The effect of COVID-19 on training in cardiology: a survey of UK cardiology trainees

Background

The COVID-19 pandemic has had an unprecedented impact on delivering cardiology services. In the UK and many other countries, cardiologists in training were redeployed to other services. To date, the impact of COVID-19 and the requisite NHS response on cardiology training has been unclear.

Purpose

To assess the impact of the COVID-19 pandemic on cardiology training in the UK.

Methods

An annual survey of UK cardiology trainees was conducted through the national trainee organisation. In 2021, trainees were asked questions regarding their training experience, procedural exposure, and how they had been impacted by COVID-19. Reported procedural numbers were compared with those reported in 2017–2019. Chi squared analyses were used to compare categorical variables with Mann-Whitney U tests used for continuous variables.

Results

A total of 576 trainees completed the survey (70% male, mean age 33±3 years). Of 545 respondents who detailed the impact of COVID-19 on training opportunities, 70.5% (n=384) reported a negative or very negative impact. A similar picture was seen when asked about procedure, diagnostic, and outpatient training opportunities (Figure 1). Those completing core cardiology training in 2021 reported performing significantly fewer coronary angiograms (median 170, IQR 85–315) compared with those completing core cardiology training 2017–2019 (median 285, IQR 165–460, p<0.001).

Fifty percent of trainees (n=285) reported being redeployed for a median duration of 4 months (IQR 3–5 months). There was substantial regional variation in both the proportion of trainees redeployed (Figure 2, p<0.001) and the median length of redeployment (p=0.008). Those redeployed were more likely to report negative training experiences as a result of COVID-19 (p<0.001). Redeployed trainees completing core cardiology training in 2021 reported undertaking significantly fewer echocardiograms (median 205, IQR 100–300) compared with those not redeployed (median 280, IQR 200–300, p=0.01). Thirty-five percent of all trainees reported being close to burnout, with redeployed trainees being more likely to feel this way (p<0.001). When asked about methods to redress lost training opportunities, 37% of trainees wanted to prolong their training time with a median of 6 months felt to be required (IQR 6–8 months).

Discussion

This large survey of the UK experience illustrates the substantial negative impact of COVID-19 on the quality of cardiology training. Redeployment alone resulted in an estimated 95 person-years of lost training time. Coordinated national and regional strategies are required to avoid the creation of a generation of under-trained consultant cardiologists.

Funding Acknowledgement

Type of funding sources: None.

Assessment of optimal thresholds for ventricular scar substrate characterization using the high density grid multipolar mapping catheter

Funding Acknowledgements

Type of funding sources: None.

Background

Voltage thresholds for ventricular scar definition are based on historic data collected using catheters with widely spaced bipoles in the absence of contact force. Modern multipolar mapping catheters employ smaller electrodes and interelectrode spacing that theoretically allows for mapping with increased resolution and reduced far-field electrogram (EGM) component. Despite the advancement in technology, historic cut-offs of &lt;0.5mV for dense scar and 0.5-1.5mV for scar borderzone continue to be used in contemporary electrophysiology.

Purpose

We aimed to assess the optimal voltage cut-offs for ventricular scar substrate characterization using the HD Grid multipolar mapping catheter. Voltage cut-offs were assessed against cardiac MRI derived scar. We compared optimal voltage cut-offs using conventional bipolar sampling, the Best Duplicate Algorithm and with the HD wave solution plus best duplicate algorithm on.

Methods

A multicentre study of twenty patients undergoing VT ablation was conducted. Substrate mapping was performed using the high-density HD-grid multipolar mapping catheter. Bipolar voltage maps were co-registered with cardiac MRI obtained prior to the procedure to assess the voltage characteristics of scar defined by cardiac MRI (CMR) (Figure 1). Pre-procedure contrast enhanced CMR data were analysed using ADAS software (Galgo medical). Data points were collected in regions of scar during (1) HD wave mapping with best duplicate algorithm on(Waveon), (2) Mapping with HD wave off and best duplicate on (Waveoff) and (3) with conventional bipolar mapping (Alloff).

