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Feature Articles
Pressure-controlled ventilation in children with severe status
asthmaticus*
Ashok P. Sarnaik, MD, FAAP, FCCM; Kshama M. Daphtary, MD; Kathleen L. Meert, MD, FAAP;
Mary W. Lieh-Lai, MD, FAAP; Sabrina M. Heidemann, MD, FAAP
Objective: The optimum strategy for mechanical ventilation in
a child with status asthmaticus is not established. Volume-controlled ventilation continues to be the traditional approach in such
children. Pressure-controlled ventilation may be theoretically
more advantageous in allowing for more uniform ventilation. We
describe our experience with pressure-controlled ventilation in
children with severe respiratory failure from status asthmaticus.
Design: Retrospective review.
Setting: Pediatric intensive care unit in a university-affiliated
children’s hospital.
Patients: All patients who received mechanical ventilation for
status asthmaticus.
Interventions: Pressure-controlled ventilation was used as the
initial ventilatory strategy. The optimum pressure control, rate,
and inspiratory and expiratory time were determined based on
blood gas values, flow waveform, and exhaled tidal volume.
Measurement and Main Results: Forty patients were admitted
for 51 episodes of severe status asthmaticus requiring mechanical ventilation. Before the institution of pressure-controlled ventilation, median pH and PCO2 were 7.21 (range, 6.65–7.39) and 65
torr (29 –264 torr), respectively. Four hours after pressure-con-
A
sthma remains one of the most
common causes of hospitalization in children despite considerable advances in our understanding of its pathophysiology and
management (1). Severe acute exacerbations of asthma, which are refractory to
pharmacologic interventions, are potentially life threatening and may require
mechanical ventilation. Morbidity and
*See also p. 191.
From the Division of Critical Care Medicine, Department of Pediatrics, Children’s Hospital of Michigan,
Wayne State University School of Medicine, Detroit, MI.
Presented, in part, at the American College of
Chest Physicians Annual Meeting, Philadelphia, PA,
November 2001.
Address requests for reprints to: Ashok P. Sarnaik,
MD, Children’s Hospital of Michigan, 3901 Beaubien,
Detroit, MI 48201. E-mail: [email protected]
Copyright © 2004 by the Society of Critical Care
Medicine and the World Federation of Pediatric Intensive and Critical Care Societies
DOI: 10.1097/01.PCC.0000112374.68746.E8
Pediatr Crit Care Med 2004 Vol. 5, No. 2
trolled ventilation, median pH increased to 7.31 (6.98 –7.45, p <
.005), and PCO2 decreased to 41 torr (21–118 torr, p < .005). For
patients with respiratory acidosis (PCO2 >45 torr) within 1 hr of
starting pressure-controlled ventilation, the median length of time
until PCO2 decreased to <45 torr was 5 hrs (1–51 hrs). Oxygen
saturation was maintained >95% in all patients. Two patients had
pneumomediastinum before pressure-controlled ventilation. One
patient each developed pneumothorax and subcutaneous emphysema after initiation of pressure-controlled ventilation. All patients survived without any neurologic morbidity. Median duration
of mechanical ventilation was 29 hrs (4 –107 hrs), intensive care
stay was 56 hrs (17–183 hrs), and hospitalization was 5 days
(2–20 days).
