Asthma in the intensive care unit: A review of patient characteristics and outcomes

Background

Most asthma-related deaths occur in low- and middle-income countries, and South Africa (SA) is ranked fifth in global asthma mortality. Little is known about the characteristics and outcome of asthma patients requiring intensive care unit (ICU) admission in SA.

Objectives

To identify and characterise patients with acute severe asthma admitted to the respiratory ICU at Groote Schuur Hospital, Cape Town, SA, in order to evaluate outcomes and identify predictors of poor outcomes in those admitted.

Methods

We performed a retrospective descriptive study of patients with severe asthma admitted to the respiratory ICU at Groote Schuur Hospital between 1 January 2014 and 31 December 2019.

Results

One hundred and three patients (110 admission episodes) were identified with an acute asthma exacerbation requiring ICU admission; all were mechanically ventilated. There was a female preponderance (53.6%; n=59/110), with a median (range) age overall of 33 (13 - 84) years. Of all admissions, 40 (36.4%) were current tobacco smokers and 16 (14.5%) patients with a history of substance abuse. Two thirds (60.0%; n=66/110) of the patients were using an inhaled corticosteroid (ICS). No predictors of mortality were evident in multivariate modelling, although those who died were older, and had higher Acute Physiology and Chronic Health Evaluation (APACHE II) scores and longer duration of admission. Only 59 of the surviving 96 individual patients (61.5%) attended a specialist pulmonology clinic after discharge.

Conclusion

Among patients admitted to the respiratory ICU at Groote Schuur Hospital for asthma exacerbations, there was a high prevalence of smokers and poor coverage with inhaled ICSs. Although mortality was low compared with general ICU mortality, more needs to be done to prevent acute severe asthma exacerbations.

Study synopsis

What the study adds. Intensive care unit (ICU) admission represents the most severe form of exacerbation of asthma. South Africa (SA) has a very high rate of asthma deaths, and this study demonstrates that admission to an ICU with a very severe asthma exacerbation frequently results in a good outcome. However, many of the patients admitted to the ICU were not adequately treated with background asthma medications prior to their admission. Implications of the findings. Death from asthma should be avoidable, and admission to an ICU is not associated with high mortality. Patients are therefore likely to be dying at home or out of hospital. Better education and access to medication and early access to health services rather than improved in-hospital care would potentially alter SA's high asthma mortality.

Exploring overcrowding trends in an inner city emergence department in the UK before and during COVID-19 epidemic

Background

The COVID-19 pandemic and the associated lockdowns have caused significant disruptions across society, including changes in the number of emergency department (ED) visits. This study aims to investigate the impact of three pre-COVID-19 interventions and of the COVID-19 UK-epidemic and the first UK national lockdown on overcrowding within University College London Hospital Emergency Department (UCLH ED). The three interventions: target the influx of patients at ED (A), reduce the pressure on in-patients’ beds (B) and improve ED processes to improve the flow of patents out from ED (C).

Methods

We collected overcrowding metrics (daily attendances, the proportion of people leaving within 4 h of arrival (four-hours target) and the reduction in overall waiting time) during 01/04/2017–31/05/2020. We then performed three different analyses, considering three different timeframes. The first analysis used data 01/04/2017–31/12–2019 to calculate changes over a period of 6 months before and after the start of interventions A-C. The second and third analyses focused on evaluating the impact of the COVID-19 epidemic, comparing the first 10 months in 2020 and 2019, and of the first national lockdown (23/03/2020–31/05/2020).

Results

Pre-COVID-19 all interventions led to small reductions in waiting time (17%, p < 0.001 for A and C; an 9%, p = 0.322 for B) but also to a small decrease in the number of patients leaving within 4 h of arrival (6.6,7.4,6.2% respectively A-C,p < 0.001).

In presence of the COVID-19 pandemic, attendance and waiting time were reduced (40% and 8%; p < 0.001), and the number of people leaving within 4 h of arrival was increased (6%,p < 0.001). During the first lockdown, there was 65% reduction in attendance, 22% reduction in waiting time and 8% increase in number of people leaving within 4 h of arrival (p < 0.001). Crucially, when the lockdown was lifted, there was an increase (6.5%,p < 0.001) in the percentage of people leaving within 4 h, together with a larger (12.5%,p < 0.001) decrease in waiting time. This occurred despite the increase of 49.6%(p < 0.001) in attendance after lockdown ended.

