or the blood, lung imaging allows for regional assessment of anatomy or function. Methods which pro- vide the capability of quantifying these regional differences in mechanically venti- lated patients are of great interest.
A lung imaging technique such as dynamic computed tomography (dCT) pro- vides valuable anatomic information about lung heterogeneity and is well validated, but lacks the bedside monitoring capabilities ideal for the ICU. Electrical impedance tomography (EIT) and vibration response imaging (VRI) have emerged as new non- invasive and radiation-free imaging tools providing real time functional lung assess- ment. The dynamic nature of these techniques provides information on lung func- tion and has advantages over static techniques and may better illustrate patient-ven- tilator interactions. Although more realistic assessment of ventilatory processes is obtained compared to static measurements, these methodologies are not currently available for routine use in clinical practice. Bedside tools for the adjustment of mechanical ventilation with the capability of measuring regional ventilation, if vali- dated, offer significant potential utility in the ICU. For example these technologies can be used to guide and manage, and to implement lung-protective ventilation strategies.
Dynamic Computerized Tomography
CT has opened a new era in our understanding of the pathophysiological and clini-
cal aspects of lung injury in mechanically ventilated patients. Although traditional
chest radiograph shows diffuse involvement of lung parenchyma in acute respiratory
distress syndrome (ARDS), our understanding of this pathology was changed after
CT studies showed heterogeneity of lung involvement in ARDS [2]. CT scan provides
axial images of the thorax, allowing visualization of lung parenchyma. Based on
acquiring CT images at a subsecond speed (50 to 500 msec) in a quasi-continuous fashion, CT scanning is used in dynamic conditions, including analysis of mechani- cal ventilation and lung perfusion [3 – 6]. dCT imaging is a highly sensitive approach to image-based analysis of the processes of spontaneous respiration and mechanical ventilation. This approach allows the variations of functional anatomy and their cor- relation with gas exchange during tidal breathing to be investigated without inter- rupting mechanical ventilation [7, 8].
Dynamic scanning can be performed during continuous respiration and raw CT data are then reconstructed with a predefined temporal increment (i.e., 100 ms). As the scanner table is immobile during the dynamic multiscan acquisition, chosen slices move slightly over time with the cranio-caudal respiratory motion of the lung thus affecting image quality. In the near future, multislice CT scanners will allow simultaneous acquisition of several slices.
In contrast to static breath-hold imaging, dCT acquisitions allow the assessment of several complete respiratory cycles. In light of this new technique, it becomes clear that the analysis of static CT images does not accurately reflect physiological reality during cyclic ventilation. Breath-holding is required to avoid image artifacts during conventional static CT imaging whereas fast dynamic acquisition allows a cine-type visualization of the lung inflation and deflation processes during contin- ued respiration. dCT can also provide valuable information about regional differ- ences in dynamic distribution during mechanical ventilation [8].
The indications for using dCT scan in clinical practice are not yet completely clear. Concerns exist about the risks of moving patients out of the ICU. The most important feature of safe transport is to ensure that adequate equipment and per- sonnel are immediately available to cope with a catastrophic emergency such as accidental extubation, interruption of critical intravenous infusions, or extraction of venous, arterial, or enteral catheters. Patients requiring vasopressors or high inspired oxygen concentrations or positive end-expiratory pressure (PEEP) are at particular risk. Although portable CT could offer the advantages of bed-side moni- toring, its availability is currently very limited. Costs of dCT imaging is another issue as well, as are the costs of transport. Using the same scanner and similar acquisition parameters, preliminary data on humans showed approximately the same radiation doses for dCT measurement and spiral CT of the thorax, which are considerable [9]. Further investigations will be necessary to optimize imaging pro- cedures, and to reduce radiation exposure during dynamic acquisition.
