Sex-specific associations between cerebrovascular blood pressure autoregulation and cardiopulmonary injury in neonatal encephalopathy and therapeutic hypothermia

Abstract

Background

Cardiopulmonary injury is common in neonatal encephalopathy, but the link with cerebrovascular dysfunction is unknown. We hypothesized that cerebral autoregulation is associated with cardiopulmonary injury in neonates treated with therapeutic hypothermia (TH) for neonatal encephalopathy.

Methods

The cerebral hemoglobin volume index (HVx) from near-infrared spectroscopy was used to identify the mean arterial blood pressure (MAP) with optimal autoregulatory vasoreactivity (MAP_OPT). We measured associations between MAP relative to MAP_OPT and indicators of cardiopulmonary injury (duration of mechanical respiratory support and administration of inhaled nitric oxide (iNO), milrinone, or steroids).

Results

We identified associations between cerebrovascular autoregulation and cardiopulmonary injury that were often sex-specific. Greater MAP deviation above MAP_OPT was associated with shorter duration of intubation in boys but longer ventilatory support in girls. Greater MAP deviation below MAP_OPT related to longer intensive care stay in boys. Milrinone was associated with greater MAP deviation below MAP_OPT in girls.

Conclusion

MAP deviation from MAP_OPT may relate to cardiopulmonary injury after neonatal encephalopathy, and sex may modulate this relationship. Whereas MAP above MAP_OPT may protect the brain and lungs in boys, it may be related to cardiopulmonary injury in girls. Future studies are needed to characterize the role of sex in these associations.

INTRODUCTION

Neonatal encephalopathy affects ~3 in 1000 births in the United States.(1, 2) Neonatal encephalopathy causes multi-organ disease that includes cardiopulmonary injury in 80% of cases despite the use of therapeutic hypothermia (TH).(1, 3, 4) Supporting cerebral autoregulation, the physiologic mechanism that maintains stable cerebral blood flow across a range of blood pressure, may protect the post-ischemic neonatal brain.(5–8) Dysfunctional cerebral autoregulation is linked to pulmonary injury during hypothermic cardiopulmonary bypass(9) and longer duration of post-operative mechanical ventilation in adults.(10) Whether strategies that support cerebral autoregulation also protect the cardiopulmonary system in neonates with neonatal encephalopathy is unknown.

The hemoglobin volume index (HVx) measures cerebral autoregulatory vasoreactivity using near infrared spectroscopy (NIRS). HVx is calculated by a continuous, moving correlation coefficient between mean arterial pressure (MAP) and the NIRS relative total tissue hemoglobin (rTHb), a surrogate measure of cerebral blood volume.(11, 12) Because the rTHb optical density is calculated as the sum of oxygenated and deoxygenated hemoglobin,(12) changes in tissue oxygen supply and metabolic rate during TH and mechanical ventilation affect HVx less than indices solely derived from oxyhemoglobin. HVx can be used to identify the range of MAP with most robust autoregulatory vasoreactivity—the optimal mean arterial blood pressure (MAP_OPT)—after neonatal encephalopathy.(5, 7, 13) Blood pressure deviation below MAP_OPT after neonatal encephalopathy is linked to greater brain injury on magnetic resonance imaging (MRI) at 2 weeks (5, 7) and worse neurocognitive outcomes at 2 years.(14)

Here, we investigated whether blood pressure deviation from MAP_OPT is associated with cardiopulmonary injury after neonatal encephalopathy and TH. Our objective was to study the associations between deviation from MAP_OPT and durations of cardiopulmonary support and stay in the neonatal intensive care unit (NICU). We analyzed all neonates and stratified the data by sex based on increasing evidence that responses to neonatal encephalopathy are sex-specific.(15–20)

METHODS

The Johns Hopkins University (JHU) Institutional Review Board (IRB) approved this study. Between September 2010 and July 2015, neonates admitted to the NICU for neonatal encephalopathy were screened using criteria reported previously.(5, 7, 14) Criteria for neonatal encephalopathy requiring TH were based on the NICHD Neonatal Research Network’s clinical trial.(21) We obtained written informed consent from parents of participants until May 2013. After that, NIRS monitoring became standard of care for neonatal encephalopathy at JHU NICU, and the IRB waived the requirement for written consent.