Conclusions

The mixed results pre-COVID-19 (significant improvements in waiting time with some interventions but not improvement in the four-hours target), may be due to indirect impacts of these interventions, where increasing pressure on one part of the ED system affected other parts. This underlines the need for multifaceted interventions and a system-wide approach to improve the pathway of flow through the ED system is necessary.

During 2020 and in presence of the COVID-19 epidemic, a shift in public behaviour with anxiety over attending hospitals and higher use of virtual consultations, led to notable drop in UCLH ED attendance and consequential curbing of overcrowding.

Importantly, once the lockdown was lifted, although there was an increase in arrivals at UCLH ED, overcrowding metrics were reduced. Thus, the combination of shifted public behaviour and the restructuring changes during COVID-19 epidemic, maybe be able to curb future ED overcrowding, but longer timeframe analysis is required to confirm this.

Inequalities in cancer screening participation: examining differences in perceived benefits and barriers

OBJECTIVE

Inequalities exist in colorectal cancer (CRC) screening uptake, with people from lower socioeconomic status backgrounds less likely to participate. Identifying the facilitators and barriers to screening uptake is important to addressing screening disparities. We pooled data from 2 trials to examine educational differences in psychological constructs related to guaiac fecal occult blood testing.

METHODS

Patients (n = 8576) registered at 7 general practices in England, within 15 years of the eligible age range for screening (45-59.5 years), were invited to complete a questionnaire. Measures included perceived barriers (emotional and practical) and benefits of screening, screening intentions, and participant characteristics including education.

RESULTS

After data pooling, 2181 responses were included. People with high school education or no formal education reported higher emotional and practical barriers and were less likely to definitely intend to participate in screening, compared with university graduates in analyses controlling for study arm and participant characteristics. The belief that one would worry more about CRC after screening and concerns about tempting fate were strongly negatively associated with education. In a model including education and participant characteristics, respondents with low emotional barriers, low practical barriers, and high perceived benefits were more likely to definitely intend to take part in screening.

CONCLUSIONS

In this analysis of adults approaching the CRC screening age, there was a consistent effect of education on perceived barriers toward guaiac fecal occult blood testing, which could affect screening decision making. Interventions should target specific barriers to reduce educational disparities in screening uptake and avoid exacerbating inequalities in CRC mortality.

Reasons for non-uptake and subsequent participation in the NHS Bowel Cancer Screening Programme: a qualitative study

BACKGROUND

Screening for bowel cancer using the guaiac faecal occult blood test offered by the NHS Bowel Cancer Screening Programme (BCSP) is taken up by 54% of the eligible population. Uptake ranges from 35% in the most to 61% in the least deprived areas. This study explores reasons for non-uptake of bowel cancer screening, and examines reasons for subsequent uptake among participants who had initially not taken part in screening.

METHODS

Focus groups with a socio-economically diverse sample of participants were used to explore participants' experience of invitation to and non-uptake of bowel cancer screening.

RESULTS

Participants described sampling faeces and storing faecal samples as broaching a cultural taboo, and causing shame. Completion of the test kit within the home rather than a formal health setting was considered unsettling and reduced perceived importance. Not knowing screening results was reported to be preferable to the implications of a positive screening result. Feeling well was associated with low perceived relevance of screening. Talking about bowel cancer screening with family and peers emerged as the key to subsequent participation in screening.

CONCLUSIONS

Initiatives to normalise discussion about bowel cancer screening, to link the BCSP to general practice, and to simplify the test itself may lead to increased uptake across all social groups.

Indications for fenestrated endovascular aneurysm repair

Background

Endovascular technology has advanced rapidly in the development of fenestrated endovascular aneurysm repair (FEVAR). Current evidence for endovascular aneurysm repair is limited to infra-renal aortic aneurysms. With increased costs and complexity of FEVAR, its current role is unclear. A national multicentre, cross-disciplinary consensus model was developed to propose indications for FEVAR.

Methods

All UK FEVAR centres and a wide selection of high-volume aneurysm treatment centres were invited to participate. The RAND appropriateness methodology was used. Five key steps were undertaken: meta-analysis of current literature; survey of current UK practice; nominal group establishment and definition of key clinical attributes; round 1—online survey of case vignettes; and round 2—nominal group consensus meeting.

Results

More than 90 per cent of UK FEVAR centres participated. Literature review showed heterogeneous case series with no clear indications for use of FEVAR. Survey of current practice showed wide variations in aneurysm management. Consensus agreement on the role of FEVAR was achieved in 68·8 per cent of cases. Consensus for FEVAR was agreed in areas of moderate risk from open repair and need for suprarenal clamping, but it was less likely to be indicated in patients aged 85 years or more with 5·5–6-cm aneurysms, or short-necked infrarenal aortic aneurysms.

