Pressure Support* Changes in Ventilatory Pattern and Components of the Work of Breathing William B. Van de Croon; M. D.;t Keith Gordey, M. D.;:I: Scott E. Dornseij; B.A.;§ David] Dries, M.D.;~ Broce S. Kleinman, M.D.;II Pankaj Kumar; M.D.;II and Mali Mathro, M.D., F.C.C.R#
To evaluate the interaction between patient and ventilator during widely varying levels of pressure support (PS) ventilation, we studied 33 patients who had undergone aortocoronary bypass. All patients were without preoperative evidence of lung disease and had left ventricuIar ejection fractions >45 percent. We assessed both changes in ventilatory pattern and the use of an extension of the Campbell technique to determine the components of the mechanical work of breathing (WOB). Patients were placed on 0, 10, 20, and 30 cm 1Is0 ofPS. We found that increasing the pressure support level (PSL) did not change minute ventilation, Pco1 , or pH despite large changes in both rate and depth of breathing. The inspiratory time fraction was consistently and progressively reduced as PS increased. Although mean inspiratory 80w (MIF) increased by 75 ± 9 (SE) percent as the PSL increased to 30 cm 1Is0, mean airway pressure rose only 3.5±0.1 cm ",0. Observed changes in the resistive and elastic components of WOB at PSL >0 were consistent with values predicted from baseline observations and changes in VT and MIF demonstrating that the Campbell technique of separating resistive and elastic components of the patient's WOB during unassisted ventilation can be extended to the analysis of WOB during
V olume-controUed assisted mechanical ventilation
and synchronized intermittent mandatory ventilation (SIMV) have proved to be both effective and reasonably safe modes of ventilatory support. However, use of these modes can be complicated by ventilator-generated alkalosis, patient ventilator dyssynchrony, and patient distress. When the inspiratory How demands of the patient are not met, the mechanical work of breathing (WOB) may even be elevated above levels required for spontaneous breathing without ventilatory assistance. 1
*From the Departments of Medicine, Surgery, and Anesthesiology, Stritch School of Medicine, Loyola University of Chicago, and Foster G. McGaw Hospital, Maywood, Illinois. t Assistant Professor of Medicine, Loyola University of Chicago. +FeIlow, Pulmonary and Critical Care Medicine. §Research Assistant, Pulmonary and Critical Care Medicine. 'Assistant Professor, Department of Surgery. IIAssistant Professor, Department of Anesthesia. # Professor, Department of Anesthesia. Manuscript received November 6; revision accepted March 15. Reprint requests: Dr. lOn de Groot Pulmonary and Critical Care Medicine, 2160 South First Avenue, Maywood, IL 60163
1082
mechanical ventilation. We were surprised to observe that although inspiratory WOB feU 67 ± 13 percent as the PSL increased to 30 em 1Is0, postinspiratory work by the inspiratory muscles (WOBpDM) did not show significant change. The persistence and substantial values ofWOBpuM in some patients suggested the presence of signmcant patient-ventilator dyssynchrony, especially at higber levels of PS. Total inspiratory WOB per minute, including both patient WOB and WOB by the ventilator, increased by 186 ± 29 percent, demonstrating that PS results in a respiratory pattern requiring substantially greater total mechanical work. (Cheat 1991; 100:1082-89) ANOVA=analysis of variance; CL=lung compliance; Ccw= compliance of the chest wall;!= respiratory frequency; MAP = mean airway pressure; MIF = mean inspiratory 8ow; PEEP = positive end-expiratory pressure; Pes = esophageal pressure; PS = pressure support; PSL = pressure support level; SIMV = synchronized intermittent mandatory ventilation; TE = expiratory time; TJ = inspiratory time; 1'vTroT = duty cycle; Tror = total time per breath; VL = lung volume; VT = tidal volume; WOB = work of breathing; WOBEL = elastic WOB; WOBEX = expiratory WOB; WOBPDM = postinspiratory inspiratory muscle WOB; WOBFfOT = patient total WOB; WOBR = resistive WOB; WOBTOf=total WOB; WOBVENT=venti1ator WOB.
