19 2
February 1977 The Journal o f P E D 1 A T R 1 C S
Lung mechanics in congenital heart disease with increased and decreased pulmonary blood flow Respiratory rate, tidal volume, dynamic lung compliance, functional residual capacity, and pulmonarv resistance were measured within 24 hours of cardiac catheterization in 25 infants, 12 of whom had increased pulmonary blood flow and 13 of whom had decreased PBF. There were no differences in the two groups of patients with respect to V, and FRC. Respiratory rate and pulmonary resistance were higher in infants with increased PBF. Lung compliance was significantly lower in infants with increased PBF (4.9 ml/cm H~O) than in those with decreased PBF (8.9 ml/cm HzO) (P < 0.01). The decrease in CL in infants with increased PBF significantly correlated with mean pulmonary artery pressure (r = O.798). No correlation was found between CL and left atrial pressure or magnitude of the left-toright shunt. Compliance was normal in patients with increased PBF and normal PAP, suggesting that PAP and not PBF is the primary factor that affects CL in patients with intracardiac left-to-right shunts.
E. Banealari, M.D.,* M. J. Jesse, M.D., H. Geiband, M.D., and O. Garcia, M.D., M i a m L Fla.
TACHYPNEA and intercostal retractions are frequently observed in infants with congenital heart disease associated with a significant increase in pulmonary blood flow. ~ On the other hand, cardiac malformations associated with a decrease in PBF rarely have respiratory symptomatology, although cyanosis may be evident due to intracardiac right-to-left shunting. Several studies have been performed in children and adults with CHD, in an attempt to correlate the presence of respiratory symptoms and the mechanical properties of the lung. The differing results reported can partially be explained by the marked differences in the patient populations studied, and to the multiple factors involved in the mechanical characteristics of the lung. For example, low values of dynamic compliance have been reported in patients with increased PBF.'-' but simultaneous
From the Divisions of Newborn Medicine and Pediatric Cardiology, Department of Pediatrics, University of Miami School of Medicine. Supported in part by the National Foundation, Research and Service Grant C-75, and the Joyce Biel Adler Neonatal'Research Grant. *Reprint address: University of Miami School of Medicine, PO Box 520875, Biscayne Annex, Miami. FL 33152.
VoL 90, No. 2, pp. 192-195
measurement of the functional residual capacity was not performed, making the interpretation of these results difficult. Thus a systematic study was performed in an attempt to determine the relationship between pulmonary hemodynamics and the mechanics of breathing in infants with CHD and associated increased or decreased PBF.
MATERIALS AND M E T H O D S Twenty-five infants less than 12 months of age, who were undergoing diagnostic cardiac catheterization for Abbreviations used ASD: atrial septal defect CHD: congenital heart disease CL: lung compliance FRC: functional residual capacity LAP: left atrial pressure PAP pulmonary artery pressure PBF: pulmonary blood flow PDA: patent ductus arteriosus PS: pulmonary stenosis R: pulmonary resistance TA: tricuspid atresia TF: tetralogy of fallot VT: tidal volume VSD: ventricutar septal defect Qp/Qs: pulmonary/systemic blood flow
Volume 90 Number 2
Lung mechanics in congenital heart disease
T a b l e I. C a t h e t e r i z a t i o n d a t a a n d p u l m o n a r y
Case No.