Clinical care

The JHU NICU whole-body TH protocol was followed as previously published.(5, 7, 14) The treating team was blinded to HVx and made all clinical decisions. For persistent pulmonary hypertension of the newborn (PPHN), the primary mode of ventilation was high-frequency oscillatory ventilation. Clinicians initiated inhaled nitric oxide (iNO) (INOmax, Mallinckrodt pharmaceuticals, St Louis, MO) by endotracheal tube for the clinical diagnosis of PPHN based on persistently low PaO₂ and oxygenation index >25 despite a FiO₂ of 1.0 and lung recruitment strategies. PPHN was confirmed by echocardiogram. Per clinical protocol, the iNO was started at 20 ppm and weaned down once the goal PaO₂ could be sustained with FiO2≤0.60. Dopamine was the first-line inotropic agent (maximum dose 20 mcg/kg/min), and clinicians determined the neonates’ hemodynamic goals. Milrinone infusion was used for ventricular dysfunction diagnosed by echocardiogram and as part of PPHN management. All neonates received morphine intravenously (IV) per NICU TH protocol. Some neonates also received benzodiazepines or additional opiates. Occasionally clonidine was administered for shivering, and vecuronium was given for neuromuscular blockade when needed. Early hydrocortisone (20 mg/m² IV) was used for arterial hypotension refractory to inotropes and/or adrenal insufficiency. Steroids were therefore considered a marker of severe systemic illness affecting multiple organ systems.

Autoregulation monitoring

Our autoregulation monitoring methodology was previously published.(5, 7, 14) Briefly, neonatal cerebral oximetry probes were placed bilaterally on the forehead and connected to an INVOS 5100 NIRS machine (INVOS; Medtronic, Minneapolis, MN). Synchronous measurements of NIRS and arterial blood pressure signals at 100 Hz were processed with ICM+ software (Cambridge Enterprises, Cambridge, UK). We analyzed time-integrated, 10-second means of MAP and rTHb, a surrogate measure of cerebral blood volume, to filter out high-frequency waves from respiration and pulse. HVx is calculated by a continuous, moving Pearson correlation coefficient between MAP and rTHb from consecutive 10-second windows, and the average from each 300-second epoch is used to calculate one HVx value.(11, 12) HVx is a continuous variable that ranges from −1 to +1. When autoregulatory vasoreactivity is functional, HVx is negative or near-zero. A positive HVx that approaches 1 indicates dysfunctional autoregulation. Artifacts in the NIRS and MAP signals were manually filtered out, and data that comprised <1% of the recording were removed as an additional method to remove artifacts.(5, 7, 14) After ruling out unilateral intracranial lesions on MRI, right and left HVx values were averaged and sorted into 5-mmHg bins. The most negative HVx (nadir) was used to identify the MAP_OPT with optimal autoregulatory vasoreactivity in each observation period (TH, rewarming, and first 6 hours of normothermia).(5, 7, 13, 14) Some neonates were coded as “unidentifiable MAP_OPT” if a clear nadir could not be identified.(14) An investigator (JKL) blinded to outcome measures identified the MAP_OPT values, which were corroborated by additional investigators (FJN, MG). We calculated an area under the curve (AUC) below MAP_OPT to combine the extent of blood pressure deviation with the amount of time spent with blood pressure below MAP_OPT. The AUC (min•mmHg/h) was calculated as time (minutes) spent with blood pressure below MAP_OPT and blood pressure deviation (mmHg) below MAP_OPT, and then normalized for the duration of monitoring (hours).(5, 7, 13, 14) In summary, the parameters derived from MAP_OPT in each period were: (i) maximal blood pressure deviation below or above MAP_OPT; (ii) duration of time spent with blood pressure below, at, or above MAP_OPT as a percentage of the monitoring period; and (iii) AUC.

Demographic and perinatal data

Clinical data were retrieved from electronic medical record by investigators blinded to the autoregulation data (RC-V and MOC). (Table 1)(22)

Table 1.

Demographic and clinical characteristics of infants with an identified optimal mean arterial blood pressure.