Conclusion

These data record areas of agreement and define the grey area of equipoise. Consequently, guidelines and recommendations can be developed on the indications for FEVAR to inform clinicians, commissioners and health economists.

Guideline for the management of chronic obstructive pulmonary disease--2011 update

OBJECTIVE

To revise the South African Guideline for the Management of Chronic Obstructive Pulmonary Disease (COPD) based on emerging research that has informed updated recommendations.

KEY POINTS

(1) Smoking is the major cause of COPD, but exposure to biomass fuels and tuberculosis are important additional factors. (2) Spirometry is essential for the diagnosis and staging of COPD. (3) COPD is either undiagnosed or diagnosed too late, so limiting the benefit of therapeutic interventions; performing spirometry in at-risk individuals will help to establish an early diagnosis. (4) Oral corticosteroids are no longer recommended for maintenance treatment of COPD. (5) A therapeutic trial of oral corticosteroids to distinguish corticosteroid responders from non-responders is no longer recommended. (6) Primary and secondary prevention are the most cost-effective strategies in COPD. Smoking cessation as well as avoidance of other forms of pollution can prevent disease in susceptible individuals and ameliorate progression. Bronchodilators are the mainstay of pharmacotherapy, relieving dyspnoea and improving quality of life. (7) Inhaled corticosteroids are recommended in patients with frequent exacerbations and have a synergistic effect with bronchodilators in improving lung function, quality of life and exacerbation frequency. (8) Acute exacerbations of COPD significantly affect morbidity, health care units and mortality. (9) Antibiotics are only indicated for purulent exacerbations of chronic bronchitis. (10) COPD patients should be encouraged to engage in an active lifestyle and participate in rehabilitation programmes.

OPTIONS

Treatment recommendations are based on the following: annual updates of the Global Obstructive Lung Disease (GOLD), initiative, that provide an evidence-based comprehensive review of management; independent evaluation of the level of evidence in support of some of the new treatment trends; and consideration of factors that influence COPD management in South Africa, including lung co-morbidity and drug availability and cost.

OUTCOME

Holistic management utilising pharmacological and nonpharmacological options are put in perspective.

EVIDENCE

Working groups of clinicians and clinical researchers following detailed literature review, particularly of studies performed in South Africa, and the GOLD guidelines. BENEFITS, HARMS AND COSTS. The guideline pays particular attention to cost-effectiveness in South Africa, and promotes the initial use of less costly options. It promotes smoking cessation and selection of treatment based on objective evidence of benefit. It also rejects a nihilistic or punitive approach, even in those who are unable to break the smoking addiction.

RECOMMENDATIONS

These include primary and secondary prevention; early diagnosis, staging of severity, use of bronchodilators and other forms of treatment, rehabilitation, and treatment of complications. Advice is provided on the management of acute exacerbations and the approach to air travel, prescribing long-term oxygen and lung surgery including lung volume reduction surgery.

VALIDATION

The COPD Working Group comprised experienced pulmonologists representing all university departments in South Africa and some from private practice, and general practitioners. Most contributed to the development of the previous version of the South African guideline. GUIDELINE SPONSOR: The meeting of the Working Group of the South African Thoracic Society was sponsored by an unrestricted educational grant from Boehringer Ingelheim and Glaxo-Smith-Kline.

Why do patients with systemic lupus erythematosus take or fail to take their prescribed medications? A qualitative study in a UK cohort

OBJECTIVE

It has been suggested that low adherence may contribute to poor clinical outcomes in patients with SLE. In this study, we explored the reasons why patients with lupus did or did not take their medications as prescribed.

METHODS

Questionnaires including a 10-cm visual analogue scale (VAS) to assess self-reported adherence to prescribed medications were distributed to 315 patients with SLE. The responses were used to select a purposive sample of subjects who participated in interviews to discuss why they did or did not take their medications.

RESULTS

Of the 315 patients, 220 (70%) completed the questionnaire. Thirty-three patients were interviewed. Themes explaining why patients took their medications regularly included: the fear of worsening disease, the belief that there was no effective therapeutic alternative to their prescribed medications, lack of knowledge about SLE to allow confidence in changing medications and feelings of moral obligation or responsibility to others. Themes explaining why patients did not take their medications regularly included: the belief that lupus could and should be controlled using alternative methods, the belief that long-term use of drugs was not necessary, the fear of drug adverse effects, practical difficulties in obtaining medications, and poor communication between patients and doctors.