Conclusions: Based on this retrospective study, we suggest
that pressure-controlled ventilation is an effective ventilatory
strategy in severe status asthmaticus in children. Pressure-controlled ventilation represents a therapeutic option in the management of such children. (Pediatr Crit Care Med 2004; 5:133–138)
KEY WORDS: status asthmaticus; ventilators; mechanical; intensive care; child; preschool; child; adolescent
mortality rates among patients receiving
mechanical ventilation can be considerable. Markedly increased airway resistance and prolonged time constant are
characteristic features of respiratory mechanics in asthma. Traditionally, volume
control has been the preferred mode of
ventilation (2– 4). However, as tidal volume is delivered with constant flow in
traditional volume-controlled ventilation
(VCV), relatively less obstructed airways
with shorter time constant are likely to
receive more volume throughout inspiration compared with more obstructed airways with longer time constant. This
would result in uneven ventilation,
higher peak inspiratory pressure, and a
decrease in dynamic compliance. It has
been suggested that pressure-controlled
mode is better suited for mechanical ventilation in asthma (5, 6). With pressurecontrolled ventilation (PCV), because of a
constant inflation pressure, relatively less
obstructed lung units with shorter time
constant would achieve pressure equilibration earlier during inspiration compared with more obstructed areas. Thus,
units with shorter time constants would
attain their final volume earlier in inspiration whereas those with longer time
constants would continue to receive additional volume later in inspiration. This
would result in more even distribution of
inspired gas, delivery of more tidal volume for the same inflation pressure, and
improved dynamic compliance compared
with VCV. A disadvantage of PCV is that
the delivered tidal volume will vary depending on the respiratory system resistance. In a disease such as asthma with
potentially rapid changes in airway resistance, the tidal volume received by the
patient may change dramatically for the
same amount of inflation pressure. This
would necessitate frequent changes in
pressure control level to accommodate
changes in resistance. Pressure-regulated
volume control may be more advanta133
geous than PCV because it guarantees
tidal volume by regulating inflation pressure in the face of changing dynamic
compliance. However, pressure-regulated
volume control is a relatively new mode
of ventilation, not available in most ventilators, and therefore not well studied.
The objective of our study is to describe
our experience using PCV in the initial
management of children with status asthmaticus according to a predetermined
strategy.
MATERIALS AND METHODS
Study Population. Medical records of children receiving mechanical ventilation for status asthmaticus, admitted to the intensive
care unit (ICU) of Children’s Hospital of Michigan between January 1, 1995, and December
31, 2000, were reviewed. Patients who were
intubated before admission and extubated
within 4 hrs of admission, and those who had
suffered cardiopulmonary arrest and met clinical criteria of brain death on admission, were
excluded. The Wayne State University Human
Investigation Committee approved the study
and waived the need for informed consent.
Ventilatory Strategy. All patients were
managed with PCV as the initial ventilatory
mode. Puritan Bennett 7200 (Nellcor Puritan
Bennett, Pleasanton, CA), VIP Bird (Bird Products, Palm Springs, CA), and Siemens 900C
(Siemens-Elma, Solna, Sweden) were used for
mechanical ventilation. All patients received a
positive end-expiratory pressure of 4 cm H2O.
The level of pressure control (peak inspiratory
pressure ⫺ positive end-expiratory pressure)
was initiated at 25–30 cm H2O for patients age
1–5 yrs and at 30 –35 cm H2O for patients ⬎5
yrs of age. Respiratory rate was initially set at
12–16 breaths/min for patients age 1–5 yrs
and at 10 –12 breaths/min in patients ⬎5 yrs.
The inspiratory to expiratory ratio was set at
1:4. Exhaled tidal volume and flow-time curve
on ventilator waveform graphics were continuously monitored. Pressure control was adjusted by changes of 2–5 cm H2O to achieve an
exhaled tidal volume of 10 –12 mL/kg. In general, the increase in pressure control was limited to 50 cm H2O. Ventilatory rate, inspiratory time, and expiratory time were selected to
ensure near-complete return of expiratory
flow to baseline and to achieve the desired
exhaled tidal volume. Pressure control, rate,
inspiratory time, and expiratory time were
subsequently adjusted to maintain pH ⬎7.3
and PCO2 ⬍50 torr or exhaled tidal volume of
10 –12 mL/kg. The FIO2 was adjusted to maintain SpO2 ⬎95%. The optimal settings for
each patient were defined as the combination
of pressure control, rate, and inspiratory time
within the initial 8 hrs that resulted in the
first observed decline in PCO2 that was sustained on subsequent blood gas determinations. Pressure control was reduced by 2–5 cm
H2O with improvement in airway obstruction
134
as indicated by exhaled tidal volume ⬎12
mL/kg and resolution of respiratory acidosis.
Patients were weaned from mechanical ventilation when their flow waveforms demonstrated an exhalation phase short enough to
allow spontaneous breathing in between machine-delivered breaths and their PCO2 values
were ⬍45 torr. Pharmacologic paralysis was
stopped before weaning. Patients were weaned
according to physician preference by decreasing PCV synchronized intermittent mandatory
ventilation rate, by providing pressure support, by changing to VCV, or by combining
VCV with pressure support.
