Papers by Johan van Schalkwyk
Blood Pressure Variability Causes Spurious Identification of Hypertension in Clinical Studies: A Computer Simulation Study
Amer J Hypertens, 2008
The blood pressure (BP) of an individual varies considerably from day to day. Hypertension is com... more The blood pressure (BP) of an individual varies considerably from day to day. Hypertension is commonly identified based on the average of two BPs taken at each of two visits, a practice consistent with current guidelines. We hypothesized that (i) in the setting of high-normal BP ("prehypertension"), this practice results in frequent spurious detection of hypertension, and (ii) that random, spurious detection of hypertension and flawed study design together explain why in the Trial of Preventing Hypertension (TROPHY) study candesartan appeared to suppress the development of hypertension for 2 years after cessation of therapy. We used Monte Carlo simulation to quantify spurious detection of hypertension at repeated clinic visits in one million subjects with unchanging usual systolic BPs (SBPs) between 130 and 139 mm Hg and normal BP variability. Criteria for identifying hypertension derived from Rosner and Polk, Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7), and the Trials of Hypertension Prevention (TOHP), TROPHY, and Framingham studies were applied to Canadian Heart Health Survey data. On the basis of these simulations, we created a three-parameter model of Kaplan-Meier curves recorded in the TROPHY study. We used the model to analyze the rates at which hypertension was identified in the TROPHY study. TROPHY criteria spuriously identify hypertension in 76% of subjects over 18 clinic visits. Higher rates of spurious detection of hypertension are seen with JNC 7, TOHP and Framingham study criteria. Even highly conservative criteria falsely identify hypertension in 27% of subjects over 18 visits. Our three-parameter model suggests no postdiscontinuation benefit of candesartan. When applied over multiple visits, current criteria for detecting hypertension are intolerant of normal BP variation. The use of conservative criteria would reduce spurious identification of hypertension. The apparent long-term beneficial effect of candesartan seen in the TROPHY study can be explained by an inadequate method for detecting hypertension, and flawed study design.
Canadian Journal of Anaesthesia Journal Canadien D Anesthesie, Jul 31, 2010
Purpose The incidence of hypoxemia in patients undergoing surgery is largely unknown and may have... more Purpose The incidence of hypoxemia in patients undergoing surgery is largely unknown and may have a clinical impact. The objective of this study was to determine the incidence of intraoperative hypoxemia in a large surgical population.
Pulmonary Barotrauma and Cerebral Arterial Gas Embolism During Hyperbaric Oxygen Therapy
Aviation Space and Environmental Medicine, Sep 1, 2010
Hyperbaric oxygen therapy (HBOT) is used to treat a variety of disorders. It is a safe treatment ... more Hyperbaric oxygen therapy (HBOT) is used to treat a variety of disorders. It is a safe treatment modality, but rare catastrophic complications may occur. In this case report, we describe the occurrence of irreversible spastic quadriparesis in a patient who suffered a cerebral arterial gas embolism (CAGE) during decompression from HBOT. The patient had a history of respiratory disease and was subsequently found to have bullous changes in the left lung, which almost certainly predisposed to this rare event. We discuss appropriate pretreatment screening to prevent such events and highlight the paradox that HBOT, the cause of the CAGE, is also the treatment of choice.
Incidence de l’hypoxémie pendant la chirurgie: données probantes de deux institutions
Lead poisoning from Ayurvedic medicines
The New Zealand medical journal, Jan 10, 2013
A case of lead poisoning with established exposure to Ayurvedic medicines is presented. This pati... more A case of lead poisoning with established exposure to Ayurvedic medicines is presented. This patient migrated from India to New Zealand 8 years previously. He regularly visits India where he purchases "herbal remedies" for his wellbeing.
American journal of hypertension, 2008
Blood pressure variability causes spurious identification of hypertension in clinical studies: a computer simulation study
American journal of hypertension, 2008
The blood pressure (BP) of an individual varies considerably from day to day. Hypertension is com... more The blood pressure (BP) of an individual varies considerably from day to day. Hypertension is commonly identified based on the average of two BPs taken at each of two visits, a practice consistent with current guidelines. We hypothesized that (i) in the setting of high-normal BP ("prehypertension"), this practice results in frequent spurious detection of hypertension, and (ii) that random, spurious detection of hypertension and flawed study design together explain why in the Trial of Preventing Hypertension (TROPHY) study candesartan appeared to suppress the development of hypertension for 2 years after cessation of therapy. We used Monte Carlo simulation to quantify spurious detection of hypertension at repeated clinic visits in one million subjects with unchanging usual systolic BPs (SBPs) between 130 and 139 mm Hg and normal BP variability. Criteria for identifying hypertension derived from Rosner and Polk, Seventh Report of the Joint National Committee on Prevention, D...