CONCLUSION

The patients' reasons for taking or not taking their medications largely related to previous experiences with the disease and/or drugs. However, improvements in communication between doctors and patients may promote better adherence in patients with SLE.

Running along parallel lines: how political reality impedes the evaluation of public health interventions. A case study of exercise referral schemes in England

Political decisions about the way that public health initiatives are implemented have a significant impact on the ability to evaluate their effectiveness. However, the influence of the political imperative has been little explored. This case study of key research, policy and practice events concerning one initiative, exercise referral schemes (ERSs), demonstrates that these schemes were encouraged to expand by the Department of Health (DH) before DH-funded evaluations had reported their findings and with little reference to National Institute for Health and Clinical Excellence (NICE) recommendations. Policy evolved in parallel rather than in conjunction with the development of evidence, and experimental evaluations in England are now unlikely. This is due to the comprehensive coverage of schemes, widespread assumptions of effectiveness, likely difficulties in obtaining research funding, indirect adverse consequences of dismantling schemes and lack of appropriate process and outcome data. Embedding a commitment to robust evaluation prior to universal adoption of new initiatives has been shown to be feasible by policy-makers in the international setting. This is required to prevent the establishment of public health interventions that do not work and may cause harm or widen health inequalities.

Factors influencing adherence to medications in a group of patients with systemic lupus erythematosus in Jamaica

The objective of this study was to examine the factors influencing adherence to medications in a group of patients with systemic lupus erythematosus (SLE) in Jamaica. A qualitative study was designed using a screening questionnaire and semi-structured interviews. The study was conducted in the rheumatology clinic at the University Hospital of the West Indies, Kingston, Jamaica. 75 patients with SLE including 20 interviewees, who had SLE for at least 1 year participated in the study. The main outcome measures were: (i) level of self-reported adherence in a sample of the clinic attendees and (ii) interviewees explanations of the reasons for taking or not taking drugs as prescribed by their physician. 56% of the 75 study participants reported taking their medications more than 85% of the time. High cost and poor availability of medications were the main reasons for poor adherence, but some patients chose not to take their medications because of side effects, perceived mild severity of their disease and/or a preference to take drugs only when symptomatic. Patients used herbal medicines to counteract side effects of Western medicines, to ‘purge the blood’ and to manage lupus symptoms when they had no medications. Religious beliefs were used as a coping strategy. Traditional use of herbal medicines is common particularly in patients from rural Jamaica, and may explain the observed use of herbal medicines in those who have emigrated to developed countries. Socio-economic constraints and poor drug availability are particularly important influences on poor adherence in Jamaican patients with SLE. Religious beliefs and use of herbal remedies do not seem to affect adherence adversely but are used when drugs cannot be obtained.

An experimental study of determinants of the extent of disagreement within clinical guideline development groups

OBJECTIVE

To assess the effect of design features and clinical and social cues on the extent of disagreement among participants in a formal consensus development process.

METHODS

Factorial design involving 16 groups consisting of 135 general practitioners (GPs) and 42 mental health professionals from England. The groups rated the appropriateness of four mental health interventions for three conditions (chronic back pain, irritable bowel syndrome, and chronic fatigue syndrome) in the context of various clinical and social cues. The groups differed in three design features: provision of a systematic literature review (versus not provided), group composition (mixed versus GP only), and assumptions about the healthcare resources available (realistic versus idealistic). Disagreement was measured using the mean absolute deviation from a group's median rating for a scenario.

RESULTS

None of the design features significantly affected the extent of disagreement within groups (all p>0.3). Disagreement did differ between treatments (closer consensus for cognitive behavioural therapy and behavioural therapy than for brief psychodynamic intervention therapy and antidepressants) and cues (closer consensus for depressed patients and patients willing to try any treatment).

CONCLUSION

In terms of the extent of disagreement in the groups in this study, formal consensus development was a robust technique in that the results were not dependent on the way it was conducted.

Does gender bias exist in the use of specialist health care?

OBJECTIVES

To investigate the evidence for the existence of gender bias (defined as care provided independently of clinical need) in the use of specialist services by critically appraising the literature.

METHODS

A computer-assisted search of the bibliographic databases PubMed, Medline, EMBASE, Healthstar and Social Science Citation Index for English language papers published from 1966 until May 1999. In addition, four journals were handsearched and the reference lists of identified papers were explored. Retrospective studies were only used when there were insufficient prospective studies.