Baseline Patient Characteristics. Age, gender, race, past medical history, current home
medications, referral source, and medications
administered before and after mechanical ventilation were recorded. Reasons for intubation
and mechanical ventilation were categorized
as a) cardiorespiratory arrest; b) respiratory
arrest (absent or agonal respirations); c) respiratory acidosis (pH ⱕ7.32 with PCO2 ⱖ50
torr), with clinical impression of respiratory
fatigue (severe retractions, inability to speak a
complete sentence, obtunded sensorium and
diaphoresis); and d) clinical impression of respiratory fatigue without documented respiratory acidosis.
Outcome Measures. Ventilator settings and
arterial or capillary blood gas values were recorded before initiation of PCV, within 1 hr of
starting PCV, and at 4, 8, and 12 hrs after
initiation of PCV. Duration of mechanical ventilation, intensive care and hospitalization,
complications, and mortality also were noted.
Complications evaluated included pneumothorax, pneumomediastinum, subcutaneous
emphysema, the need for fluid resuscitation
and vasopressor agents, and neurologic morbidity. Fluid resuscitation was provided for
hypotension and/or clinical impression of hypovolemia (cold, clammy extremities and decreased urine output). Vasopressor agents
were started for hypotension resistant to fluid
resuscitation.
Statistical Analysis. Categorical variables
are represented by the absolute count and
percentage in each category. Continuous variables are represented by the median and
range. Both capillary and arterial blood gas
values were used for analysis of pH and PCO2.
Since all patients received albuterol nebulization with 100% oxygen delivered through the
inspiratory limb of the ventilator, the actual
FIO2 delivered to the patient could not be accurately determined. Therefore, PaO2/FIO2 values were omitted from statistical interpretation. Blood gas measurements over time were
compared using Wilcoxon signed ranks test.
We considered p ⬍ .05 to be significant.
RESULTS
During the study period, 278 patients
were admitted to the ICU for management of status asthmaticus. Of these, 56
episodes in 45 patients received mechanical ventilation. Four patients were rapidly weaned off mechanical ventilation
and extubated within 4 hrs of admission
and were excluded from the analysis. One
patient sustained cardiorespiratory arrest
at home, was resuscitated, but was clinically brain dead on arrival to the ICU and
also was excluded from analysis. The remaining 40 patients were admitted for 51
episodes of status asthmaticus; 34 patients were admitted once, two patients
were admitted twice, three patients were
admitted three times, and one patient
was admitted four times. Twenty-one patients (41%) were intubated and mechanically ventilated for respiratory acidosis,
19 (37%) for clinical impression of fatigue, and 11 (22%) for respiratory arrest.
Baseline characteristics of patients are
shown in Table 1. The racial distribution
of patients reflects our referral base for
medical/surgical emergencies.
Medical therapy administered before
and during institution of PCV is shown in
Table 2. All episodes were managed with
sedatives and analgesics, and 46 episodes
were managed with neuromuscular
blocking agents from the initiation of
PCV. Sedation and analgesia were
achieved with midazolam and morphine.
Ketamine was used as an adjunct sedative
in six patients. Vecuronium was used for
pharmacologic paralysis. All episodes
were treated with inhaled albuterol and
intravenous corticosteroids, and 94% received intravenous theophylline (7). Sodium bicarbonate or tris-(hydroxymethyl) aminomethane was administered
in 11 episodes for metabolic acidosis. Antibiotics were given in five episodes for
suspected bacterial infection. Arterial
catheters were placed in 45 episodes.
Bronchoscopy was performed in five for
management of mucus plugging.
Ventilator settings that achieved the
most satisfactory gas exchange within the
first 8 hrs in age groups 1–5 yrs, 6 –10
yrs, and 11–18 yrs are shown in Table 3.