Electrical Impedance Tomography
EIT has emerged as a non-invasive and radiation-free imaging technique for poten- tial bedside use in the ICU [10, 11]. EIT uses the variability in electrical impedance between tissue, air, and fluid to provide a map of impedance. The EIT hardware injects small amounts of electrical current sequentially, using electrodes applied cir- cumferentially to the patient’s chest. A standard set up of 16 electrodes receiving small currents in a rotating fashion is currently used and can generate up to 44 cross-sectional images per second with a typical resolution of more than 10 Hz [12].
The receiving electrode calculates the voltage differential and determines the imped-
ance between the transmitting and receiving electrodes [13]. This creates a tomo-
gram depicting the distribution of tissue electrical properties in a cross-sectional
image. Air is a poor conductor of electric current and causes high impedance,
whereas water or blood are good conductors. The difference makes it possible to detect changes in air and tissue content, enabling the assessment of ventilation dis- tribution. The shape of the lungs is indirectly accessible by the use of the functional images, showing regions with very low changes outside and high changes within the lung. In general, an increase in aerated lung volume results in a positive impedance change and a decrease in aerated lung volume produces a negative impedance change. The cardiac-related impedance signal can be significant and may, therefore, interfere with the accuracy of the ventilation-induced impedance signal. During mechanical ventilation, the displacement of the blood away from the thorax will increase measured impedance [14]. In a recent study to limit the effect of cardiac events on measured impedance changes, a low-pass filter of 2 Hz (120 cycles per minute) was applied and the center of the EIT image containing the heart was excluded in the analysis [15].
In pigs, EIT-derived impedance changes correlated very closely with whole lung pressure-volume relationships quantified by strain gauge plethysmography [16]. The ratio of anteroposterior lung impedance changes during recruitment and derecruit- ment was also described in the same animal model [17]. In an animal model of acute lung injury (ALI), comparison of atelectasis using EIT and CT (as a reference method) revealed the correlation of impedance changes and volume changes [11]. A dynamic approach to assess regional recruitment was also described in patients with ALI. Victorino et al have presented results in 10 severely ill patients with marked in- homogeneity of ventilation distribution [18]. There was excellent reproducibility of the measurement of ventilation distribution when partitioning the lungs into four zones, with a variation of only 4 to 7 %. Bias was minor and the difference between the EIT and CT was less than 10 % for detecting imbalances between the right and left lung. The only disadvantage of this investigation was that EIT and CT images were not obtained simultaneously due to electromagnetic interference of the EIT equipment in the CT scanner. However, this study confirmed that regional imped- ance changes are closely correlated with regional volume changes identified by CT.
The spatial resolution of EIT is generally low and cannot be easily increased by an increase in the number of electrodes because the physical limitation of the current flow through the tissue is not changed by the number of electrodes. It is best in the area near the electrodes and worsens in the targeted regions deeper in the thorax which contain a large part of the lung volume. The transverse area of the thorax, was trapezoid in the studied patients, but the algorithms are based on a circular struc- ture and, therefore, require modification [19]. Reliable recordings of absolute air content would be valuable. Interference by other electrical devices commonly found in the intensive care environment has not been systematically investigated. Some investigators have recently applied a software modification that shifts the EIT cur- rent out of the range of the electrocardiograph (EKG) electrodes, and the signal showed less interference [20]. The difficulty in fixing electrodes equidistant from one another and maintaining the connections throughout the recording is also a concern.
Compared with dCT, EIT is cheaper, smaller, and requires no ionizing radiation.
EIT can in principle produce thousands of images per second. Its major limitations
are its low spatial resolution, and large variability of images among subjects. To
obtain reasonable images, at least one hundred, and preferably several thousand
measurements must be made.
Vibration Response Imaging
The use of acoustic signals from the thorax to evaluate the functioning of the lungs is not a new concept. In the early 1800s, Laennec invented the stethoscope and described lung sounds and this practice continues today but is considered more an art than science due to its subjective nature [21]. Attempts have been made to move this clinical tool more towards the realm of science by some investigators whose research revealed that lung sounds are associated with inspiratory flow rate [22], patient position [23], and position of sensors [24]. In this way, distribution of regional ventilation was able to be measured using breath sounds [25]. Although interesting, the computational capabilities are not yet available to develop this con- cept into a viable imaging modality.