Results

The median bipolar voltage for regions of dense CMR scar using (Waveon) HD wave solution and best duplicate algorithm was 0.27mV (IQR 0.14 – 0.46). The median voltage with (Waveoff) HD wave off was 0.29mV (0.15 – 0.45). The median voltage with (Alloff) HD wave off and best duplicate off was 0.32mV (0.19 – 0.5). ROC analysis using AUC suggested the optimal cut-off for endocardial dense scar using (Waveon) HD wave mapping and best duplicate algorithm was 0.30mV (sensitivity: 69.6%, specificity: 60.74%), (Waveoff) cut-off with the best duplicate and without the HD wave mapping was 0.34mV (sensitivity: 69.78%, specificity: 64.46%) and (Alloff) without wave mapping or best duplication was 0.36mV (sensitivity: 84%, specificity: 52%) Figure 2.

Conclusion

Ventricular substrate characterization with newer mapping technology using narrow electrode spacing and smaller electrode size suggests that traditional voltage cut-offs may need revision for delineation of scar characteristics. Additionally, the ability to repeat sample in a region to obtain the best signal (Best Duplicate), and the ability to obviate the effect of wavefront direction using the HD wave solution omnipolar technology, may further increase the fidelity of scar characterization. This has important implications for mapping VT and characterizing channels in order to identify VT circuits.

Integration of structural and functional data in VT ablation -- SENSE2 protocol mapping

Funding Acknowledgements

Type of funding sources: None.

Background

We have previously developed the sense protocol functional substrate mapping technique for VT ablation(1). However, functional substrate characterizaiton can involve protracted mapping time.

Purpose

We incorporated the integration of MRI data using ADAS-3D software into the mapping workflow, to integrate structural mapping information into the functional mapping substrate characterization, in order to improve procedural efficiency.

Methods

CMRs were performed in 20 patients with ischemic related VT and VT therapy in the previous 6 months. These were processed with the ADAS-3D software to characterize the extent of ventricular scars and also ADAS corridors which may correlate with VT channels. Focused substrate maps were then performed in patients, guided by the extent of ADAS scar and corridors, looking at the scar substrate in intrinsic rhythm and then functional channels using single extra pacing from the RV at 20ms above ERP (SENSE2 Protocol). Specifically healthy areas 2cm beyond the scar borderzone based on ADAS were not mapped, in order to reduce substrate mapping time and complete geometries were not created. Following delineation of functional channels pacemapping and entrainment mapping were used to confirm targets for ablation.

The ADAS 3D MRI was integrated into the into the VT substrate map on Ensite-Precision with alignment to the aorta, RV and PA (Figure 1). We compared our data with previous functional mapping data without the integration of MRI.

Results

20 patients (age 70 years; 19 male subjects) underwent ablation. Mean EF 28%. Median procedure time was 161 minutes compared with 246 minutes (in our previous study)(p=&lt;0.001) Mean substrate mapping time was 32 mins vs 63 mins (p=&lt;0.001). Mean ablation time was 22 mins vs 32 mins (p=0.11). 85% (17 of 20) patients were free from symptomatic VT/ anti-tachycardia pacing or implantable cardioverter defibrillator shocks at a median follow-up of 171 days. The mean VT burden was reduced from 22 events per patient in the 6 months’ pre-ablation to 1 event per patient in the median follow up period of 171 days post ablation (p=0.02). Mean shocks per-patient burden decreased from 3.5 to 0.08 in the same time period(p=0.03).

Conclusion

The SENSE2 protocol involves the integration of structural and functional data into the VT workflow for substrate characterization. It enables focused substrate maps to be performed without the need for complete geometry to be created in large ventricles. Outcomes compare favourably with our previous data but with significantly shorter procedure times. This streamlined workflow has the potential to improve care in VT ablation by shortening procedure times with similar outcomes which may reduce risks for the patient.

Figure 1: Comparison of Voltage Map with MRI scar & corridors using ADAS

UK national and regional trends in cardiovascular magnetic resonance usage – the British Society of CMR survey results

Background

CMR is an imaging modality recommended for multiple indications. Access to CMR is a key issue for its clinical use. We surveyed all CMR units in the UK.

Methods

An online survey of CMR units in the UK, with responses analysed by region and compared with population data.

Results

Response rate was 100% (Table). The UK performed a total of 102,886 scans in 2017, and 117,967 in 2018 representing a 1-year 14.7% increase and a 10-year increase of 573% compared to 2008 data (20,597).By head of population in 2018 there were 1,776 CMR scans per million people, with significant variation nationally and regionally, e.g. 4,256 per million in London vs. 396 per million in Wales (Figure). Mean number of scans per unit was 1,404, (range 98–10,000) with wide variation in referral to diagnostic times (mean 45.7 days, range 5–180) (Figure).