Blood pH and PCO2 on admission and at 1,
4, 8, and 12 hrs after institution of PCV
are shown in Table 4. Seven patients had
PCO2 ⬍45 torr, 21 patients (41%) had
PCO2 between 45 and 70 torr, 12 patients
(24%) had PCO2 between 71 and 99 torr,
and eight patients (16%) had PCO2 ⱖ100
torr before initiation of PCV. Blood gases
were not obtained before PCV in three
patients because emergent intubation
and mechanical ventilation were clinically deemed necessary. Of the ten patients with PCO2 ⬍45 torr or nondocuPediatr Crit Care Med 2004 Vol. 5, No. 2
Table 1. Baseline characteristics (n ⫽ 51)
Age, yrs
Males
Race
African-American
White
Hispanic
Reason for intubation
Respiratory arrest
Respiratory acidosis
Clinical impression of fatigue
Previous ICU admission for status asthmaticus
Previous mechanical ventilation for status asthmaticus
Home medications
Inhaled ␤-agonists
Inhaled corticosteroids
Oral corticosteroids
Theophylline
Referral source
Emergency room
General ward
Another hospital
a
9.0 (0.8–17)
32 (63)b
49 (96)
1 (2)
1 (2)
11 (22)
21 (41)
19 (37)
26 (51)
18 (35)
51 (100)
19 (37)
4 (8)
6 (12)
26 (51)
5 (10)
20 (39)
ICU, intensive care unit.
a
Data are median (range); bdata are absolute counts (%).
Table 2. Medical therapies (n ⫽ 51)
Inhaled
Albuterol
Ipratropium bromide
Isoproterenol
Helium-oxygen
Intravenous
Corticosteroids
Theophylline
Isoproterenol
Magnesium sulfate
Ketamine
Terbutaline
Subcutaneous
Epinephrine
Terbutaline
Before
PCV
During
PCV
51 (100)
21 (41)
8 (16)
0
51 (100)
24 (47)
0
25 (49)
51 (100)
28 (55)
6 (12)
7 (14)
0
3 (6)
51 (100)
48 (94)
21 (41)
9 (18)
6 (12)
0
33 (65)
3 (6)
0
0
PCV, pressure-controlled ventilation.
Data are absolute counts and percentages.
mented PCO2 before PCV, eight were
intubated for clinical impression of respiratory fatigue and two for respiratory arrest. Noninvasive ventilation was not
used in any of these patients. For patients
with PCO2 ⬎45 torr within 1 hr of starting
PCV, the median length of time until
PCO2 decreased to ⬍45 torr was 5 hrs
(1–51 hrs). SpO2 was maintained above
95% in all patients throughout the ICU
stay.
Patients were weaned from mechanical ventilation by decreasing the PCV synchronized intermittent mandatory ventilation rate in 28 episodes, by providing
pressure support in eight episodes, by
changing to VCV in five episodes, and by
Pediatr Crit Care Med 2004 Vol. 5, No. 2
combining VCV with pressure support in
ten episodes. The duration of mechanical
ventilation was 29 hrs (4 –107 hrs), intensive care 56 hrs (17–183 hrs), and hospitalization 5 days (2–20 days). Complications encountered are shown in Table 5.
Of note, fluid resuscitation was provided
in 78% of episodes. Two patients had
pneumomediastinum at admission before
institution of PCV. In both patients,
pneumomediastinum resolved during
PCV. One patient each developed pneumothorax and subcutaneous emphysema
after initiation of PCV. Subcutaneous emphysema resolved spontaneously and
pneumothorax resolved after insertion of
a single chest tube while PCV was continued. All patients survived without neurologic morbidity including polyneuropathy associated with the use of steroids
and neuromuscular blocking agents.
DISCUSSION
Despite maximum medical therapy,
mechanical ventilation may be needed to
manage respiratory insufficiency resulting from airway obstruction in severe status asthmaticus. Mechanical ventilation
in patients with airway obstruction may
be associated with significant morbidity
and mortality rates. The goal of mechanical ventilation in status asthmaticus is
two-fold: to provide sufficient gas exchange to ensure survival until reversal
of airway obstruction is accomplished,
and to minimize complications associated with such support. The two modes of
mechanical ventilation that are com-
monly used are PCV and VCV. In PCV, a
predetermined airway pressure is maintained for a set period of inspiratory time.