Pressure support (PS) ventilation may eliminate some of these problems in patients who can reliably trigger the ventilator. It is a patient-triggered, pressure-assisted, How-cycled mode of mechanical ventilation. PS has been reported to increase patient comfort2-4 and decrease respiratory muscle stress5 during spontaneous breathing. Few complications of PS have been reported. 6 To better understand the scope of patient-ventilator interactions during PS ventilation, we prospectively observed patients after aortocoronary bypass surgery receiving widely varying levels of PS. We studied changes in the pattern of breathing as well as changes in the WOB by both the patient and the ventilator. To study changes in components of the WOB and the degree of patient-ventilator synchrony, we extended the method of Campbell7 •8 to the analysis of the WOB during mechanically assisted ventilation. METHODS
lbtient Population The study was approved by our institutional review board. Pressur8 Support (Van de Grasff et 81)
Informed consent was obtained from each patient before surge~ Thirty-three patients undergoing aortocoronary bypass were studied (26 men and 7 women). Their average age was 66 years (range, 44 to 82 years). Patients were excluded for any evidence of pulmonary dysfunction by history, physical examination, or chart revie~ or for a left ventricular ejection fraction O em 11.0, WOBEX and WOBPIIM were determined in the same manner. ~ tested our ability to continue separating WOBR and WOBEL with a line drawn between the end expiratory
were compared against values estimated by assuming volume
Bow independence
respiratory system. Under such conditions, total (patient plus ventilator) WOBEL will vary from levels observed with a PSL of 0 em R.O in proportion to the square of the &actional change in VT. Total (patient plus ventilator) WOBR will be changed by the product of the &actional changes in MIF and VT. From these predicted total values of WOBEL and WOBB, we calculated what &actions each component, resistive and elastic, should constitute the total inspiratory work. We multiplied these fractions by the observed patient inspiratory work to obtain our estimated values of WOBEL and WOBBe
DtIIa Analr/a. Statistical analysis was conducted (SYSTAT venion 4.2). The data are reported as means ± SE except as noted. Significance of change was determined by analysis of variance (ANOVA) considering pS 0.05 as significant. When significance was observed, po8t hoc comparisons of 10, 20, and 30 em 11.0 values with PSL=O values were made usiDg an F test with Bonferroni correction for multiple determinations (n = 3). E&"ects on Peo. and pR of the order in which levels of PS were applied were evaluated usiDg two-way ANOVA. Estimated and observed values ofWOBa. and WOB. were correlated using linear regression. Because these values were not normally distributed, this analysis was conducted after logarithmic transformation. Work is reported in joules. RESULTS
All patients tolerated the PS levels applied without apparent discomfort. As the PSL rose from 0 em 810, VT ehanged from 0.802±0.049 L to O.856±O.048 L at 10 em 810' 1.106±0.052 I at 20 em 810
A.
2
.30
• • •
!
...-
e ~
,
... .15
0
FIGURE 2. Mean values of A, inspiratory time (TJ), B, inspiratory time &action (TvTrar), C, mean inspiratory Bow (MIF), and D, mean airway pressure (MAP) for each level of PS. Two asterisks = p
To evaluate the interaction between patient and ventilator during widely varying levels of pressure support (PS) ventilation, we studied 33 patients who had undergone aortocoronary bypass. All patients were without preoperative evidence of lung disease and had left ventricuIar ejection fractions >45 percent. We assessed both changes in ventilatory pattern and the use of an extension of the Campbell technique to determine the components of the mechanical work of breathing (WOB). Patients were placed on 0, 10, 20, and 30 cm 1Is0 ofPS. We found that increasing the pressure support level (PSL) did not change minute ventilation, Pco1 , or pH despite large changes in both rate and depth of breathing. The inspiratory time fraction was consistently and progressively reduced as PS increased. Although mean inspiratory 80w (MIF) increased by 75 ± 9 (SE) percent as the PSL increased to 30 cm 1Is0, mean airway pressure rose only 3.5±0.1 cm ",0. Observed changes in the resistive and elastic components of WOB at PSL >0 were consistent with values predicted from baseline observations and changes in VT and MIF demonstrating that the Campbell technique of separating resistive and elastic components of the patient's WOB during unassisted ventilation can be extended to the analysis of WOB during
V olume-controUed assisted mechanical ventilation
and synchronized intermittent mandatory ventilation (SIMV) have proved to be both effective and reasonably safe modes of ventilatory support. However, use of these modes can be complicated by ventilator-generated alkalosis, patient ventilator dyssynchrony, and patient distress. When the inspiratory How demands of the patient are not met, the mechanical work of breathing (WOB) may even be elevated above levels required for spontaneous breathing without ventilatory assistance. 1