Clinical indications for CMR were: heart failure 21%, cardiomyopathy 27%, function and viability 22%, stress 24%, vascular disease 5%, valvular 5%, myocarditis/pericardial 10%, paediatric /congenital 10%, others e.g. transplant/masses 4%, with overlap. There were 358 consultants reporting CMR in 2018 (234 (65%) cardiologists and 124 (35%) radiologists). 81% of units had a CMR service for patients with pacemakers and defibrillators.

Conclusion

The survey shows the state of CMR in the UK. The 10-year growth has been remarkable, but there are wide disparities in terms of use, access and wait times with potential implications for clinical care. Action is needed to make access equitable across the UK.

Figure 1

Funding Acknowledgement

Type of funding source: None

Investigating the role of CD44 and hyaluronate in embryo-epithelial interaction using an in vitro model

Implantation failure is an important impediment to increasing success rates in assisted reproductive technologies. Knowledge of the cascade of morphological and molecular events at implantation remains limited. Cell surface CD44 and hyaluronate (HA) have been reported in the uterus, but a role in intercellular interaction at implantation remains to be evaluated. Mouse embryos were co-cultured with human Ishikawa endometrial epithelial monolayers over 2 days. Attachment was tenuous during the first 24 h, after which it became stable, leading to breaching of the monolayer. The effects of enzymatically reducing the density of HA, or introducing a function-blocking antibody to CD44, were monitored during progression from weak to stable embryonic attachment. Hyaluronidase-mediated removal of surface HA from the epithelial cells enhanced the speed of attachment, while a similar treatment of embryos had no effect. The antibody to CD44 caused retardation of initial attachment. These results suggest that CD44-HA binding could be employed by embryos during initial docking, but the persistence of HA in epithelial cells might be detrimental to later stages of implantation by retarding attainment of stable attachment.

28Impact of non-invasive rapid cardiac magnetic resonance for the assessment of cardiomyopathies in developing countries

Background

Cardiovascular mortality is higher in developing countries. Part of that is suboptimal testing. Cardiac magnetic resonance (CMR) is the gold standard for measuring structure, function of the heart and adds incremental value by imaging scarring and to assess iron level. Despite the existence of MRI units, CMR is identified as a complex test, with poor training and availability in developing countries.

Purpose

To assess the potential impact of a faster CMR protocol at a multicentre level in developing countries; implementing it with an education program, for the assessment cardiomyopathies.

Methods

An international partnership. A rapid CMR protocol for the evaluation of cardiac volumes, function and tissue characterization (Cardiac Iron T2* and LGE for scar) Figure 1a. We deployed the protocol as a multicentre study: Argentina, Peru, India and South Africa accompanied by a program of education. Pre-scan clinical information, scanning data: complications, image quality and post-scan follow-up of participants for the assessment on impact, between 3 to 24 months.

Results

510 scans (4 countries, 6 cities, 12 centres) were performed with the rapid CMR protocol. Contrast studies in 378 (74%). There were no scan-related complications. Quality of the studies was maintained in a high level as an average of 89%. 97% of studies responded referral's question. All patients with contrast CMR scan have had at least one 2D echocardiogram before CMR. Average scan duration was 21±6 mins for contrast studies and 12±3 for non-contrast T2* protocol. The most common underlying diagnoses were non-ischaemic cardiomyopathy in 73% of participants (including cardiac iron level assessment in 26%, HCM in 17%, DCM in 15%), 27% for ischaemic cardiomyopathy and 15% for other pathologies. 4 of the 12 participant centres started to incorporate CMR for the first time. Findings impacted management in 60% of patients, including new diagnosis in 21% of participants. See table 1, figure 1b. For just cardiac iron assessment: 1/3 of participants had iron deposited in the heart with 14% of patients in severe levels.

Conclusions

CMR can be delivered faster and easier. When this abbreviated protocol is enabled with education, it can be implemented in developing countries with existing technology. This protocol shows high quality exam, with an important impact on patient's management.