The delivered tidal volume depends on
airway resistance and dynamic compliance. In VCV, a predetermined tidal volume is delivered throughout the inspiratory phase. The resultant peak airway
pressure is a dependent variable determined by airway resistance and dynamic
compliance. Dynamic compliance is
greatly influenced by flow-resistive properties of the lung. In diseases with increased airway resistance, dynamic compliance can be sharply reduced at higher
respiratory rates because of an increase in
air flow. Another important consideration in mechanical ventilation is that of
time constant, a product of static compliance and resistance. Time constant is a
reflection of the amount of time that is
necessary for equilibration of proximal
airway pressure and alveolar pressure.
Asthma is a predominantly obstructive
airway disease characterized by long time
constant requiring a relatively long time
for approximation of pressure at proximal
airway and alveoli during inspiration and
expiration. Expiratory time constant is
prolonged even more because of a greater
increase in expiratory resistance resulting from an upstream (closer to the terminal airways) displacement of the equal
pressure point and narrowing of airways
during expiration. As opposed to disorders of static compliance with short time
constants such as acute respiratory distress syndrome that can be managed with
relatively faster respiratory rates, disorders of resistance such as asthma require
relatively slow rates for adequate ventilation. Insufficient inspiratory time would
result in decreased tidal volume, whereas
incomplete exhalation would result in dynamic hyperinflation and auto positive
end-expiratory pressure. Ventilator strategies for patients with asthma should include relatively low respiratory rates with
long expiratory times.
The degree of airway obstruction is
not uniform in asthma and varies in different lung units. Thus, there may be
considerable regional variation in time
constants. It could be argued that in status asthmaticus, PCV is a better strategy
than VCV, which is the traditionally recommended mode of ventilation (2– 4).
With VCV, tidal volume is delivered at a
relatively constant flow rate so that relatively less obstructed airways with
shorter time constants are likely to receive more volume throughout inspira135
Table 3. Optimal ventilator settings
1–5 Yrs
(n ⫽ 20)
36 (20–45)
14 (10–20)
1.0 (0.6–1.2)
1:4 (1:3–1:5)
4
Peak inspiratory pressure, cm H2O
Rate, breaths/min
Inspiratory time, secs
I:E ratio
PEEP, cm H2O
6–10 Yrs
(n ⫽ 13)
11–18 Yrs
(n ⫽ 18)
38 (26–49)
12 (10–14)
1.0 (1.0–1.25)
1:4 (1:3–1:5)
4
40 (30–60)
12 (8–12)
1.2 (1.0–1.8)
1:4 (1:2–1:5)
4
I:E, inspiratory/expiratory; PEEP, positive end-expiratory pressure.
Data are median (range).
Table 4. Blood gas values over time
pH
PCO2, torr
Before PCV
Within 1 Hr
of PCV
4 Hrs
After PCV
8 Hrs
After PCV
12 Hrs
After PCV
7.21
(6.65–7.39)
(n ⫽ 48)
65
(29–264)
(n ⫽ 48)
7.22
(6.95–7.47)
(n ⫽ 51)
59
(24–115)
(n ⫽ 51)
7.31a
(6.98–7.45)
(n ⫽ 51)
41a
(21–118)
(n ⫽ 51)
7.34a
(7.07–7.49)
(n ⫽ 46)
38a
(23–98)
(n ⫽ 46)
7.38a
(7.17–7.55)
(n ⫽ 43)
37a
(16–76)
(n ⫽ 43)
PCV, pressure-controlled ventilation.
a
p ⬍ .01 by Wilcoxon’s signed ranks test compared with values obtained before PCV. Data are
median and range.
Table 5. Outcomes (n ⫽ 51)
Duration of mechanical ventilation, hrs
Duration of ICU stay, hrs
Duration of hospitalization, days
Complications
Need for fluid resuscitation
Need for vasopressors
Pneumothorax
Pneumomediastinum
Subcutaneous emphysema
Neurologic morbidity
Death
29 (4–107)a
56 (17–183)
5 (2–20)
40 (78)b
4 (8)
1 (2)
0c
1 (2)
0
0
a
Data are median (range); bdata are absolute counts (%); ctwo patients had pneumomediastinum
before initiation of pressure-controlled ventilation.
tion compared with more obstructed airways with longer time constants. This
will result in uneven ventilation, higher
peak inspiratory pressure, and decrease
in dynamic compliance. With PCV, because of the constant inflation pressure,
relatively less obstructed lung units with
shorter time constants will achieve pressure equilibration during early inspiration and more obstructed areas with
longer time constants will continue to
receive additional volume in late inspiration. This will result in more even distribution of inspired gas, delivery of more
tidal volume for the same inflation pressure, and improved dynamic compliance.
