In this study, the relationship between LVO and PI derived from a new generation pulse oximeters in healthy late preterm infants was investigated for the first time. It is believed that PI may be a direct predictor of peripheral perfusion, as well as a useful and practical method for determining critical patients in NICU and evaluating the effect of treatment on peripheral perfusion [15, 16]. Normal values of PI were evaluated during the postnatal transition period and early neonatal period in healthy term infants [17, 18]. Cresi et al  was investigated the PI reference values in clinically and hemodynamically stable preterm infants during the first week of life. They reported average PI values measured from the feet of infants at 28 to 36 gestational weeks (mean 32.5 weeks) as 0.90 on the postnatal 1st day, 1.22 on the 3rd day and 1.36 on the 7th day. Although they found a significant difference in PI values between 1st day and 3rd day, there wasn’t a significant difference between 3rd day and 7th day. They suggested that this trend of changes in peripheral PI values reflects physiological changes in peripheral microvascular blood flow that begin immediately after birth and may be associated with intrinsic hemodynamic adaptation that occurs on the first day of life. A study of systemic blood flow in preterm infants reported that perfusion is achieved even in the presence of low blood flow and high vascular resistance within the first 24 hours of life. In the following days, it was noted that vascular resistance decreased due to vasodilation and blood flow returned to normal . In our study, the median PI value measured from the preterm babies’ feet at the 48th hour of life was higher than the PI values from the study of Cresi et al . This may have been caused by the fact that the gestation age of preterm infants in our study was higher than that of infants in the study of Cresi et al, and the completion of physiological changes in peripheral microvascular blood flow within the first 24 hours in our cases.
In a study conducted by Granelli et al , pre - and post-ductal PI reference values were investigated in 10.000 healthy newborns whose postnatal ages ranged from 1 to 120 hours. In this study, the measured PI 5th percentile value of both the right hand and foot was reported as 0.7. It is noted that a PI value of < 0.7 can be a critical CHD indicator. They suggested that only pre-and post-ductal arterial oxygen saturation screening can miss the diagnosis of infants with left ventricular obstruction, and that this screening method may be insufficient in the diagnosis of critical CHD associated with low systemic perfusion. But as far as we know, there is only one study in the English literature that evaluates the correlation between PI and LVO in the newborn. Corsini et al  showed for the first time that there is a correlation between PI and LVO in healthy term infants. In their study on 49 healthy term infants at the 2nd day of life, when postnatal transition circulation was completed, the average PI value of both pre-ductal and post-ductal was 1.9. In our study, the pre-ductal PI value was similar to the Corsini et al study, but the post-ductal PI value was lower. Corsini et al  reported the LVO/kg, RVO/ kg and SVCf/kg as 139, 160 and 132 mL/kg/min, respectively. In our study, both LVO/kg, RVO/kg and SVC/kg values were higher than those reported by Corsini et al . They reported that there was a positive correlation between pre- / post-ductal PI and LVO in term infants, and also the correlation between PI and LVO/kg continued when LVO was normalized according to body weight (LVO/kg). Therefore, they noted that PI may have a role in detecting critical CHD with low perfusion. Contrary to the findings of Corsini et al , our study found no correlation between pre- / post-ductal PI and LVO in healthy late preterm infants. There was also no correlation between PI and LVO/kg when LVO was normalized according to body weight (LVO/kg). These data suggested that LVO, as a hemodynamic parameter, in healthy late preterm infants was not sufficiently sensitive to reflect peripheral microvascular blood flow. It has been thought that this difference between term and preterm infants may be due to the fact that the maturation of the sympathetic nervous system, which plays a role in the regulation of peripheral perfusion, in preterm infants is not yet complete. Although studies in adult patients on this topic are limited, a study conducted by Lima et al  in adult patients reported no association between changes in cardiac output and peripheral PI or clinical signs of poor peripheral perfusion.
In our study, there was no correlation between pre- / post-ductal PI and RVO or RVO/kg, similar to the study of Corsini et al . This may be because the foramen ovale has not yet closed. Similar to the results of Corsini et al (10), there was no association between pre- / post-ductal PI and SVCf or SVCf/kg in our study. Takahaski et al  reported that they found a correlation between PI and SVC flow in their cohort of preterm infants. They noted that SVC flow reflects systemic blood flow, especially in small preterm infants with PDA. In our study, there was a moderate correlation between SVC flow and LVO/kg. The reason why our study result differs with the findings of Takahaski et al  may be that our study population does not cover very low birth weight babies with PDA.
As a result of observations, it has been reported that PI shows circadian rhythm, and can be affected by nutrition, intravenous therapy, jaundice, sleep-awake status and sleep position [11, 22]. A study conducted by Sahni et al  in low birth weight infants reported that the PI value measured in the supine sleep position was higher than in the supine position. In our study, peripheral perfusion in all infants was evaluated after feeding, in supine position and while awake. Although the infants with physiological jaundice were included in the study, the infants with hyperbilirubinemia requiring phototherapy were not included in the study. In our study, circadian rhythm was not taken into account in the time period during which the measurement was performed, due to the fact that babies had different birth times. It has been reported that PI measurement is not affected by physiological variables such as heart rate, SpO2, oxygen consumption, blood pressure, and body temperature . Similarly, no association between PI and heart rate, SpO2, body temperature, and blood pressure (systolic, diastolic, and mean blood pressure) was found in our study.
Our study had some limitations: i) the number of babies included in the study was small, and ii) we did not examine whether there was a relationship between PI and LVO in infants with CHD.
In conclusion, a correlation between pre- / post-ductal PI and LVO in healthy late preterm infants wasn’t found in our study. We believe that a systemic hemodynamic parameter such as LVO cannot adequately reflect peripheral microvascular blood flow due to incomplete maturation of the sympathetic nervous system in preterm infants, which is involved in regulating peripheral perfusion. New studies with larger populations are needed to determine the actual role of the synergistic effect of SPO2 with PI in screening for critical CHD, especially in preterm infants.