Characteristics and impact on management Contrast studies Non-contrast studies All patients (%) 378 (74) 132 (36) Age, mean (range) years 54 (16–93) 24 (13–41) Male (%) 151 (39) 64 (48) Pre-echocardiography exam (%) 370 (98) 42 (32) Scanning duration mean (SD) 21 (6) 12 (3) Good quality exam (%) 329 (87) 120 (91) Impact on management Total All patients (%) 510 (100) Completely new diagnosis (%) 105 (21) Change/Addition of Medication (%) 128 (25) Intervention/ Surgery (%) 31 (6) Invasive angiography/biopsy (%) 25 (5) Hospital discharge/admission (%) 15 (3) TOTAL 306 (60%) SD: Standard Deviation.

Acknowledgement/Funding

Global Engagement UCL, UK Foreign & Commonwealth Office and The Peruvian Scientific, Technological Development and Technological Innovation (FONDECYT)

P680The clinical value of CMR in the management of Cardio-Oncology patients - a tertiary centre experience

Background

There is increasing awareness of cardiotoxicity arising from cancer treatments. Early diagnosis and treatment is key, to ensure patients receive optimal oncological management. Cardiovascular magnetic resonance (CMR) offers gold standard measurement of cardiac function, alongside tissue characterisation and myocardial perfusion, thereby potentially providing additive value in the context of cardio-oncology.

Purpose

We sought to understand the clinical value of CMR in cardio-oncology at a tertiary cardio-oncology centre.

Methods

We retrospectively reviewed CMR scans requested in cardio-oncology patients at our institution within a ten-month period. We categorised clinical indications and assessed the impact on clinical management using previously-published criteria.

Results

102 CMR studies were requested in 93 cardio-oncology patients (mean age 56 (range 18 to 82), 49% male) between (March to December 2018). 41% of patients had haematological malignancies, 59% solid tumours.

15% of requests were for risk stratification prior to initiation of cancer therapy, 21% for screening for cardio-toxicity in patients currently receiving cardiotoxic agents (3% anthracyclines, 13% HER2 monoclonal antibodies, 4% fluoropyrimidines), 15% for investigation of patients with cardiac complications during cancer treatment, 35% assessment for late effects post cancer treatment, and 14% for cardiac malignancies/ infiltration.

The most common indications for CMR were monitoring of left ventricular ejection fraction (LVEF) in patients where quantification by echocardiography was non-diagnostic or significantly different between imaging studies (39%) and ischaemia assessment including for patients due to receive fluoropyrimidines (26%). Others were aetiology of LV dysfunction/cardiomyopathy (13%) and tissue characterisation (23%), including assessment for cardiac AL amyloid (11 patients), myocarditis (2), cardiac metastases (1), cardiac masses (6), and cardiac iron loading (1).

CMR findings had clinical impact in 61% of patients and assisted in adjudicating a new diagnosis in 29% of patients. 88% of patients were able to continue anthracycline/anti-HER2 therapies based on CMR findings of stable LVEF (93% of whose echocardiograms had suggested reductions). LVEF had reduced significantly in 12% of patients meaning chemotherapy was held/discontinued. 3 patients were recommended to receive non-fluoropyrimidine chemotherapy based on perfusion CMR (pCMR) findings, with one patient permitted to receive capecitabine following normal pCMR.

Conclusion

CMR provides a comprehensive assessment of myocardial structure and function with utility within the context of cardio-oncology for risk stratification pre-chemotherapy, screening for cardiotoxicity during treatment and investigation of cardiac complications of cancer treatment. The additional information derived from CMR generally provides reassurance enabling administration of optimal cancer therapies.

Guidelines for training in cardiovascular magnetic resonance (CMR)

These "Guidelines for training in Cardiovascular Magnetic Resonance" were developed by the Certification Committee of the Society for Cardiovascular Magnetic Resonance (SCMR) and approved by the SCMR Board of Trustees.