	TOTAL		BOYS		GIRLS		P-value
Demographic - antenatal data	n		n		n
Gestational age, weeks (mean, SD)	65	38 6/7 (16/7)	38	38 6/7 (15/7)	27	39 (16/7)	0.61a
Birth weight, grams (mean, SD)	65	3287 (622)	38	3452 (660)	27	3054 (486)	0.007b*
Race, White (n, %)	65	31 (48%)	38	21 (55%)	27	10 (37%)	0.35c
Race, Black (n, %)	65	24 (37%)	38	12 (32%)	27	12 (44%)	0.50c
Ethnicity, Hispanic (n, %)	65	8 (11%)	38	3 (8%)	27	5 (19%)	0.32c
Adequate for gestational age (n, %)	65	51 (78%)	38	28 (74%)	27	23 (85%)	0.85c
In utero growth restriction (n, %)	65	1 (1.5%)	38	0 (0%)	27	1 (3.7%)	1.00 e
Maternal gestational diabetes (n, %)	65	6 (9%)	38	5 (13%)	27	1 (4%)	0.19c
Born at outside facility (n, %)	65	57 (88%)	38	34 (89%)	27	23 (85%)	0.60c
Delivery mode, c-section (n, %)	65	50 (77%)	38	27 (71%)	27	23 (85%)	0.18c
Emergency delivery (n, %)	65	48 (74%)	38	26 (68%)	27	22 (82%)	0.24c
Perinatal data
Apgar score, 1 min (median, IQR)	65	1 (1–2)	38	1 (1–3)	27	1(0–2)	0.04d*
Apgar score, 5 min (median, IQR)	65	3 (2–6)	38	4 (2–6)	27	3 (2–4)	0.11d
Cord pH (mean, SD)	47	6.93 (0.17)	28	6.93 (0.17)	19	6.94 (0.19)	0.95a
Cord base deficit (mean, SD)	41	−15.9 (7.4)	23	−15.8 (6.9)	18	−15.9 (8.3)	0.96a
First hour pH (mean, SD)	64	7.11 (0.15)	38	7.11 (0.15)	26	7.09 (0.14)	0.45a
First hour base deficit (mean, SD)	55	−16.8 (5.4)	34	−16.6 (5.4)	21	−17.1 (5.5)	0.69a
Admission Sarnat score (median, IQR)	65	2 (2–2)	38	2 (2–2)	27	2 (2–2)	0.78d
Severe encephalopathy (n, %)	65	13 (20%)	38	8 (21%)	27	5 (19%)	0.56c
Hemoglobin (g/dL) (mean, SD)	63	15.5 (1.7)	36	15.5 (1.5)	27	15.5 (1.9)	0.95a
Delta hemoglobin (g/dL) (mean, SD)	59	2.5 (1.5)	34	2.4 (1.5)	25	2.6 (1.5)	0.56a
Cardiopulmonary outcomes
Mech. ventilation, days (median, IQR)	65	1 (1–7)	38	1 (0–6)	27	2 (1–7)	0.74d
Pressure support, days (median, IQR)	65	2 (1–7)	38	1 (1–7)	27	3 (1–7)	0.19d
Oxygen support, days (median, IQR)	65	9 (5–15)	38	8 (5–15)	27	10 (5–15)	0.69d
PPHN (n, %)	65	25 (39%)	38	15 (40%)	27	10 (37%)	0.84c
iNO use (n, %)	65	13 (20%)	38	7 (18%)	27	6 (22%)	0.71c
Any steroids first 7 days of life (n, %)f	65	13 (20%)	38	7 (18%)	27	6 (22%)	0.71c
Steroid days (median, IQR)	65	0 (0–3)	38	0 (0–2)	27	0 (0–6)	0.77d
Ventricular dysfunction (n, %)	65	9 (14%)	38	4 (11%)	27	5 (19%)	0.36c
Milrinone (n, %)	65	9 (14%)	38	5 (13%)	27	4 (15%)	0.67c
NICU LOS, days (median, IQR)	65	13 (9–28)	38	13 (9–29)	27	14 (9–22)	0.98d
Confounders included in analysis
Vasopressor, any (n, %)	65	43 (66%)	38	24 (63%)	27	19 (70%)	0.54c
Seizures, any (n, %)	65	23 (35%)	38	13 (34%)	27	10 (37%)	0.81c
Benzodiazepine, any (n, %)	65	9 (11%)	38	5 (13%)	27	4 (15%)	0.85c
Clonidine, any (n, %)	65	9 (11%)	38	5 (13%)	27	4 (15%)	0.85c
PaCO2 All 35 – 45 (n, %)	65	5 (8%)	38	4 (11%)	27	1 (4%)	0.39e
Some < 35, all < 45 (n, %)	65	13 (20%)	38	7 (18%)	27	6 (22%)	0.95c
None < 35, some > 45 (n, %)	65	26 (40%)	38	14 (38%)	27	12 (44%)	0.72c
Some < 35, some > 45 (n, %)	65	21 (32%)	38	13 (34%)	27	8 (30%)	0.90c