*From the Departments of Medicine, Surgery, and Anesthesiology, Stritch School of Medicine, Loyola University of Chicago, and Foster G. McGaw Hospital, Maywood, Illinois. t Assistant Professor of Medicine, Loyola University of Chicago. +FeIlow, Pulmonary and Critical Care Medicine. §Research Assistant, Pulmonary and Critical Care Medicine. 'Assistant Professor, Department of Surgery. IIAssistant Professor, Department of Anesthesia. # Professor, Department of Anesthesia. Manuscript received November 6; revision accepted March 15. Reprint requests: Dr. lOn de Groot Pulmonary and Critical Care Medicine, 2160 South First Avenue, Maywood, IL 60163
1082
mechanical ventilation. We were surprised to observe that although inspiratory WOB feU 67 ± 13 percent as the PSL increased to 30 em 1Is0, postinspiratory work by the inspiratory muscles (WOBpDM) did not show significant change. The persistence and substantial values ofWOBpuM in some patients suggested the presence of signmcant patient-ventilator dyssynchrony, especially at higber levels of PS. Total inspiratory WOB per minute, including both patient WOB and WOB by the ventilator, increased by 186 ± 29 percent, demonstrating that PS results in a respiratory pattern requiring substantially greater total mechanical work. (Cheat 1991; 100:1082-89) ANOVA=analysis of variance; CL=lung compliance; Ccw= compliance of the chest wall;!= respiratory frequency; MAP = mean airway pressure; MIF = mean inspiratory 8ow; PEEP = positive end-expiratory pressure; Pes = esophageal pressure; PS = pressure support; PSL = pressure support level; SIMV = synchronized intermittent mandatory ventilation; TE = expiratory time; TJ = inspiratory time; 1'vTroT = duty cycle; Tror = total time per breath; VL = lung volume; VT = tidal volume; WOB = work of breathing; WOBEL = elastic WOB; WOBEX = expiratory WOB; WOBPDM = postinspiratory inspiratory muscle WOB; WOBFfOT = patient total WOB; WOBR = resistive WOB; WOBTOf=total WOB; WOBVENT=venti1ator WOB.
Pressure support (PS) ventilation may eliminate some of these problems in patients who can reliably trigger the ventilator. It is a patient-triggered, pressure-assisted, How-cycled mode of mechanical ventilation. PS has been reported to increase patient comfort2-4 and decrease respiratory muscle stress5 during spontaneous breathing. Few complications of PS have been reported. 6 To better understand the scope of patient-ventilator interactions during PS ventilation, we prospectively observed patients after aortocoronary bypass surgery receiving widely varying levels of PS. We studied changes in the pattern of breathing as well as changes in the WOB by both the patient and the ventilator. To study changes in components of the WOB and the degree of patient-ventilator synchrony, we extended the method of Campbell7 •8 to the analysis of the WOB during mechanically assisted ventilation. METHODS
lbtient Population The study was approved by our institutional review board. Pressur8 Support (Van de Grasff et 81)
Informed consent was obtained from each patient before surge~ Thirty-three patients undergoing aortocoronary bypass were studied (26 men and 7 women). Their average age was 66 years (range, 44 to 82 years). Patients were excluded for any evidence of pulmonary dysfunction by history, physical examination, or chart revie~ or for a left ventricular ejection fraction O em 11.0, WOBEX and WOBPIIM were determined in the same manner. ~ tested our ability to continue separating WOBR and WOBEL with a line drawn between the end expiratory
were compared against values estimated by assuming volume
Bow independence
respiratory system. Under such conditions, total (patient plus ventilator) WOBEL will vary from levels observed with a PSL of 0 em R.O in proportion to the square of the &actional change in VT. Total (patient plus ventilator) WOBR will be changed by the product of the &actional changes in MIF and VT. From these predicted total values of WOBEL and WOBB, we calculated what &actions each component, resistive and elastic, should constitute the total inspiratory work. We multiplied these fractions by the observed patient inspiratory work to obtain our estimated values of WOBEL and WOBBe
DtIIa Analr/a. Statistical analysis was conducted (SYSTAT venion 4.2). The data are reported as means ± SE except as noted. Significance of change was determined by analysis of variance (ANOVA) considering pS 0.05 as significant. When significance was observed, po8t hoc comparisons of 10, 20, and 30 em 11.0 values with PSL=O values were made usiDg an F test with Bonferroni correction for multiple determinations (n = 3). E&"ects on Peo. and pR of the order in which levels of PS were applied were evaluated usiDg two-way ANOVA. Estimated and observed values ofWOBa. and WOB. were correlated using linear regression. Because these values were not normally distributed, this analysis was conducted after logarithmic transformation. Work is reported in joules. RESULTS
All patients tolerated the PS levels applied without apparent discomfort. As the PSL rose from 0 em 810, VT ehanged from 0.802±0.049 L to O.856±O.048 L at 10 em 810' 1.106±0.052 I at 20 em 810
A.
2
.30
• • •
!
...-
e ~
,
... .15
0
FIGURE 2. Mean values of A, inspiratory time (TJ), B, inspiratory time &action (TvTrar), C, mean inspiratory Bow (MIF), and D, mean airway pressure (MAP) for each level of PS. Two asterisks = p