Anaphylaxis is more common with rocuronium and succinylcholine than with atracurium
Anesthesiology, 2015
Intraoperative anaphylaxis is a rare but serious occurrence, often triggered by neuromuscular-blo... more Intraoperative anaphylaxis is a rare but serious occurrence, often triggered by neuromuscular-blocking drugs (NMBDs). Previous reports suggest that the rates of anaphylaxis may be greater for rocuronium than for other NMBDs, but imprecise surrogate metrics for new patient exposures to NMBDs complicate interpretation. This was a retrospective, observational cohort study of intraoperative anaphylaxis to NMBDs at two hospitals between 2006 and 2012. Expert anesthetic and immunologist collaborators investigated all referred cases of intraoperative anaphylaxis where NMBDs were administered and identified those where a NMBD was considered responsible. New patient exposures for each NMBD were extracted from electronic anesthetic records compiled during the same period. Anaphylaxis rates were calculated for each NMBD using diagnosed anaphylaxis cases as the numerator and the number of new patient exposures as the denominator. Twenty-one patients were diagnosed with anaphylaxis to an NMBD. T...
The agreement between reaction-board measurements and kinematic estimation of adult male human whole body centre of mass location during running
Physiological Measurement, 2004
The segmental method for estimating the centre of mass (COM) location of the human body has been ... more The segmental method for estimating the centre of mass (COM) location of the human body has been widely used since 1889. How closely this method agrees with direct measurements of the location and movement of COM during activity however, remains unclear. To test this, a novel reaction-board utilizing life sized projections of human subjects is designed for measuring COM location. Agreement between the segmental method and the more direct reaction-board measurement method is then assessed. Our data demonstrate that the reaction-board system has a physical maximum error of 1.28 cm and 1.95 cm for locating COM along the vertical (board length) and horizontal (board width) axes respectively, and show that the reaction-board and segmental methods agree to within limits of 6.0 cm for the location of COM and to within 5.6 cm for the movement of COM between two points, in recumbent individuals. Applied to running, the segmental method agrees to within limits of 4.8 cm for oscillation of COM and 5.3 cm for stride median COM height. The segmental method agrees with a more direct technique of known accuracy, the reaction-board method, most closely when measuring averaged oscillation over repeated strides, where it displays a measurement error range of 5.1 cm to 0.1 cm in runners.
Pediatric Pulmonology, 1997
Amplitude and phase frequency response characteristics of infant air-balloon catheters (IABC) of ... more Amplitude and phase frequency response characteristics of infant air-balloon catheters (IABC) of differing French gauge (FG) sizes and brands were quantified to determine their suitability for measuring dynamic intra-esophageal pressure (P es ) accurately. Frequency response performances of matching IABC and water-filled catheters (WFC) were also compared using the swept sine wave technique. The maximum respiratory rate within which IABCs could potentially measure P es within a 5% error limit was calculated (F RR ).
The Journal of Clinical Hypertension, 2013
The Journal of Clinical Hypertension, 2013
Diagnosing Hypertension in Children and Adolescents
JAMA, 2008
... 4. Moore WE, Stephens A, Wilson T, Wilson W, Eichner JE. ... To the Editor: In August 2005, H... more ... 4. Moore WE, Stephens A, Wilson T, Wilson W, Eichner JE. ... To the Editor: In August 2005, HurricaneKatrina caused a public health emergency by displacing more than 4400 phy-sicians in the greater New Orleans area and leading to the clo-sure of 13 of 16 hospitals in New ...
Predicting air-balloon and water-filled infant catheter frequency responses
IEEE Engineering in Medicine and Biology Magazine, 1997
... ac ace ad e de - abde - ... ratio of Ar, d, SOFC - ORC phase-shift difference of PAS, e, SOFC... more ... ac ace ad e de - abde - ... ratio of Ar, d, SOFC - ORC phase-shift difference of PAS, e, SOFC-ORC phase-shift difference of Pfr, w/aPor Adc Ven Jag, water-fillediair-balloon Portex Adcock Ven Erich-Jaeger catheter brands and FG size, Variables ae indicated are P < 0 05 between ...
Hypertension, 2011
Current guidelines emphasize the importance of identification and reversal of lifestyle factors i... more Current guidelines emphasize the importance of identification and reversal of lifestyle factors in the management of resistant hypertension. 1 This position is supported by evidence that dietary modification, particularly a decrease in salt intake, is effective not only in mild-to-moderate hypertension 2 but also in resistant hypertension. 3 A recent study in which renal sympathetic innervation was irreversibly disrupted by radiofrequency ablation is, therefore, of concern. 4 We believe that, before heroic measures are taken to control resistant hypertension, it is appropriate not only to attempt dietary modification but also to provide assurance that dietary modification is effective by performing 24-hour urinary sodium measurements. The absence of such information from the study of Krum et al 4 raises the possibility that adequate dietary intervention might have spared at least some of the subjects not only the inconvenience of the intervention but also the 3% complication rate of this experimental procedure. It is possible that the authors, or Ardian Inc (who participated in trial design, data management, data analysis, article preparation, and review), may have such data, in which case we would invite them to provide evidence that maximal attempts at dietary modification were used before radiofrequency ablation.