m e c h a n i c s in i n f a n t s with i n c r e a s e d a n d d e c r e a s e d P B F
Specific CL (ral/craH,~O/ L-FRC)
FRC/wt CL (ml/kg) (ml/cmHzO)
19 3
Pulmonary resistance inspiration
Pulmonary resistance expiration
(craH~O/L/sec)
PA P system Qp/Qs (ram Hg)
Diagnosis
Infants with increased PBF 1 2 3 4 5 6 7* 8 9 10 11" 12
36 23 23 30 16 34 29 25 20 32 18.7 27 26.1 1.8 26.6 2.02
Mean SE Mean (PS excluded) SE
3.6 3 2.3 2.27 1.68 4.2 13.3 3.02 3.25 7.5 10 5.1 4.94 1.02 3.6 1.69
26 29 30 13 38 48 86 40 44 44 97 52 45.6 6.95 36.4 3.74
120 35 23 94 22 52 21.9 83 53 36 36 52.4 9.9 57.5 11.4
71 125 88 90 60 56 8.8 100 84 -11.3 53 67.9 10.7 80.7 7.8
1.5 3.7 6.3 2.2 4.5 2 1.8 2.7 4 1.3
42 65 85 70 52 50 20 40 45 42 40
VSD VSD-ASD VSD PDA VSD-ASD VSD-ASD-PDA VSD-PS PDA VSD VSD-PDA VSD-PS ASD
17 56 42 18 7.1 53 20 13.8
37 62 37 32 32 160 63 12.5
0.86 0.50 0.90 0.75 0.90 -
-
20 --
0.25 0.29 0.25 0.30
---
TA-PA hypoplasia PA atresia-ASD-PDA TF TA-ASD-VSD TF TA TF TF PA atresia-single ventricle TF TF PA atresia TF
Infants with decreased PBF 1 2 3 4 5 6 7 8 9 10 11 12 13 Mean SE P ( + PBF vs ~, PBF) P (PS excluded)
22 28 28 32 28 34 42 35
4.1 10.4 6.2 10.8 8.5 5.4 10.8 8.6
49 101 68 64 47 63 102 67
35 -31.6 1.9 n/s n/s
7,6 10.2 21.2 6.4 6 8.94 1.19 0.019 0.01
69 --. . 70 6.5 0.02 0.0003
10 -. 26.8 6.1 0.053 0.03
.
. 50.6 14.8 n/s n/s
*Pulmonic stenosis.
CHD+ w e r e s e l e c t e d for t h e s t u d y . T w e l v e h a d l e s i o n s with
infant
intracardiac left-to-right shunts and resultant increased
W h i t e Plains, N Y ) a n d a f a c e m a s k . E s o p h a g e a l p r e s s u r e
PBF;
13 h a d r i g h t - t o - l e f t s h u n t s a n d
(Electronics
for
Medicine,
PBF.
w a s m e a s u r e d u s i n g a t h i n latex b a l l o o n p l a c e d in t h e
P a t i e n t s with e v i d e n c e o f p u l m o n a r y c o m p l i c a t i o n s or a
m i d p o r t i o n o f the e s o p h a g u s a n d c o n n e c t e d to a p r e s s u r e
clinical
transducer (Statham Instruments+ Oxnard, CA). Inspira-
diagnosis
of
congestive
decreased
pneumotachograph
heart
failure
were
excluded from the study. Lung mechanics were evaluated on the day of the c a r d i a c c a t h e t e r i z a t i o n w i t h t h e i n f a n t s in a n o n s e d a t e d
tory a n d e x p i r a t o r y p u l m o n a r y r e s i s t a n c e w e r e c a l c u l a t e d from
the pressure volume
loop
using
the nonelastic
p r e s s u r e at m i d i n s p i r a t i o n a n d e x p i r a t i o n , a n d t h e s i m u l -
s l e e p i n g state. D y n a m i c c o m p l i a n c e w a s c a l c u l a t e d at e n d
taneous
i n s p i r a t i o n f r o m s i m u l t a n e o u s t r a c i n g s o f tidal v o l u m e
c a p a c i t y w a s d e t e r m i n e d in all i n f a n t s w i t h i n c r e a s e d P B F
and esophageal pressure. Tidal volume was obtained by
a n d in n i n e with d e c r e a s e d P B F b y m e a n s o f t h e h e l i u m
electrical i n t e g r a t i o n o f t h e flow s i g n a l m e a s u r e d w i t h a n
d i l u t i o n t e c h n i q u e in a c l o s e d s y s t e m . ;
measurements
of
flow.
Functional
residual
194
Bancalari et aL
The Journal o f Pediatrics February 1977
ml/cm
ml/cm H 2 0 /
H20
L-FRC p = 0.01
p = 0.0003
10 - -
100 . . . . . . . . . . . . .