VRI is a novel dynamic imaging technique that measures vibration energy of lung sounds generated during respiration and mechanical ventilation [26, 27] (Dellinger et al., unpublished data). As air enters and leaves the lungs, the vibrations propagate through the lung tissue and are recorded by surface sensors. The current device uses 36 surface skin sensors (6 rows) which are spatially distributed and attached to the patient’s back. A protoptype device with 7 rows is shown in Figure 1. The vibration energy signal is transmitted to the VRI device where it is processed and a dynamic digital image is created. Each frame of the dynamic image represents 0.17 seconds of the respiratory cycle. In the dynamic image, left and right lungs are depicted side by side and the image simulates size and structure of the lungs and spine. In addition, a graph is produced that represents the average vibration energy as a function of time and is displayed under the image. Numerical raw values for vibration energy are also available and can be used to analyze and compare any regions of interest.
Areas with greatest vibration energy are depicted as dark colors (black) and low energy areas are shown in light colors (light gray); minimum energy areas are defined as “white”. The maximal energy frame (MEF) of the inspiratory phase typi- cally shows the maximum area of the vibration distribution in the VRI image. The MEF image of a normal healthy non-smoker is shown in Figure 2. The VRI technol- ogy is totally non-invasive and requires no radiation.
The dynamic image produced from the VRI recording provides a sense of the air movement in the lungs. The VRI was designed to diagnose lung pathologies that
Fig. 1. Attachment of VRI sensors.
Fig. 2. Vibration response image recorded on healthy non-smoker 30 year old male during one respiratory cycle. Both image and total vibration energy graph are displayed over time. R = right lung; L = left lung
Fig. 3. VRI images and chest radiographs before and after drainage of pleural effusion.
influence lung vibration energy, such as consolidation, atelectasis, asthma, crackles,
and wheezes. It allows demonstration of the effect on lung vibration when a large
pleural effusion is drained (Fig. 3). It also characterizes different characteristics of
vibration in chronic obstructive pulmonary disease (COPD) patients. In mechani-
Fig. 4. Maximal vibration energy during inspiration versus expiration in patients with chronic obstructive pulmonary disease compared to patients with no pulmonary disease.
Fig. 5. VRI images of an ARDS patient before and after recruitment maneuver and increased PEEP setting.
Note the ability to quantitate regional vibration energy. In table, upper boxes represent top 2 rows of sen- sors, the middle boxes the middle 2 rows of sensors, and the lower boxes the bottom 2 rows of sensors.
cally ventilated patients with COPD, the VRI image typically shows greater intensity of vibration during expiration, the reverse of non-COPD patients where intensity of vibration is typically greatest during inspiration (Fig. 4) [28].
In mechanically ventilated patients in the ICU, VRI has other potential uses. Our group has demonstrated changes in geographical distribution of lung vibration in different modes of mechanical ventilation [26] (Dellinger et al., unpublished data).
Research is ongoing to correlate these results with effectiveness of ventilation and
oxygenation and potentially clinical outcome. VRI offers potential utility in assess-
ing the effectiveness of recruitment and PEEP settings in ARDS patients [29]. Figure
5 shows changes in vibration distribution when an ARDS patient at PEEP 5 cmH
2O
underwent a recruitment maneuver and PEEP was increased to 10 cmH
2O. The abil-
ity to visualize regional distribution of vibration during recruitment may assist in
judging the level of dependent lung opening. VRI offers the potential for use in the
Fig. 6. Recently extubated patient with left atelectasis before and during incentive spirometry.
ICU in non-mechanically ventilated patients as well. The immediate images pro- vided with the VRI might serve as feedback and performance incentive for patients using incentive spirometry. Figure 6 shows the extension of vibration in a patient with left lower lobe atelectasis following incentive spirometer breath [30].