There are isolated case reports of PCV and
pressure support ventilation in patients
with status asthmaticus who required
mechanical ventilation (6, 8). This is the
136
only study reported that describes the use
of PCV exclusively in managing severe
respiratory failure from status asthmaticus. It is also the second largest study
describing the outcome of children with
status asthmaticus who were intubated
and mechanically ventilated.
Risk factors for intubation in asthmatics include psychosocial problems, family
dysfunction, low socioeconomic status,
second-hand smoke exposure, parental
history of allergy, prior intubation, intercurrent respiratory infection, prior hospitalization for asthma in the past year,
excessive use of ␤2-agonist inhalers, and
steroid dependence (9 –11). A number of
these risk factors were evident in our
patients (Table 1). Most children with status asthmaticus are medically managed
without intubation and mechanical ven-
tilation. Mechanical ventilation is most
often reserved for children with actual or
impending respiratory failure. Of 278 patients with status asthmaticus admitted
to the ICU during the study period, only
56 (20%) required intubation. These findings are similar to those of Roberts et al.
(12), who reported a 17% incidence of
mechanical ventilation in asthmatic children from 11 pediatric ICUs. Little information is available in the literature regarding the specific indications for
intubation in patients with asthma. Stein
et al. (13) reported that 10% of asthmatic
children intubated in the ICU had preceding respiratory arrest. Among adult studies, approximately 50% of asthmatics requiring intubation had cardiopulmonary
arrest or coma (14, 15).
It has been suggested that mechanical
ventilation itself may be a cause of mortality and morbidity in patients with
acute severe asthma. Studies of patients
with severe asthma who received mechanical ventilation during the 1970s and
1980s have reported mortality rates as
high as 23–38% (16 –18). More recent
studies suggest that the mortality rate for
ventilated asthmatics may be decreasing
(19, 20). The greatest threats to survival
in such patients are ineffective oxygenation and ventilation, and barotrauma.
Darioli and Perret (21) proposed using
permissive hypercapnia to minimize
barotrauma in patients with asthma who
need mechanical ventilation. Purported
benefits of hypercapnia include a reduction in lung stretch, dilation of small
airways, improvement in collateral ventilation, and attenuation of inflammatory
processes (12). Several authors have reported decreased mortality rates with the
use of controlled hypoventilation (14, 19,
20). However, there are no controlled
studies demonstrating benefits of permissive hypercapnia compared with normocapnia. Moreover, hypercapnia and the
resultant respiratory acidosis may be associated with undesirable consequences
such as decreased catecholamine responsiveness, blunting of the effects of bronchodilators (22, 23), and increased intracranial pressure. We aimed at achieving
normocapnia in our patients using the
pressure control, respiratory rate, inspiratory time, and expiratory time required to deliver the necessary tidal volume and minute alveolar ventilation.
More than 80% of our patients had PCO2
⬎45 torr before the institution of PCV.
All these patients had a steady decline in
PCO2 after institution of PCV. All patients
Pediatr Crit Care Med 2004 Vol. 5, No. 2
O
ur retrospective
study of 51 episodes in 40 pa-
tients demonstrates the efficacy of pressure-controlled
ventilation in treating children with acute respiratory
failure from severe status
asthmaticus. Pressure-controlled ventilation represents
a therapeutic option in the
management
of
such
children.
survived to be discharged from the hospital.
The median duration of ventilation in
our patients was 29 hrs (mean, 30 hrs;
range, 4 –107 hrs). This compares favorably with that reported in other studies.
Malmstrom et al. (24) reviewed the
records of 59 children admitted on 66
occasions to the ICU who received mechanical ventilation for severe asthma.