t6p 9 ,6p 5--
50
- -
0 - -
DYNAMIC COMPLIANCE
- -
SPECIFICCOMPLIANCE
Fig. 1. Values of dynamic and specific lung compliance in infants with increased and decreased pulmonary blood flow. O
=
PULMONIC
loo
r
"' u
75
O U "r
50
uL,I.I E
25
-0.7987
(p = 0,0098)
0
_< ~_
=
STENOSIS
9
*
9
~O
I
I
I
I
I
I
10
20
30
40
50
60
MEAN PULMONARY ARTERY PRESSURE (ram klg)
Fig. 2. Relation between mean pulmonary artery pressure and specific lung compliance in infants with increased pulmonary blood flow. (One infants with VSD and PS is not included because the pulmonary artery was not entered during cardiac catheterization.) RESULTS The physical characteristics of the patients studied were similar in both groups. The mean age in infants with increased PBF was 3.8 months (15 days to 9 months) as compared to 2.1 months (3 days to 7 months) in those patients with decreased PBF. The weight at the time of the study was also similar in both groups (4,081 + 334 gm versus 3,930 -2-_371 gm). The respiratory rate was higher in infants with increased PBF (64 versus 45; P < 0.05), whereas the tidal volume was similar in the two groups.
Catheterization data and the results of the pulmonary mechanics in the two groups are shown in Table I. Functional residual capacity was similar in both groups, and dynamic compliance was significantly lower in the group with increased PBF (4.94 versus 8.94 ml/cm H..O: P < 0.02). The difference in C,, was therefore due to changes in the elastic properties of the lung and not to differences in lung volume. When dynamic compliance was corrected to one liter of FRC to obtain specific compliance, the difference between the two groups remained significant. Two infants with increased PBF (Cases 7 and 11) had associated mild pulmonic stenosis; they were the only patients in this group who had a specific compliance greater than 60 ml/cm H~O/L-FRC. If these two patients are not included in the statistical analysis, the difference in compliance is even greater (Fig. 1). Total pulmonary resistance was higher in infants with increased PBF; during inspiration this difference reached statistical significance (57.5 versus 26.8 cm H~O/L/sec: P < 0.05). No correlation was found between specific compliance and left atrial pressure in seven patients in whom LAP was measured. Likewise no correlation was found between specific compliance and the magnitude of the left-to-right shunt in infants in whom PBF was shown to be increased. However, a significant correlation was obtained between specific compliance and mean pulmonary artery pressure in these patients (r = 0.798: P < 0.01) (Fig. 2). DISCUSSION The results of compliance and FRC obtained in the present study in infants with decreased PBF are similar to those previously reported in normal infants." : it appears, therefore, that a diminished PBF does not alter the mechanical properties of the lung. The finding of a decreased dynamic lung compliance in patients with increased PBF has been reported previously in infants and adults with intracardiac left-to-right shunts/ ' The mechanism for this change, however, is not clear, since many hemodynamic factors can influence the mechanical properties of the lung. An increase in left atrial and pulmonary venous pressure is known to produce a decrease in lung compliance in experimental animals,'" but in the present study no significant correlation was found between these two measurements. This may be explained by the fact that all of our patients had relatively low left atrial pressures, because infants with heart failure were excluded from the study. Both cardiomegaly and an increase in intrathoracic blood volume may displace the effective lung volume.
Vohtme 90 Number 2
These mechanisms have been implicated as possible causes of the decreased lung compliance in patients with a significant left-to-right shunts " These explanations seem unlikely because the FRC was similar in both of our patient groups. The two other mechanisms which may be responsible for the decrease in lung compliance are the increase in PBF and/or the increase in pressure in the pulmonary system. An increase in flow without an increase in pressure can be excluded as a possibility since no correlation was found in this and other studies between the magnitude of left-to-right shunts and lung complianceS "~ Moreover, patients with atrial septal defects and a large increase in PBF have been found to have normal C I j Thus it appears that PAP is primarily responsible for the changes observed in the mechanical properties of the lung. The findings in our two patients with left-to-right shunts and pulmonary stenosis support this hypothesis; these were the only patients who had normal lung compliance values in spite of increased pulmonary blood flow. The mechanism by which the increase in pulmonary vascular pressure alters the mechanical properties of the lung can only be theorized. One possibility is that the increase in PAP may result in an increase in the tension of the walls of the pulmonary vascular system. This "stiffer" vasculature may then oppose pulmonary expansion and result in the observed decrease in lung compliance. The same mechanism may be in part responsible for the transient decreased compliance observed in the normal newborn infant and the subsequent increase in C~. observed as PAP decreases after birth. An increase in pulmonary artery pressure due to an increase in pulmonary vascular resistance with normal PBF may have the same effect. An additional factor that can contribute to the decrease in lung compliance in infants with a left-to-right shunt is the observed higher total pulmonary resistance associated with an increased respiratory rate. Lung compliance
Lung mechanics in congenital heart disease
19 5
becomes frequency dependent with an increase in small airway resistance. The cause of this increased pulmonary resistance in infants with a left-to-right shunt can be either extrinsic compression of the airways by enlarged pulmonary vessels or congestion of the bronchial wall. The determination of lung compliance is a simple bedside procedure which could be utilized as an adjunct in the clinical evaluations of patients with intracardiac left-to-right shunts, since it may be an indicator of changes occurring in the pulmonary artery pressure.