RESULTS

One hundred and thirty-eight studies were identified covering five major topics: coronary artery disease; renal transplantation; human immunodeficiency virus (HIV) and acquired immune deficiency syndrome (AIDS); mental illness; and other (mainly invasive) procedures. The majority (94) examined coronary artery disease. It appears that men are more likely to undergo non-invasive investigations than women, but that subsequent investigation and treatment shows no clear evidence of gender differences. Men are more likely to undergo renal transplantation and, for those with HIV and AIDS, to receive azidothymidine (zidovudine, AZT) than women. There are some indications that disparities in favour of men also occur for those suffering from cardiac arrhythmias and cerebrovascular disease, and for those undergoing vascular surgery, hip replacement and heart transplantation. In contrast, women are more likely to undergo liver transplantation and cataract surgery. Mental health services may be provided differently for men and women. All these findings are limited by a lack of accurate denominator information and insufficient ability to adjust for prognostic factors.

CONCLUSIONS

Differences in health care use can be due to demand factors (e.g. differences in the prevalence and severity of disease or in patient preferences), supply factors (particularly clinical judgement), or both. There is a need to examine these explanations thoroughly for gender inequalities in order to ensure that equity (lack of bias) is achieved. There is also a need for higher quality studies if differences are to be attributed conclusively to bias or not.

Patient determinants of mental health interventions in primary care

BACKGROUND

A large proportion of a general practitioner's (GP's) caseload comprises patients with mental health problems. It is important to ensure that care is provided appropriately, on the basis of clinical need. It is therefore necessary to investigate the determinants of the use of mental health care in the primary care sector and, in particular, to identify any non-clinical characteristics of patients that affect the likelihood of their receiving appropriate care.

AIM

To identify and compare the influence of non-clinical patient factors on GPs' acknowledgement of mental problems and on their provision of mental health care.

METHOD

Cross sectional study of adults aged 16 to 65 years old (n = 802) attending one of eight practices (20 GPs in total) in inner west London.

RESULTS

Multivariable analysis showed that the combination of factors that best predict GPs' acknowledgement of the presence of mental problems are general health questionnaire (GHQ) scores (odds ratio [OR] = 1.10 per unit increase in score, 95% confidence interval [CI] = 1.07 to 1.13), previous mental symptoms (OR = 7.5, 95% CI = 4.3 to 12.9), increasing age (OR = 1.03 per one-year increase, 95% CI = 1.01 to 1.04) and physical health status (OR = 0.98 per unit increase in short form-36 (SF-36) score, 95% CI = 0.96 to 1.00). Multivariable analysis showed that the combination of factors that best predict intervention (prescription for psychotropic medication; return visit to GP; referral to psychiatric inpatients/outpatients; referral to other [specified] health professionals, or social services) are previous symptoms (OR = 7.4, 95% CI = 3.8 to 14.4), white ethnic group (OR = 2.2, 95% CI 0.9 to 5.5); and not owning a property (OR = 2.1, 95% CI = 1.1 to 4.0). Life events influenced intervention only in the presence of low GHQ scores (OR = 8.1, 95% CI = 2.7 to 24.0).

CONCLUSIONS

Mental problems are common in primary care and their acknowledgement is a necessary but not a sufficient condition for intervention. Our results show that GPs' decisions about mental health interventions can be influenced by non-clinical patient factors, regardless of patients' clinical needs. The results suggest that current practice may not always be equitable, and point to the need for better understanding of the basis of these potential inequalities and for focused training.

Long-term respiratory health effects of the herbicide, paraquat, among workers in the Western Cape

OBJECTIVE

To evaluate the possible effects of paraquat spraying among workers on deciduous fruit farms in the Western Cape, South Africa. Paraquat is a commonly used herbicide world wide and is a well documented cause of pulmonary fibrosis in studies of laboratory animals and in humans after exposure to a high dose (usually accidental or as parasuicide). The respiratory effects of long term, low dose exposure to paraquat have not been fully evaluated.

METHODS

A cross sectional study of 126 workers. Administered questionnaires generated information on exposure, respiratory symptoms, and potential confounding variables. Spirometry and gas transfer were measured and chest radiographs performed. Oxygen desaturation on exercise testing was by oximetry during a modified stage one exercise test.

RESULTS

No association was found between long term exposure to paraquat and reported symptoms, spirometry (forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), FEV1/FVC) and gas transfer (TLCO and KCO) or chest radiography. Multivariate analysis showed a significant relation between measures of long term exposure to paraquat and arterial oxygen desaturation during exercise independent of short term exposure.