^*p<0.05;

^aunpaired T-test;

^bunpaired T-test with Levene’s test;

^cχ² test;

^dMann-Whitney U test;

^eFisher’s exact test;

^fused either as outcome or confounder in modeling.

Markers of cardiopulmonary injury

The duration of invasive mechanical ventilation, pressure support, and oxygen support were defined as the number of days with an endotracheal tube, with any support other than low-flow nasal cannula, and FiO₂>0.21, respectively. PaCO₂ measurements classified neonates into those with:(23) (i) PaCO₂ levels 35–45 mmHg; (ii) some <35 but none >45 mmHg; (iii) none <35 but some >45 mmHg; and (iv) some <35 and some >45 mmHg. Data were also collected on the presence of electrical seizures; use of iNO, inotropes, milrinone, benzodiazepines, or clonidine during HVx monitoring; and receipt of steroids.

Statistical analysis

Analyses were conducted with SASv9.2 (SAS Institute, Cary, NC). We determined associations between the autoregulation parameters (MAP_OPT and blood pressure in relation to MAP_OPT) in all neonates and stratified by sex. Linear regression for the logarithmic transformation of the average number of days of NICU stay, invasive mechanical ventilation, pressure support, and oxygen support was used to find both the crude (unadjusted) (data not shonw) and adjusted associations with the autoregulation parameters from each of three observation periods (TH, rewarming, and first 6 hours of normothermia). These analyses were adjusted for PaCO₂ category, seizures, and receipt of a vasopressor, benzodiazepine, clonidine, or steroids. Binary outcomes (milrinone, iNO, or steroids) were analyzed as longitudinal outcomes over time, by autoregulation parameters within the same observation period. We used logistic regression for these binary measurements with generalized estimating equations to control for each measurement (milrinone, iNO, or steroid use) within the same neonate.(24) Temporal associations between autoregulation parameters and subsequent and preceding milrinone and iNO use were analyzed by logistic regression. The analysis of milrinone, iNO, and steroid use were controlled for presence of seizures, and the analysis of iNO and steroids were additionally adjusted for PaCO₂ category during autoregulation monitoring.

RESULTS

We screened 122 newborns from September 27, 2010 to July 27, 2015. Forty-six neonates were ineligible for the study because of an unreliable arterial blood pressure tracing (16), parents’ refusal to consent (9), death (5), transfer to the pediatric ICU for extracorporeal membrane oxygenation (ECMO) (6), technical difficulties (5), inadequate monitoring resources (3), coagulopathy (1), complex heart disease (1), and language barriers (1). In total, 75 neonates received HVx monitoring (Supplemental Table S1, online). These patients received continuous HVx monitoring for 45.8 ±20.6 hours [mean ±SD] (n=31 girls/44 boys) during TH, 6.4±2.3 hours (n=26 girls/39 boys) during rewarming, and 5.6±0.9 hours (n=25 girls/38 boys) during normothermia. Ten neonates did not have HVx monitoring during rewarming because of ECMO (5), withdrawal of care (3), and technical difficulties (2). Two additional neonates did not have HVx monitoring during normothermia due to removal of the arterial catheter (1) or NIRS (1). Fifty-two neonates received vasopressors: 63% (33/52; 21 boys and 12 girls) received dopamine only, 28% (15/52; 7 boys and 8 girls) received dopamine and dobutamine, and 9% (5/52; 2 boys and 3 girls) received dopamine, dobutamine, and epinephrine.