REFERENCES
1. Lees MH, Way RC, and Ross BB: Ventilation and respiratory gas transfer of infants with increased pulmonary blood flow, Pediatrics 40:259, 1967. 2. GriffinAJ, Ferrara JD, Lax JO, and Cassels DE: Pulmonary compliance. An index of cardiovascular status in !nfancy, Am J Dis Child 123:89, 1972. 3. Wallgren (3, Geubelle F, and Koch G: Studies of the mechanics of breathing in children with congenital heart lesions, Acta Paediatr 49:415, 1960. 4. Saxton GA. Rabinowitz M, Dexter L, and Haynes F: The relationship of pulmonary compliance to pulmonary vascular pressures in patients with heart disease, J Clin Invest 35:611, 1956. 5. Krauss AN, and Auld PAM: Measurements of functional residual capacity in distressed neonates by helium rebreathing, J PEDIATR77:228, 1970. 6. Phelan PD, and Williams HE: Ventilatory studies in healthy infants, Pediatr Res 3:425, 1909. 7. Krieger 1: Studies of mechanics of respiration in infancy, Am J Dis Child 105:439, 1963. 8. HowlettG: Lung mechanics in normal infants and infants with congenital heart disease, Arch Dis Child 47:707, 1972. 9. Phelan PD, Gillam GL, Menahem SA, Coombs E, and Venables AW: Respiratory function in infants with a ventricular septal defect, Aust Paediat J 8:79, 1972. 10. Borst HG, Berglund E, Whittenberger JL, Mead J, McGregor M, and Collier C: The effect of pulmonary vascular pressures on the mechanical properties of the lungs of anesthetized dogs, J Clin Invest 36:1708, 1957.
February 1977 The Journal o f P E D 1 A T R 1 C S
Lung mechanics in congenital heart disease with increased and decreased pulmonary blood flow Respiratory rate, tidal volume, dynamic lung compliance, functional residual capacity, and pulmonarv resistance were measured within 24 hours of cardiac catheterization in 25 infants, 12 of whom had increased pulmonary blood flow and 13 of whom had decreased PBF. There were no differences in the two groups of patients with respect to V, and FRC. Respiratory rate and pulmonary resistance were higher in infants with increased PBF. Lung compliance was significantly lower in infants with increased PBF (4.9 ml/cm H~O) than in those with decreased PBF (8.9 ml/cm HzO) (P < 0.01). The decrease in CL in infants with increased PBF significantly correlated with mean pulmonary artery pressure (r = O.798). No correlation was found between CL and left atrial pressure or magnitude of the left-toright shunt. Compliance was normal in patients with increased PBF and normal PAP, suggesting that PAP and not PBF is the primary factor that affects CL in patients with intracardiac left-to-right shunts.
E. Banealari, M.D.,* M. J. Jesse, M.D., H. Geiband, M.D., and O. Garcia, M.D., M i a m L Fla.
TACHYPNEA and intercostal retractions are frequently observed in infants with congenital heart disease associated with a significant increase in pulmonary blood flow. ~ On the other hand, cardiac malformations associated with a decrease in PBF rarely have respiratory symptomatology, although cyanosis may be evident due to intracardiac right-to-left shunting. Several studies have been performed in children and adults with CHD, in an attempt to correlate the presence of respiratory symptoms and the mechanical properties of the lung. The differing results reported can partially be explained by the marked differences in the patient populations studied, and to the multiple factors involved in the mechanical characteristics of the lung. For example, low values of dynamic compliance have been reported in patients with increased PBF.'-' but simultaneous
From the Divisions of Newborn Medicine and Pediatric Cardiology, Department of Pediatrics, University of Miami School of Medicine. Supported in part by the National Foundation, Research and Service Grant C-75, and the Joyce Biel Adler Neonatal'Research Grant. *Reprint address: University of Miami School of Medicine, PO Box 520875, Biscayne Annex, Miami. FL 33152.