Identification of the functional pathways altered by placental cell exposure to high glucose: lessons from the transcript and metabolite interactome

The specific consequences of hyperglycaemia on placental metabolism and function are incompletely understood but likely contribute to poor pregnancy outcomes associated with diabetes mellitus (DM). This study aimed to identify the functional biochemical pathways perturbed by placental exposure to high glucose levels through integrative analysis of the trophoblast transcriptome and metabolome. The human trophoblast cell line, BeWo, was cultured in 5 or 25 mM glucose, as a model of the placenta in DM. Transcriptomic analysis using microarrays, demonstrated 5632 differentially expressed gene transcripts (≥± 1.3 fold change (FC)) following exposure to high glucose. These genes were used to generate interactome models of transcript response using BioGRID (non-inferred network: 2500 nodes (genes) and 10541 protein-protein interactions). Ultra performance-liquid chromatography-mass spectrometry (MS) and gas chromatography-MS analysis of intracellular extracts and culture medium were used to assess the response of metabolite profiles to high glucose concentration. The interactions of altered genes and metabolites were assessed using the MetScape interactome database, resulting in an integrated model of systemic transcriptome (2969 genes) and metabolome (41 metabolites) response within placental cells exposed to high glucose. The functional pathways which demonstrated significant change in response to high glucose included fatty acid β-oxidation, phospholipid metabolism and phosphatidylinositol phosphate signalling.

Effectiveness of the Cobb-Stainsby excision arthroplasty

BACKGROUND

Dislocated metatarsophalangeal joints from clawed or hammer toes can be a disabling consequence of several conditions. The Cobb-Stainsby forefoot arthroplasty combines partial phalangectomy (Stainsby) with extensor tendon transfer to the metatarsal head (Cobb). We present a retrospective, three surgeon case series of 215 toes in 126 patients.

METHODS

Early results and complications were gathered from the medical charts of 126 patients who met the inclusion criteria. Seventy-five patients were contactable by phone with a follow up range of 12-82 months (median follow up 45 months). Primary outcome measures were improvement of pain and function, reduction in plantar callosities and cosmetic improvement of the deformity.

RESULTS

Pre-operatively all patients presented with pain and shoe wear problems. Post-operatively seventy-two patients (96%) were satisfied, 72 (96%) reported pain relief, 55 (73%) were happy with toe control, 61 (81%) were pleased with cosmesis and 56 (75%) reported unlimited daily activities. Superficial wound infections were observed in 13 of the 126 patients (10%) and two in 75 patients (2%) developed recurrent clawing.

CONCLUSION

Our case series demonstrates improved outcomes over alternatives such as the Weil's osteotomy.

Cost-effectiveness analysis of new generation coronary CT scanners for difficult-to-image patients

AIMS

New generation dual-source coronary CT (NGCCT) scanners with more than 64 slices were evaluated for patients with (known) or suspected of coronary artery disease (CAD) who are difficult to image: obese, coronary calcium score > 400, arrhythmias, previous revascularization, heart rate > 65 beats per minute, and intolerance of betablocker. A cost-effectiveness analysis of NGCCT compared with invasive coronary angiography (ICA) was performed for these difficult-to-image patients for England and Wales.

METHODS AND RESULTS

Five models (diagnostic decision model, four Markov models for CAD progression, stroke, radiation and general population) were integrated to estimate the cost-effectiveness of NGCCT for both suspected and known CAD populations. The lifetime costs and effects from the National Health Service perspective were estimated for three strategies: (1) patients diagnosed using ICA, (2) using NGCCT, and (3) patients diagnosed using a combination of NGCCT and, if positive, followed by ICA. In the suspected population, the strategy where patients only undergo a NGCCT is a cost-effective option at accepted cost-effectiveness thresholds. The strategy of using NGCCT in combination with ICA is the most favourable strategy for patients with known CAD. The most influential factors behind these results are the percentage of patients being misclassified (a function of both diagnostic accuracy and the prior likelihood), the complication rates of the procedures, and the cost price of a NGCCT scan.

CONCLUSION

The use of NGCCT might be considered cost-effective in both populations since it is cost-saving compared to ICA and generates similar effects.

The burden, epidemiology, costs and treatment for Duchenne muscular dystrophy: an evidence review

BACKGROUND

Duchenne Muscular Dystrophy (DMD) is a rapidly progressive, lethal neuromuscular disorder, present from birth, which occurs almost exclusively in males. We have reviewed contemporary evidence of burden, epidemiology, illness costs and treatment patterns of DMD. This systematic review adhered to published methods with information also sought from the web and contacting registries. Searches were carried out from 2005 to June 2015. The population of interest was individuals with clearly defined DMD or their carers.