We identified MAP_OPT in 65 infants (87%) during TH (MAP_OPT = 45±7.5 mmHg); in 58 infants (89%) during rewarming (MAP_OPT = 50±7.5 mmHg); and in 60 (95%) during normothermia (MAP_OPT = 50±7.5 mmHg). Clinical data about the infants with identified MAP_OPT are presented in Table 1. Ten neonates (6 or 60% male, 4 or 40% female) were excluded because their MAP_OPT could not be identified. Boys were heavier than girls (mean ± SD = 3452 ± 660g vs. 3054 ± 486g, p=0.007). Although the boys had higher 1-min Apgar scores than did girls (p=0.04), the median 1-min Apgar score was 1 for both sexes (Table 1). MAP and MAP_OPT were similar in both sexes in each of the observation periods (Figure 1 and Table 2). During normothermia, girls spent more time with blood pressure below MAP_OPT (p=0.05) than did boys. Accordingly, girls spent less time with blood pressure at MAP_OPT than did boys during normothermia (p=0.02). (Table 2 and Figure 2)

Figure 1.

Box and whisker plots of the percentage of time that girls (white boxes) and boys (black boxes) spent at each level of mean arterial blood pressure (MAP, mmHg). The observation periods were therapeutic hypothermia (a, girls [n=31], and boys [n=44]), rewarming (b, girls [n=26], and boys [n=39]), and the first 6 hours of normothermia (c, girls [n=25], and boys [n=38]). Box and whiskers plots, boxes represent the interquartile range (IQR) limited by the 25^th and 75^th percentile (lower and upper limit, respectively), line inside the box indicates the median and whiskers extend up the last datapoint within 1.5 times the IQR from the median. Outliers are not represented.

Table 2.

Optimal MAP (MAP_OPT) and blood pressure parameters in relation to MAP_OPT

Parameter	n	TOTAL median (IQR)	n	BOYS median (IQR)	n	GIRLS median (IQR)	P-valuea
MAPOPT (mmHg)
Hypothermia	65	45 (40–55)	38	45 (40–55)	27	45 (40–55)	0.97
Rewarming	58	50 (45–55)	33	50 (45–55)	25	50 (45–58)	0.91
Normothermia	60	50 (45–55)	38	50 (45–55)	22	55 (44–60)	0.23
Area under the curve below MAPOPT (min•mmHg/h)
Hypothermia	65	22.9 (1.3–236)	38	48.6 (0.6–220)	27	19.6 (3.4–287)	0.39
Rewarming	58	180.3 (21.5–474.8)	33	175.5 (24.8–386)	25	197.9 (8.9–556)	0.59
Normothermia	60	102.7 (3–482.7)	38	59 (1–296)	22	452 (8.1–585)	0.07
Duration of BP below MAPOPT (% of the autoregulation monitoring period)
Hypothermia	65	6 (0.05–45.8)	38	13.4 (0–45.4)	27	4.5 (0.6–54.2)	0.48
Rewarming	58	42.4 (4.7–85.7)	33	41.3 (6.6–79.7)	25	43.5 (1.7–87.9)	0.65
Normothermia	60	35.9 (0–93)	38	19 (0–64)	22	90 (1.5–96.7)	0.05*
Duration of BP at MAPOPT (% of the autoregulation monitoring period)
Hypothermia	65	10.5 (4.5–22.5)	38	12.6 (4.3–26)	27	9.5 (94.8–18.8)	0.78
Rewarming	58	12.5 (3.6–30.8)	33	12 (3.6–32.2)	25	13 (3.9–25.8)	0.59
Normothermia	60	10 (2.6–30.6)	38	19.9 (3.5–33.6)	22	4.6 (1.6–16.6)	0.02*
Duration of BP above MAPOPT (% of the autoregulation monitoring period)
Hypothermia	65	73.6 (25.8–94.3)	38	63.7 (31.8–94.6)	27	79.2 (20.7–93.8)	0.88
Rewarming	58	23.5 (5.4–62)	33	22.4 (5.8–62.4)	25	29 (3.7–61.7)	0.89
Normothermia	60	26.7 (0.9–84)	38	35.3 (2.7–89.9)	22	2.1(0–64)	0.06
Maximal BP deviation below MAPOPT (mmHg)
Hypothermia	65	10 (2.5–15)	38	10 (0–15)	27	10 (5–20)	0.33
Rewarming	58	10 (5–15)	33	10 (5–15)	25	10 (5–20)	0.51
Normothermia	60	10 (0–20)	38	10 (0–15)	22	15 (3.8–20)	0.11
Maximal BP deviation above MAPOPT (mmHg)
Hypothermia	65	25 (15–35)	38	22.5 (15–31.3)	27	25 (20–35)	0.18
Rewarming	58	15 (10–28)	33	15 (10–28)	25	15 (8–28)	0.87
Normothermia	60	15 (5–25)	38	15 (8–23)	22	13 (0–25)	0.36