VoL 90, No. 2, pp. 192-195
measurement of the functional residual capacity was not performed, making the interpretation of these results difficult. Thus a systematic study was performed in an attempt to determine the relationship between pulmonary hemodynamics and the mechanics of breathing in infants with CHD and associated increased or decreased PBF.
MATERIALS AND M E T H O D S Twenty-five infants less than 12 months of age, who were undergoing diagnostic cardiac catheterization for Abbreviations used ASD: atrial septal defect CHD: congenital heart disease CL: lung compliance FRC: functional residual capacity LAP: left atrial pressure PAP pulmonary artery pressure PBF: pulmonary blood flow PDA: patent ductus arteriosus PS: pulmonary stenosis R: pulmonary resistance TA: tricuspid atresia TF: tetralogy of fallot VT: tidal volume VSD: ventricutar septal defect Qp/Qs: pulmonary/systemic blood flow
Volume 90 Number 2
Lung mechanics in congenital heart disease
T a b l e I. C a t h e t e r i z a t i o n d a t a a n d p u l m o n a r y
Case No.
m e c h a n i c s in i n f a n t s with i n c r e a s e d a n d d e c r e a s e d P B F
Specific CL (ral/craH,~O/ L-FRC)
FRC/wt CL (ml/kg) (ml/cmHzO)
19 3
Pulmonary resistance inspiration
Pulmonary resistance expiration
(craH~O/L/sec)
PA P system Qp/Qs (ram Hg)
Diagnosis
Infants with increased PBF 1 2 3 4 5 6 7* 8 9 10 11" 12
36 23 23 30 16 34 29 25 20 32 18.7 27 26.1 1.8 26.6 2.02
Mean SE Mean (PS excluded) SE
3.6 3 2.3 2.27 1.68 4.2 13.3 3.02 3.25 7.5 10 5.1 4.94 1.02 3.6 1.69
26 29 30 13 38 48 86 40 44 44 97 52 45.6 6.95 36.4 3.74
120 35 23 94 22 52 21.9 83 53 36 36 52.4 9.9 57.5 11.4
71 125 88 90 60 56 8.8 100 84 -11.3 53 67.9 10.7 80.7 7.8
1.5 3.7 6.3 2.2 4.5 2 1.8 2.7 4 1.3
42 65 85 70 52 50 20 40 45 42 40
VSD VSD-ASD VSD PDA VSD-ASD VSD-ASD-PDA VSD-PS PDA VSD VSD-PDA VSD-PS ASD
17 56 42 18 7.1 53 20 13.8
37 62 37 32 32 160 63 12.5
0.86 0.50 0.90 0.75 0.90 -
-
20 --
0.25 0.29 0.25 0.30
---
TA-PA hypoplasia PA atresia-ASD-PDA TF TA-ASD-VSD TF TA TF TF PA atresia-single ventricle TF TF PA atresia TF
Infants with decreased PBF 1 2 3 4 5 6 7 8 9 10 11 12 13 Mean SE P ( + PBF vs ~, PBF) P (PS excluded)
22 28 28 32 28 34 42 35
4.1 10.4 6.2 10.8 8.5 5.4 10.8 8.6
49 101 68 64 47 63 102 67
35 -31.6 1.9 n/s n/s
7,6 10.2 21.2 6.4 6 8.94 1.19 0.019 0.01
69 --. . 70 6.5 0.02 0.0003
10 -. 26.8 6.1 0.053 0.03
.
. 50.6 14.8 n/s n/s
*Pulmonic stenosis.
CHD+ w e r e s e l e c t e d for t h e s t u d y . T w e l v e h a d l e s i o n s with
infant
intracardiac left-to-right shunts and resultant increased
W h i t e Plains, N Y ) a n d a f a c e m a s k . E s o p h a g e a l p r e s s u r e
PBF;
13 h a d r i g h t - t o - l e f t s h u n t s a n d
(Electronics
for
Medicine,
PBF.