CONCLUSION

Previous studies have not shown a significant relation between measures of exposure to paraquat and standard tests of lung function. Arterial oxygen desaturation during exercise represents a more sensitive test. The findings indicate that working with paraquat under usual field conditions is associated with abnormal exercise physiology in a dose dependent fashion independent of recent exposure and acute poisoning events.

Evidence-based policy: rhetoric and reality

Evidence-based policy has been hailed as the current zeitgeist in health care. Its intuitive appeal is clear but its implementation is problematic. This case study demonstrates the practical difficulties of using this approach for purchasing health care. New technologies suffer from limited knowledge about both their long-term benefits and their adverse effects. This undermines attempts to assess effectiveness and efficiency in local populations. Furthermore, the use of evidence should not be assumed to eliminate the need to apply value judgements, which has repercussions for the attainment of equity of access to health care. It is therefore unrealistic to view this approach as a panacea. Rather, evidence-based medicine should contribute to the decision-making process, in conjunction with other considerations such as equity and patient preference.

Autonomic neuropathy in extra-hepatic portal vein thrombosis: evidence for impaired autonomic reflex arc

BACKGROUND/AIMS

Autonomic dysfunction is common in cirrhosis, and may be associated with increased mortality and hyperdynamic circulatory changes. Our aim was to investigate whether autonomic disturbances occur in extrahepatic portal vein thrombosis and their correlation with hemodynamic abnormalities.

PATIENTS AND METHODS

Heart rate variation in response to standing, deep breathing, and Valsalva maneuver, and blood pressure response to sustained handgrip and to standing, were studied in 16 subjects with portal vein thrombosis (10 males, 30.8+/-2.8 years: mean+/-SE), 12 with cirrhosis (7 males, 52+/-2.3 years), and 10 healthy controls (7 males, 30.8+/-3.0 years). Supine resting, and 10- and 30-min standing epinephrine and norepinephrine levels were measured and results correlated with cardiac output.

RESULTS

Autonomic dysfunction occurred in 62% of portal vein thrombosis and 75% of cirrhosis subjects, but in no controls (p<0.02). Similarly, postural hypotension occurred in portal vein thrombosis (-10.25+/-0.65 mmHg, p=0.003) and cirrhosis (-7.42+/-0.82 mmHg, p=0.007) but not in controls. All groups had similar baseline epinephrine and norepinephrine concentrations. Epinephrine increased significantly in controls (45%, p<0.01 and 49%, p<0.02) after 10 of 30 min standing but not in the portal vein thrombosis or the cirrhotic group, and norepinephrine increased after 10 and 30 min standing in cirrhotics (128%, p<0.004 and 130%, p<0.008) and controls (129%, p<0.002 and 116%, p<0.004), but not portal vein thrombosis (34.5% and 39%, NS vs baseline). Portal vein thrombosis and cirrhosis groups had increased cardiac output (4441+/-509 and 3262+/-292) vs controls (1763+/-212 ml/min/m2, p<0.002), but there was no correlation with autonomic neuropathy or with catecholamine levels.

CONCLUSIONS

Autonomic dysfunction and impaired catecholamine response to orthostatic stress occur commonly in portal vein thrombosis and suggest an impairment of the autonomic reflex arc, but changes do not correlate with hemodynamic abnormalities.

Natural Convection: A Mechanism for Transporting Oxygen to Incubating Salmon Eggs

Respiratory exchange at the surface of developing fish eggs is influenced by two processes: diffusion and convection. The respiring egg acts as an oxygen sink, removing dissolved oxygen from the diffusion layer surrounding the outer surfaces of the egg capsule. To maintain the oxygen gradient in the diffusion layer, oxygen must be delivered to it by convection. This paper describes two components of convective transport: that provided by forced convection — the bulk transport of oxygenated water to the egg; and a newly recognized component, natural convection. Salmon normally spawn in gravel environments. Mass transfer of oxygen to incubating eggs, resulting from forced convection, is provided by subsurface water moving with a velocity related primarily to properties of the gravel and the hydraulic gradient. Egg-induced natural convection is seen as a component embedded in these relations. Acting as an emergency oxygen transport and metabolite disposal mechanism at low water velocities, natural convection could maintain perfusion of respiring eggs at some minimum water velocity in the event of diminution or interruption of natural water flow. The relevance of natural convection as a possible mechanism affecting several other respiring systems is discussed. Key words: respiration, forced convection, density of water, bulk transport, diffusion, diffusion layer, oxygen gradient