^*p≤0.05;

^aMann-Whitney U test.

BP, blood pressure; MAP, mean arterial pressure.

Figure 2.

Box and whisker plots of the area under the curve (AUC) below MAP_OPT (a), and percent time spent below MAP_OPT (b), at MAP_OPT (c), and above MAP_OPT (d) for girls (white boxes) and boys (black boxes) during 6 hours of normothermia (girls n=22, and boys n=38). Box and whiskers plots, boxes represent the interquartile range (IQR) limited by the 25^th and 75^th percentile (lower and upper limit, respectively), line inside the box indicates the median and whiskers extend up the last datapoint within 1.5 times the IQR from the median. Outliers (open circles) are those datapoints within 1.5 to 3 times the IQR from the median and extremes (closed circles) are those datapoint beyond 3 times the IQR form the median. *, p<0.05. Analysis performed by Mann Whitney U Test.

Sex-specific associations between cerebral autoregulation and duration of respiratory support

Greater maximal blood pressure deviation above MAP_OPT during normothermia was associated with 13% shorter duration of mechanical ventilation (days) among boys (β = –0.142, p = 0.018) (Supplemental Table S2, online). In girls, however, more time spent with blood pressure above MAP_OPT during rewarming was associated with 3% more days of positive pressure support (β=0.033, p = 0.044). Greater AUC (β = –0.004, p = 0.046) and greater duration with blood pressure below MAP_OPT (β = –0.036, p= 0.024) during rewarming were associated with 1% and 4% fewer days of positive pressure support in girls, respectively. (Supplemental Table S3, online). Greater AUC (β = –0.003, p = 0.007), longer duration with blood pressure below MAP_OPT (β = –0.022, p = 0.005), and greater maximal blood pressure deviation below MAPT_OPT (β = −0.081, p = 0.019) during normothermia were also associated with fewer days of oxygen support in girls (Supplemental Table S4, online).

Blood pressure below MAP_OPT is associated with length of NICU stay

The median duration of NICU stay was 13 days (IQR: 9, 28) and was similar between boys and girls (Table 1). Among boys, more time spent with blood pressure above MAP_OPT during rewarming and normothermia (β = −0.008 for both, p = 0.016 and 0.004, respectively) was associated with a shorter NICU stay. Greater AUC (β=0.001, p = 0.018), longer duration of blood pressure below MAP_OPT (β=0.007, p = 0.004), and greater maximal blood pressure deviation below MAP_OPT (β=0.024, p = 0.018) during normothermia were associated with longer NICU stays in boys. In contrast, greater AUC (β = −0.001, p = 0.036), longer duration of blood pressure below MAP_OPT (β = −0.006, p = 0.012), and greater maximal blood pressure deviation below MAP_OPT (β = −0.024, p = 0.024) during normothermia were associated with shorter NICU stays in girls (Supplemental Table S5, online).

Cerebral blood pressure autoregulation may be associated with milrinone use

Nine neonates (14%; 5 boys and 4 girls) received milrinone for ventricular dysfunction or PPHN (Table 1). Nine (12%) received milrinone during TH, 6 (8%) during rewarming, and 4 (5%) during the first 6 hours of normothermia. Milrinone use was associated with the maximal blood pressure deviation below MAP_OPT in the same time period in girls (β=0.157; p 0.005); Supplemental Table S6, online). Longitudinal analysis of autoregulation and milrinone use in preceding or subsequent periods were not associated (data not shown).