The main limitation of this technology for ICU use is that, for technical reasons, the current VRI recordings are done with patients supported in the near sitting position and not the supine or intermediate (30° – 45°) position where they are maintained for care. Lung sounds and vibrations would be expected to change with position due to shift in fluid and gravity effect in blood flow. However, vibration energy distribution in the near seated position is nevertheless of interest as it relates to position-independent effects of mechanical ventilation. New sensors have recently been developed that will allow VRI recording in the supine to 30° position. The patient would be able to comfortably lie on a mat of sensors at the position of mechanical ventilation. This would allow VRI to be potentially used as a continuous monitoring tool as well as to integrate it into the ICU suite. New automatic analysis tools such as regional assessment, breath to breath variability and harmony of venti- lated patient with mechanical ventilator will help uncover potential clinical informa- tion captured in these recordings and may have significant impact for patient care.
Comparison of Techniques
The imaging techniques discussed have different strengths and weaknesses and all have potential application to the mechanically ventilated patient in the ICU (See Table 1 for comparison of the techniques). All three techniques provide a glimpse into the lungs during the entire respiratory cycle as the patient is being ventilated.
As air moves in and out of the lungs, size, density, airflow, and conductivity change.
The dynamic images produced by these techniques, unlike their static counterparts,
illustrate the movement of air and can, therefore, provide information on lung func-
tion and on ventilation in the various lung regions, not just anatomy. This represents
a new era in lung imaging where ventilation can be visualized directly and not
assessed through remotely measured parameters in the blood or at the ventilator.
Table 1. Comparison of dynamic lung imaging techniques.
dCT EIT VRI
Bedside Radiation free Real-time results View
Spatial resolution Portion examined
Training requirement for technician Cost
No No No Axial High One slice Weeks to months High
Yes Yes No Axial Low One slice Weeks High
Yes Yes Yes Frontal Low Whole Days Low dCT: Dynamic computed tomography
EIT: Electrical impedance tomography VRI: Vibration response imaging
Dynamic CT and EIT use radiation and electrical impedance signals, respectively, to reconstruct an anatomical image of an axial slice of the thorax. These techniques force some assumptions to be made concerning lung that is not visualized. Sequen- tial images are then taken to produce a dynamic image. VRI, on the other hand, is measuring something that is inherently dynamic, vibration due to lung airflow. Since the sensors are placed over the whole lung, the resulting VRI image is a frontal view similar to a chest radiograph allowing examination of the entire lung, not just a sin- gle slice. As such, the VRI provides display of information on the function of the entire lung, not just a single slice.
While dCT provides the best resolution of lung anatomy, the greatest impediment to its use in this patient population is the current need to transport these critically ill patients to another part of the hospital to perform the test. The radiation used in dCT is also of some concern in this or any other patient group. EIT and VRI are radiation free and have no known side effects but do not produce precise anatomical images. EIT and VRI can be performed at the bedside, providing quick results while the patient remains connected to all their monitoring devices in the ICU. EIT or VRI offer the maximal potential for bedside titration of treatment or as a lung monitor- ing tool.
The lungs of ICU patients are not homogeneous. The new dynamic imaging tech- niques discussed make it possible to examine the functioning of different regions independently and begin to elucidate clinical relevance of these findings and how they might impact treatment possibilities.
Conclusion
An important need with today’s sophisticated ventilatory management strategies and
equipment is to determine regional ventilation for optimizing lung function, for exam-
ple with recruitment, maintaining an open lung, and limiting over-distension. The
novel imaging techniques discussed offer the possibility of evaluating regional lung
function. Although dCT, EIT, and VRI offer significant potential utility in the ICU, all
of them have limitations. There is currently no direct lung monitoring technique at the
bedside but the methods examined here represent the first generation of dynamic lung
imaging techniques with the potential for widespread use in the ICU. Further studies
will help determine how these techniques might be integrated into ICU care.
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