The details of the mode of ventilation or
the ventilatory strategy were not mentioned. The mean duration of mechanical
ventilation was 51 hrs with a range of
6 –216 hrs (24). Two other studies employing VCV as the mode of ventilation
reported the mean duration of mechanical ventilation to be 23.9 hrs (range,
1.5–78 hrs) and 54 hrs (range, 20 –185
hrs) (25, 26). Studies in children combining VCV and permissive hypercapnia documented the mean duration of mechanical ventilation to be 32–54.3 hrs (4, 13,
27, 28). The duration of mechanical ventilation in adults with severe asthma is
much longer, and the mean duration
ranges from 36.1 hrs to 4.9 days (14, 15,
21, 29 –31). The median time for the PCO2
to return to normal was 5 hrs (1–51 hrs)
in this study. This was considerably less
than that reported by Dworkin and Kattan (28) (20.1 hrs) and Shugg et al. (27)
(14.9 hrs), who used VCV and controlled
hypoventilation in children with status
asthmaticus.
Pediatr Crit Care Med 2004 Vol. 5, No. 2
Despite eschewing permissive hypercapnia, the incidence of barotrauma (4%)
was relatively low in our patients. It is of
note that another two (4%) patients had
pulmonary airleak before initiation of
PCV. Furthermore, the pulmonary airleak in our patients was easily managed
and still allowed continuation of PCV
mode. In pediatric studies that used VCV
without controlled hypoventilation, the
incidence of barotrauma was 16 –33%
(25, 26). Studies using permissive hypercapnia and limited peak inflation pressures reported the incidence of barotrauma ranging from zero in one study
(24) to 15–20% in others (4, 13, 27, 32).
The corresponding rates of barotrauma in
adults who received VCV for asthma, with
and without controlled hypoventilation
strategies, were 4 –27% (14, 15, 19 –21)
and 2–18% (29, 30), respectively. The incidence of barotrauma in our study using
PCV strategy aimed at achieving normocapnia compares favorably with that reported in the literature. A large percentage (78%) of patients exhibited
circulatory compromise manifested by
cool, clammy extremities and reduced
urine output. Circulatory compromise
may have resulted from positive pressure
ventilation, preexisting hypovolemia secondary to poor oral intake and increased
insensible water losses, and increased venous capacitance following the use of
sedative and paralytic agents. Patients
with circulatory compromise improved
after intravascular volume expansion
with isotonic fluids. No other complications attributed to mechanical ventilation
were encountered.
A low tidal volume, high ventilatory
rate strategy has been shown to be effective in reducing mortality rates in patients with acute respiratory distress syndrome (33). It should be noted, however,
that acute respiratory distress syndrome
is predominantly a disease of low compliance and short time constant as opposed
to increased airway resistance as in status
asthmaticus. The ventilatory strategy in
our study was aimed at attaining a given
minute alveolar ventilation necessary to
achieve pH ⬎7.30 and PCO2 ⬍50 torr.
This was accomplished by a combination
of using a pressure control variable, using a relatively low ventilatory rate, and
adjusting inspiratory/expiratory time to
achieve exhaled tidal volume of 10 –12
mL/kg. Ventilator graphic waveform was
continuously monitored to ensure near
completion of exhalation to avoid auto
positive end-expiratory pressure and decreased dynamic compliance. Older patients required higher peak inspiratory
pressure and lower rates. Pressure control was decreased as exhaled tidal volumes increased and PCO2 decreased. Once
sufficiently improved, the patients were
rapidly weaned off the mechanical support by withdrawing pharmacologic paralysis and allowing spontaneous respirations in PCV or VCV modes with or
without pressure support and decreasing
the synchronized intermittent mandatory
ventilation rate.
Our retrospective study of 51 episodes
in 40 patients demonstrates the efficacy
of PCV in treating children with acute
respiratory failure from severe status
asthmaticus. PCV represents a therapeutic option in the management of such
children. Randomized controlled trials
are needed to compare the efficacy and
complications of PCV and VCV. Furthermore, the superiority of normocapnic vs.
hypercapnic strategy regarding duration
of mechanical ventilation and complications needs to be investigated.
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