RESULTS

Nine thousand eight hundred fifty titles were retrieved from searches. Fifty-eight studies were reviewed with three assessed as high, 33 as medium and 22 as low quality. We found two studies reporting birth and four reporting point prevalence, three reporting mortality, 41 reporting severity and/or progression, 18 reporting treatment patterns, 12 reporting quality of life, two reporting utility measures, three reporting costs of illness and three treatment guidelines. Birth prevalence ranged from 15.9 to 19.5 per 100,000 live births. Point prevalence per 100,000 males was for France, USA, UK and Canada, 10.9, 1.9, 2.2 and 6.1 respectively. A study of adult DMD patients at a centre in France found median survival for those born between 1970 and 1994 was 40.95 years compared to 25.77 years for those born between 1955 and 1969. Loss of ambulation occurred at a median age of 12 and ventilation starts at about 20 years. There was international variation in use of corticosteroids, scoliosis surgery, ventilation and physiotherapy. The economic cost of DMD climbs dramatically with disease progression - rising as much as 5.7 fold from the early ambulatory phase to the non-ambulatory phase in Germany.

CONCLUSIONS

This is the first systematic review of treatment, progression, severity and quality of life in DMD. It also provides the most recent description of the burden, epidemiology, illness costs and treatment patterns in DMD. There are evidence gaps, particularly in prevalence and mortality. People with DMD seem to be living longer, possibly due to corticosteroid use, cardiac medical management and ventilation. Future research should incorporate registry data to improve comparability across time and between countries and to investigate the quality of life impact as the condition progresses.

Pharmacogenomics related to growth disorders

Growth disorders resulting in short stature are caused by a wide range of underlying pathophysiological processes. To improve height many of these conditions are treated with recombinant human growth hormone (rhGH). However, substantial inter-individual variability in growth response both in the short and long-term is recognised. Over the last decade, disease-specific growth prediction models have been developed that the clinician can use to define a child's potential response to rhGH and to optimise starting and maintenance doses of rhGH. These models, however, are not able to predict all the variations in treatment response. There has, therefore, been recent interest in using genetic information to contribute to the evaluation of responses to rhGH, including high-throughput technologies for assessing DNA markers (genome) and mRNA transcripts (transcriptome) as pharmacogenomic tools. This review will focus on how these pharmacogenomic approaches are being applied to growth disorders.

Translational neuroendocrinology: control of human growth

Human growth is driven by both basic cell processes as well as hormones, in particular the growth hormone (GH)-insulin-like growth factor (IGF)-1 axis. Understanding how these mechanisms are coordinated is not only critical to achieving a normal growth rate, but also to recognising potential new causes of disordered growth and how they might be treated. We have demonstrated in healthy children that height is gained by periods of rapid growth interspersed by periods of very slow growth or even stasis. We have also shown that a lower order organism, Caenorhabditis elegans, grows in a similar manner. By contrast, secretion of GH from somatotrophs occurs on a daily basis in discrete pulses over a 24-h period. We have used the measurement of GH in urine as a surrogate marker of GH secretion to show that there are rhythms of GH output with frequencies of several days. We then assessed which attributes of these GH profiles were related to growth and found that disorderliness in the GH profile (as measured by approximate entropy) was related to better growth rate. This feature was then tested in the dwarf rat using different GH regimens to introduce variation into the administration of daily GH injections. Better long bone growth was associated with week-to-week or even random dose variation compared to the same amount of GH delivered as a standard daily dose. Understanding the control of growth has implications in clinical practice for modelling GH treatment regimens based on physiological principles.

Placental alkaline phosphatase de-phosphorylates insulin-like growth factor (IGF)-binding protein-1

BACKGROUND

Insulin-like growth factors (IGF) regulate fetal growth through their effects on placenta. Their actions are influenced by IGF binding protein-1. Phosphorylated IGFBP-1 (pIGFBP-1) has high affinity for IGF-I and usually inhibits IGF-I activity but during pregnancy, it is de-phosphorylated to generate lower affinity isoforms and consequently, increased IGF bioavailability. Here we investigate the role of placenta in this process.

RESULTS

Our data show that term human placental explants, but not their conditioned medium, can de-phosphorylate IGFBP-1 through the action of placental alkaline phosphatase (PLAP).

DISCUSSION

PLAP-mediated de-phosphorylation of IGFBP-1 may provide a mechanism for controlling IGF-I bioavailability and action at the maternal/fetal interface.