w a s m e a s u r e d u s i n g a t h i n latex b a l l o o n p l a c e d in t h e
P a t i e n t s with e v i d e n c e o f p u l m o n a r y c o m p l i c a t i o n s or a
m i d p o r t i o n o f the e s o p h a g u s a n d c o n n e c t e d to a p r e s s u r e
clinical
transducer (Statham Instruments+ Oxnard, CA). Inspira-
diagnosis
of
congestive
decreased
pneumotachograph
heart
failure
were
excluded from the study. Lung mechanics were evaluated on the day of the c a r d i a c c a t h e t e r i z a t i o n w i t h t h e i n f a n t s in a n o n s e d a t e d
tory a n d e x p i r a t o r y p u l m o n a r y r e s i s t a n c e w e r e c a l c u l a t e d from
the pressure volume
loop
using
the nonelastic
p r e s s u r e at m i d i n s p i r a t i o n a n d e x p i r a t i o n , a n d t h e s i m u l -
s l e e p i n g state. D y n a m i c c o m p l i a n c e w a s c a l c u l a t e d at e n d
taneous
i n s p i r a t i o n f r o m s i m u l t a n e o u s t r a c i n g s o f tidal v o l u m e
c a p a c i t y w a s d e t e r m i n e d in all i n f a n t s w i t h i n c r e a s e d P B F
and esophageal pressure. Tidal volume was obtained by
a n d in n i n e with d e c r e a s e d P B F b y m e a n s o f t h e h e l i u m
electrical i n t e g r a t i o n o f t h e flow s i g n a l m e a s u r e d w i t h a n
d i l u t i o n t e c h n i q u e in a c l o s e d s y s t e m . ;
measurements
of
flow.
Functional
residual
194
Bancalari et aL
The Journal o f Pediatrics February 1977
ml/cm
ml/cm H 2 0 /
H20
L-FRC p = 0.01
p = 0.0003
10 - -
100 . . . . . . . . . . . . .
t6p 9 ,6p 5--
50
- -
0 - -
DYNAMIC COMPLIANCE
- -
SPECIFICCOMPLIANCE
Fig. 1. Values of dynamic and specific lung compliance in infants with increased and decreased pulmonary blood flow. O
=
PULMONIC
loo
r
"' u
75
O U "r
50
uL,I.I E
25
-0.7987
(p = 0,0098)
0
_< ~_
=
STENOSIS
9
*
9
~O
I
I
I
I
I
I
10
20
30
40
50
60
MEAN PULMONARY ARTERY PRESSURE (ram klg)
Fig. 2. Relation between mean pulmonary artery pressure and specific lung compliance in infants with increased pulmonary blood flow. (One infants with VSD and PS is not included because the pulmonary artery was not entered during cardiac catheterization.) RESULTS The physical characteristics of the patients studied were similar in both groups. The mean age in infants with increased PBF was 3.8 months (15 days to 9 months) as compared to 2.1 months (3 days to 7 months) in those patients with decreased PBF. The weight at the time of the study was also similar in both groups (4,081 + 334 gm versus 3,930 -2-_371 gm). The respiratory rate was higher in infants with increased PBF (64 versus 45; P < 0.05), whereas the tidal volume was similar in the two groups.
Catheterization data and the results of the pulmonary mechanics in the two groups are shown in Table I. Functional residual capacity was similar in both groups, and dynamic compliance was significantly lower in the group with increased PBF (4.94 versus 8.94 ml/cm H..O: P < 0.02). The difference in C,, was therefore due to changes in the elastic properties of the lung and not to differences in lung volume. When dynamic compliance was corrected to one liter of FRC to obtain specific compliance, the difference between the two groups remained significant. Two infants with increased PBF (Cases 7 and 11) had associated mild pulmonic stenosis; they were the only patients in this group who had a specific compliance greater than 60 ml/cm H~O/L-FRC. If these two patients are not included in the statistical analysis, the difference in compliance is even greater (Fig. 1). Total pulmonary resistance was higher in infants with increased PBF; during inspiration this difference reached statistical significance (57.5 versus 26.8 cm H~O/L/sec: P < 0.05). No correlation was found between specific compliance and left atrial pressure in seven patients in whom LAP was measured. Likewise no correlation was found between specific compliance and the magnitude of the left-to-right shunt in infants in whom PBF was shown to be increased. However, a significant correlation was obtained between specific compliance and mean pulmonary artery pressure in these patients (r = 0.798: P < 0.01) (Fig. 2). DISCUSSION The results of compliance and FRC obtained in the present study in infants with decreased PBF are similar to those previously reported in normal infants." : it appears, therefore, that a diminished PBF does not alter the mechanical properties of the lung. The finding of a decreased dynamic lung compliance in patients with increased PBF has been reported previously in infants and adults with intracardiac left-to-right shunts/ ' The mechanism for this change, however, is not clear, since many hemodynamic factors can influence the mechanical properties of the lung. An increase in left atrial and pulmonary venous pressure is known to produce a decrease in lung compliance in experimental animals,'" but in the present study no significant correlation was found between these two measurements. This may be explained by the fact that all of our patients had relatively low left atrial pressures, because infants with heart failure were excluded from the study. Both cardiomegaly and an increase in intrathoracic blood volume may displace the effective lung volume.