Canadian Journal of Anesthesia/Journal canadien d'anesthésie, 2010
Purpose The incidence of hypoxemia in patients undergoing surgery is largely unknown and may have... more Purpose The incidence of hypoxemia in patients undergoing surgery is largely unknown and may have a clinical impact. The objective of this study was to determine the incidence of intraoperative hypoxemia in a large surgical population.
Lax sphygmomanometer standard causes overdetection and underdetection of hypertension: a computer simulation study
Blood Pressure Monitoring, 2008
To quantify overdetection and underdetection of hypertension caused by systematic sphygmomanomete... more To quantify overdetection and underdetection of hypertension caused by systematic sphygmomanometer errors permitted by the current European standard (EN 1060 &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;noninvasive sphygmomanometers&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;). We carried out Monte Carlo simulation of measurement of blood pressure (BP) of the adult Australian population using sphygmomanometers showing systematic errors compliant with the EN 1060 standard. We repeated the simulations limiting systematic sphygmomanometer errors to +/-1 mmHg. Simulated BP measurements included systematic sphygmomanometer error, random intraindividual BP variability and random measurement error. After three visits, underdetection of hypertension is common owing to variability of BP measurements and systematic errors of sphygmomanometers. After three visits, the wide tolerances of EN 1060 are responsible for approximately 4.9 and 11% of underdetection of systolic and diastolic hypertension, respectively. Underdetection is worse in some groups, for example, permitted sphygmomanometer error causes 20% of all undetected systolic hypertension in 18-24 year-old women. The current standard also results in overdetection of hypertension. Permitted sphygmomanometer error causes 5.8 and 14% of the overdetection of systolic and diastolic hypertension, respectively, after three visits. Overdetection is worse in some groups, for example, after three visits, permitted sphygmomanometer error causes 19% of falsely detected diastolic hypertension in 18-24 year-old men. For all adults, reduction of permitted sphygmomanometer error to +/-1 mmHg achieves approximately the same improvement in hypertension detection as at least one additional visit to the clinician. Systematic sphygmomanometer errors permitted by the current standard are a preventable cause of clinically significant overdetection and underdetection of hypertension. The standard should be revised to make the effects of equipment related systematic errors negligible compared with the effects of physiological variability.
Pulmonary Barotrauma and Cerebral Arterial Gas Embolism During Hyperbaric Oxygen Therapy
Aviation, Space, and Environmental Medicine, 2010
Hyperbaric oxygen therapy (HBOT) is used to treat a variety of disorders. It is a safe treatment ... more Hyperbaric oxygen therapy (HBOT) is used to treat a variety of disorders. It is a safe treatment modality, but rare catastrophic complications may occur. In this case report, we describe the occurrence of irreversible spastic quadriparesis in a patient who suffered a cerebral arterial gas embolism (CAGE) during decompression from HBOT. The patient had a history of respiratory disease and was subsequently found to have bullous changes in the left lung, which almost certainly predisposed to this rare event. We discuss appropriate pretreatment screening to prevent such events and highlight the paradox that HBOT, the cause of the CAGE, is also the treatment of choice.
Primate Pleuroesophageal Tissue Barrier Frequency Response and Esophageal Pressure Waveform Bandwidth in Health and Acute Lung Injury
Anesthesiology, 2000
Dynamic intraesophageal pressure (Pes) is used to estimate intrapleural pressure (Ppl) to calcula... more Dynamic intraesophageal pressure (Pes) is used to estimate intrapleural pressure (Ppl) to calculate lung compliance and resistance. This study investigated the nonhuman primate Ppl-Pes tissue barrier frequency response and the dynamic response requirements of Pes manometers. In healthy monkeys and monkeys with acute lung injury undergoing ventilation, simultaneous Ppl and Pes were measured directly to determine the Ppl-Pes tissue barrier amplitude frequency response, using the swept-sine wave technique. The bandwidths of physiologic Pes waveforms acquired during conventional mechanical ventilation were calculated using digital low-pass signal filtering. The Ppl-Pes tissue barrier is amplitude-uniform within the bandwidth of conventional Pes waveforms in healthy and acute lung injury lungs, and does not significantly attenuate Ppl-Pes signal transmission between 1 and 40 Hz. At Pes frequencies higher than conventional clinical regions of interest the Ppl-Pes barrier resonates significantly, is pressure amplitude dependent at low-pressure offsets, and is significantly altered by acute lung injury. Allowing for 5% or less Pes waveform error, the maximum Pes bandwidths during conventional ventilation were 1.9 Hz and 3.4 Hz for physiologic and extreme-case waveforms in healthy lungs and 4.6 Hz and 8.5 Hz during acute lung injury. In monkeys, the Ppl-Pes tissue barrier has a frequency response suitable for Ppl estimation during low-frequency mechanical ventilation, and Pes manometers should have a minimum uniform frequency response up to 8.5 Hz. However, the Ppl-Pes tissue barrier adversely affects the accurate estimation of dynamic Ppl at high frequencies, with varied airway pressure amplitudes and offsets, such as the Ppl encountered during high-frequency oscillatory ventilation.
Errors in Assessment of Resident Performance
Anesthesiology, 2012
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Papers by Johan van Schalkwyk