The cerebral autoregulation parameters were not associated with clinical use of iNO or steroids

Twenty-nine infants received iNO for clinical diagnoses of PPHN. Nineteen neonates received iNO during HVx monitoring (32% of girls and 21% of boys, Supplemental Table S1, online), and 13 of these neonates had an identified MAP_OPT (22% of girls and 18% of boys, Table 1). The iNO was initiated at 20 ppm (maximum dose) in all infants and weaned per clinical protocol. iNO was started within the first 24 h of life in 84% of neonates (16/19), between 24 and 48h of life in 5% (1/19), and soon after rewarming in 11% (2/19). Among neonates who started iNO during TH, 88% (15/17) received iNO for at least the first 7 days of life, and the other 12% (2/17) were weaned off iNO before the end of TH. Thus, 23% of neonates undergoing HVx monitoring received iNO during TH and rewarming and 27% during normothermia. Blood pressure parameters in relation to MAP_OPT were not associated with the simultaneous and clinical use of iNO for PPHN (Supplemental Table S6, online). Similarly, longitudinal analysis of autoregulation and iNO in preceding or subsequent periods were not associated (data not shown).

Girls and boys received steroids for a similar length of time. Twelve (16%) neonates received steroids during TH, 15 (20%) during rewarming, and 16 (21%) during the first 6 hours of normothermia. Thirteen infants (20%; 6 girls and 7 boys) with an identified MAP_OPT received steroids during the first 7 days of life (Table 1). Blood pressure in relation to MAP_OPT was not associated with steroid use (Supplemental Table S6, online).

DISCUSSION

We investigated the association between cerebral autoregulatory vasoreactivity measured by HVx and cardiopulmonary outcomes in neonates treated with TH for neonatal encephalopathy. Although girls and boys had similar MAP_OPT values, the relationships between blood pressure autoregulation and lung injury were often sex-specific. While blood pressure above MAP_OPT during normothermia was associated with shorter duration of intubation in boys, it was associated with longer durations of intubated or noninvasive respiratory pressure support in girls. We previously demonstrated that blood pressure below MAP_OPT is associated with more severe brain injury on MRI (5, 7) at 2 weeks and worse neurocognitive outcomes at 2 years.(14) Our current findings suggest that maintaining blood pressure above MAP_OPT protects both the brain and lungs in boys but not in girls. Additionally, blood pressure deviation below MAP_OPT during normothermia was associated with a longer NICU stay in boys but shorter stay in girls. These findings provide new evidence for sex differences in the relationships between cerebral autoregulation and cardiopulmonary injury with neonatal encephalopathy.

Even though boys and girls had similar MAP_OPT during TH, rewarming, and the first 6 hours of normothermia, girls had greater blood pressure deviation below MAP_OPT than did boys during normothermia. Boys also spent more time with blood pressure within MAP_OPT during normothermia, which indicates that boys had better autoregulatory function than girls in this pilot study cohort. The reasons behind this sex difference are not clear. We controlled for vasopressor use in the analysis given the confounding effects of vasopressors on autoregulation. Although MAP and thus time spent above MAP_OPT are affected by vasopressor and inotropic medications, boys and girls with identified MAP_OPT were treated in equal proportions with these agents (63% vs. 70%, respectively, p=0.54). The type of vasopressor used should be considered given potential sex-dependent differences in autoregulatory responses to different vasopressors. For example, phenylephrine and norepinephrine protect autoregulation only in female piglets with traumatic brain injury, (25, 26) whereas dopamine preserves autoregulation in both sexes.(27) In our study, vasopressor treatment was similar between boys and girls. Dopamine was universally used as the first-line agent, and dobutamine was added for boys and girls in equal proportions. The reciprocal interactions between cerebral and cardiopulmonary injuries are not well studied in neonatal encephalopathy, and the influence of different vasopressors on this complex relationship deserves further study.