Vohtme 90 Number 2
These mechanisms have been implicated as possible causes of the decreased lung compliance in patients with a significant left-to-right shunts " These explanations seem unlikely because the FRC was similar in both of our patient groups. The two other mechanisms which may be responsible for the decrease in lung compliance are the increase in PBF and/or the increase in pressure in the pulmonary system. An increase in flow without an increase in pressure can be excluded as a possibility since no correlation was found in this and other studies between the magnitude of left-to-right shunts and lung complianceS "~ Moreover, patients with atrial septal defects and a large increase in PBF have been found to have normal C I j Thus it appears that PAP is primarily responsible for the changes observed in the mechanical properties of the lung. The findings in our two patients with left-to-right shunts and pulmonary stenosis support this hypothesis; these were the only patients who had normal lung compliance values in spite of increased pulmonary blood flow. The mechanism by which the increase in pulmonary vascular pressure alters the mechanical properties of the lung can only be theorized. One possibility is that the increase in PAP may result in an increase in the tension of the walls of the pulmonary vascular system. This "stiffer" vasculature may then oppose pulmonary expansion and result in the observed decrease in lung compliance. The same mechanism may be in part responsible for the transient decreased compliance observed in the normal newborn infant and the subsequent increase in C~. observed as PAP decreases after birth. An increase in pulmonary artery pressure due to an increase in pulmonary vascular resistance with normal PBF may have the same effect. An additional factor that can contribute to the decrease in lung compliance in infants with a left-to-right shunt is the observed higher total pulmonary resistance associated with an increased respiratory rate. Lung compliance
Lung mechanics in congenital heart disease
19 5
becomes frequency dependent with an increase in small airway resistance. The cause of this increased pulmonary resistance in infants with a left-to-right shunt can be either extrinsic compression of the airways by enlarged pulmonary vessels or congestion of the bronchial wall. The determination of lung compliance is a simple bedside procedure which could be utilized as an adjunct in the clinical evaluations of patients with intracardiac left-to-right shunts, since it may be an indicator of changes occurring in the pulmonary artery pressure.
REFERENCES
1. Lees MH, Way RC, and Ross BB: Ventilation and respiratory gas transfer of infants with increased pulmonary blood flow, Pediatrics 40:259, 1967. 2. GriffinAJ, Ferrara JD, Lax JO, and Cassels DE: Pulmonary compliance. An index of cardiovascular status in !nfancy, Am J Dis Child 123:89, 1972. 3. Wallgren (3, Geubelle F, and Koch G: Studies of the mechanics of breathing in children with congenital heart lesions, Acta Paediatr 49:415, 1960. 4. Saxton GA. Rabinowitz M, Dexter L, and Haynes F: The relationship of pulmonary compliance to pulmonary vascular pressures in patients with heart disease, J Clin Invest 35:611, 1956. 5. Krauss AN, and Auld PAM: Measurements of functional residual capacity in distressed neonates by helium rebreathing, J PEDIATR77:228, 1970. 6. Phelan PD, and Williams HE: Ventilatory studies in healthy infants, Pediatr Res 3:425, 1909. 7. Krieger 1: Studies of mechanics of respiration in infancy, Am J Dis Child 105:439, 1963. 8. HowlettG: Lung mechanics in normal infants and infants with congenital heart disease, Arch Dis Child 47:707, 1972. 9. Phelan PD, Gillam GL, Menahem SA, Coombs E, and Venables AW: Respiratory function in infants with a ventricular septal defect, Aust Paediat J 8:79, 1972. 10. Borst HG, Berglund E, Whittenberger JL, Mead J, McGregor M, and Collier C: The effect of pulmonary vascular pressures on the mechanical properties of the lungs of anesthetized dogs, J Clin Invest 36:1708, 1957.