The relationships between cerebral autoregulation and cardiopulmonary outcomes were often sex-specific. In boys, higher blood pressure above MAP_OPT was associated with fewer days of intubation, and each 1 mmHg increase in maximal MAP above MAP_OPT during normothermia was associated with a 13% decrease in days of intubation. In contrast, more time with blood pressure above MAP_OPT during rewarming related to longer periods of intubation or noninvasive mechanical ventilation in girls. More blood pressure deviation below MAP_OPT during normothermia corresponded to fewer days of supplemental oxygen in girls but not boys. Blood pressure deviation below MAP_OPT is associated with more severe brain injury on MRI at 2 weeks(5, 7) and worse neurocognitive outcomes at 2 years(14) in neonatal encephalopathy. Hence, it appears that maintaining blood pressure at or above MAP_OPT in boys may protect both the brain and lungs, whereas blood pressure above MAP_OPT could worsen pulmonary injury and prolong the need for respiratory support in girls. Raising MAP to support cerebral perfusion pressure may cause cardiogenic strain with pulmonary edema or lung injury as previously reported in a preclinical model of neonatal encephalopathy.(28) However, it is unclear why girls might be more susceptible to these mechanisms than boys.

Blood pressure in relation to MAP_OPT did not affect the likelihood of receiving iNO for PPHN or ventricular dysfunction. While it is possible that cerebral delivery of nitrite from iNO(29) may cause cerebral vasodilation,(30) iNO did not affect MAP_OPT or increase blood pressure deviation below MAP_OPT during clinical use for PPHN in our cohort. The iNO dosing by endotracheal tube was determined by cardiopulmonary clinical indications rather than for potential cerebral vasodilation. Therefore, it is possible that higher doses of iNO or nasal administration could affect cerebral vasoactivity. Milrinone may decrease systemic blood pressure or dilate the cerebral vasculature.(31) In this pilot study, milrinone use was associated with greater maximal decrease in blood pressure below MAP_OPT among girls. Further studies are needed to determine the influence of milrinone on cerebral blood pressure autoregulation.

Our prior research in neonatal encephalopathy identified important relationships between autoregulation during TH and rewarming and neurologic outcomes.(5, 7, 14) In the current study, we identified several significant associations between the cardiopulmonary outcomes and blood pressure in relation to MAP_OPT during rewarming and normothermia. Many of these relationships were observed solely in one sex but not the other. The absence of associations during hypothermia may be related to our methods of examining blood pressure in relation to MAP_OPT during the entire hypothermic period which, while permitting direct comparisons between hypothermia, rewarming and normothermia, may sacrifice some granularity in the data across time.

Our study had several limitations. A causal relationship between blood pressure autoregulation and cardiopulmonary outcomes cannot be determined with the current design. Selection bias may have led to inclusion of less-ill neonates because parents of the sickest babies and those deemed likely to die were not consented for the study during the initial period of data collection, and HVx was not monitored in neonates who died, were withdrawn from life-sustaining treatments, or were transferred for possible ECMO. Larger studies are required to further define the multifactorial nature of the complex interactions between cerebral autoregulation and the cardiopulmonary system. Our observational data suggest that sex may play a role in this relationship. We did not measure HVx during the first 24 h of life in many neonates, and we did not evaluate the trajectory of MAP_OPT across time in short epochs. Lastly, we acknowledge that multiple testing of outcome data derived from single patients may introduce statistical limitations in the interpretation of the results.

In conclusion, we report the potential use of cerebral autoregulation monitoring with HVx to identify blood pressures that support cerebral and cardiopulmonary recovery during and after TH for neonatal neonatal encephalopathy. MAP_OPT measurements from HVx may provide information about the risk of lung injury, and sex may modulate cerebral vascular-pulmonary interactions to some degree in neonatal encephalopathy. Maintaining blood pressure above MAP_OPT may protect both the brain and lungs in boys, but it is associated with respiratory compromise in girls. Girls who receive milrinone may also be more likely to have blood pressure deviation below MAP_OPT. Growing pre-clinical and clinical data suggest sex-dimorphism in physiological responses to neonatal HI injury. Understanding these potentially divergent injury mechanisms in relation to autoregulation would improve treatment strategies in neonatal encephalopathy. Furthermore, our results support the need for large, prospective studies that are powered to evaluate sex-differences in pathophysiologic responses to neonatal encephalopathy as well as the interactions between neurologic and non-neurologic systems in neonates.